diff --git a/examples/integrations/kibana/elastic_connector.py b/examples/integrations/kibana/elastic_connector.py
index 3da1d2ac..a0ca9f5d 100644
--- a/examples/integrations/kibana/elastic_connector.py
+++ b/examples/integrations/kibana/elastic_connector.py
@@ -2,7 +2,7 @@
 
 
 class ElasticConnector:
-    def __init__(self):
+    def __init__(self) -> None:
         self.host = "localhost"
         self.port = 9200
         self.es = None
diff --git a/popmon/alerting/alerts_summary.py b/popmon/alerting/alerts_summary.py
index 126814cd..f82abc35 100644
--- a/popmon/alerting/alerts_summary.py
+++ b/popmon/alerting/alerts_summary.py
@@ -38,11 +38,11 @@ class AlertsSummary(Module):
     def __init__(
         self,
         read_key,
-        store_key="",
+        store_key: str = "",
         features=None,
         ignore_features=None,
-        combined_variable="_AGGREGATE_",
-    ):
+        combined_variable: str = "_AGGREGATE_",
+    ) -> None:
         """Initialize an instance of AlertsSummary module.
 
         :param str read_key: key of input data to read from datastore.
diff --git a/popmon/alerting/compute_tl_bounds.py b/popmon/alerting/compute_tl_bounds.py
index deea60ee..3b62f4ee 100644
--- a/popmon/alerting/compute_tl_bounds.py
+++ b/popmon/alerting/compute_tl_bounds.py
@@ -30,7 +30,7 @@
 from popmon.base import Module, Pipeline
 
 
-def traffic_light_summary(row, cols=None, prefix=""):
+def traffic_light_summary(row, cols=None, prefix: str = ""):
     """Make a summary of traffic light alerts present in the dataframe
 
     Count number of green, yellow and red traffic lights.
@@ -60,7 +60,9 @@ def traffic_light_summary(row, cols=None, prefix=""):
     return pd.Series(x)
 
 
-def traffic_light(value, red_high, yellow_high, yellow_low=0, red_low=0):
+def traffic_light(
+    value, red_high, yellow_high, yellow_low: int = 0, red_low: int = 0
+) -> int:
     """Get the corresponding traffic light given a value and traffic light bounds.
 
     :param float value: value to check
@@ -120,17 +122,17 @@ def __init__(
         self,
         read_key,
         monitoring_rules=None,
-        store_key="",
+        store_key: str = "",
         features=None,
         ignore_features=None,
-        apply_funcs_key="",
+        apply_funcs_key: str = "",
         func=None,
-        metrics_wide=False,
-        prefix="traffic_light_",
-        suffix="",
-        entire=False,
+        metrics_wide: bool = False,
+        prefix: str = "traffic_light_",
+        suffix: str = "",
+        entire: bool = False,
         **kwargs,
-    ):
+    ) -> None:
         """Initialize an instance of TrafficLightBounds module.
 
         :param str read_key: key of input data to read from datastore
@@ -171,7 +173,7 @@ def get_description(self):
         """Returns the traffic light function as description."""
         return self.traffic_light_func.__name__
 
-    def _set_traffic_lights(self, feature, cols, pattern, rule):
+    def _set_traffic_lights(self, feature, cols, pattern, rule) -> None:
         process_cols = fnmatch.filter(cols, pattern)
 
         for pcol in process_cols:
@@ -250,10 +252,10 @@ def pull_bounds(
     row,
     red_high,
     yellow_high,
-    yellow_low=0,
-    red_low=0,
-    suffix_mean="_mean",
-    suffix_std="_std",
+    yellow_low: int = 0,
+    red_low: int = 0,
+    suffix_mean: str = "_mean",
+    suffix_std: str = "_std",
     cols=None,
 ):
     """Calculate traffic light pull bounds for list of cols
@@ -307,10 +309,10 @@ def df_single_op_pull_bounds(
     df,
     red_high,
     yellow_high,
-    yellow_low=0,
-    red_low=0,
-    suffix_mean="_mean",
-    suffix_std="_std",
+    yellow_low: int = 0,
+    red_low: int = 0,
+    suffix_mean: str = "_mean",
+    suffix_std: str = "_std",
     cols=None,
 ):
     """Calculate traffic light pull bounds for list of cols on first row only
@@ -341,10 +343,10 @@ def __init__(
         self,
         read_key,
         rules,
-        store_key="",
-        suffix_mean="_mean",
-        suffix_std="_std",
-    ):
+        store_key: str = "",
+        suffix_mean: str = "_mean",
+        suffix_std: str = "_std",
+    ) -> None:
         """Initialize an instance of DynamicTrafficLightBounds.
 
         :param str read_key: key of input data to read from data store, only used to extract feature list.
@@ -385,10 +387,10 @@ def __init__(
         self,
         read_key,
         rules,
-        store_key="",
-        suffix_mean="_mean",
-        suffix_std="_std",
-    ):
+        store_key: str = "",
+        suffix_mean: str = "_mean",
+        suffix_std: str = "_std",
+    ) -> None:
         """Initialize an instance of StaticBounds.
 
         :param str read_key: key of input data to read from data store, only used to extract feature list.
@@ -432,7 +434,9 @@ class TrafficLightAlerts(Pipeline):
     - Apply them to profiled test statistics data
     """
 
-    def __init__(self, read_key, store_key, rules, expanded_rules_key=""):
+    def __init__(
+        self, read_key, store_key, rules, expanded_rules_key: str = ""
+    ) -> None:
         """Initialize an instance of TrafficLightBounds.
 
         :param str read_key: key of input data to read from data store, only used to extract feature list.
diff --git a/popmon/analysis/apply_func.py b/popmon/analysis/apply_func.py
index 36aaa5b8..5fbcccff 100644
--- a/popmon/analysis/apply_func.py
+++ b/popmon/analysis/apply_func.py
@@ -40,14 +40,14 @@ class ApplyFunc(Module):
     def __init__(
         self,
         apply_to_key,
-        store_key="",
-        assign_to_key="",
-        apply_funcs_key="",
+        store_key: str = "",
+        assign_to_key: str = "",
+        apply_funcs_key: str = "",
         features=None,
         apply_funcs=None,
         metrics=None,
-        msg="",
-    ):
+        msg: str = "",
+    ) -> None:
         """Initialize an instance of ApplyFunc.
 
         :param str apply_to_key: key of the input data to apply funcs to.
diff --git a/popmon/analysis/comparison/comparisons.py b/popmon/analysis/comparison/comparisons.py
index 29cbb83b..df10d32f 100644
--- a/popmon/analysis/comparison/comparisons.py
+++ b/popmon/analysis/comparison/comparisons.py
@@ -168,7 +168,7 @@ def ks(p, q, *_):
     dim=1,
     htype="cat",
 )
-def unknown_labels(hist1, hist2):
+def unknown_labels(hist1, hist2) -> bool:
     # check consistency of bin_labels
     labels1 = hist1.keySet
     labels2 = hist2.keySet
diff --git a/popmon/analysis/comparison/hist_comparer.py b/popmon/analysis/comparison/hist_comparer.py
index ba069a9f..8adc272d 100644
--- a/popmon/analysis/comparison/hist_comparer.py
+++ b/popmon/analysis/comparison/hist_comparer.py
@@ -40,7 +40,7 @@
 from popmon.hist.hist_utils import COMMON_HIST_TYPES, is_numeric
 
 
-def hist_compare(row, hist_name1="", hist_name2=""):
+def hist_compare(row, hist_name1: str = "", hist_name2: str = ""):
     """Function to compare two histograms
 
     Apply statistical tests to compare two input histograms, such as:
@@ -107,11 +107,11 @@ def __init__(
         read_key,
         store_key,
         assign_to_key=None,
-        hist_col="histogram",
-        suffix="comp",
+        hist_col: str = "histogram",
+        suffix: str = "comp",
         *args,
         **kwargs,
-    ):
+    ) -> None:
         """Initialize an instance of RollingHistComparer.
 
         :param func_hist_collector: histogram collection function
@@ -160,10 +160,10 @@ def __init__(
         read_key,
         store_key,
         window,
-        shift=1,
-        hist_col="histogram",
-        suffix="roll",
-    ):
+        shift: int = 1,
+        hist_col: str = "histogram",
+        suffix: str = "roll",
+    ) -> None:
         """Initialize an instance of RollingHistComparer.
 
         :param str read_key: key of input data to read from data store
@@ -201,9 +201,9 @@ def __init__(
         self,
         read_key,
         store_key,
-        hist_col="histogram",
-        suffix="prev1",
-    ):
+        hist_col: str = "histogram",
+        suffix: str = "prev1",
+    ) -> None:
         """Initialize an instance of PreviousHistComparer.
 
         :param str read_key: key of input data to read from data store
@@ -228,10 +228,10 @@ def __init__(
         self,
         read_key,
         store_key,
-        shift=1,
-        hist_col="histogram",
-        suffix="expanding",
-    ):
+        shift: int = 1,
+        hist_col: str = "histogram",
+        suffix: str = "expanding",
+    ) -> None:
         """Initialize an instance of ExpandingHistComparer.
 
         :param str read_key: key of input data to read from data store
@@ -267,9 +267,9 @@ def __init__(
         reference_key,
         assign_to_key,
         store_key,
-        hist_col="histogram",
-        suffix="ref",
-    ):
+        hist_col: str = "histogram",
+        suffix: str = "ref",
+    ) -> None:
         """Initialize an instance of ReferenceHistComparer.
 
         :param str reference_key: key of input data to read from data store
@@ -306,10 +306,10 @@ def __init__(
         read_key,
         store_key,
         assign_to_key=None,
-        hist_col="histogram",
+        hist_col: str = "histogram",
         *args,
         **kwargs,
-    ):
+    ) -> None:
         """Initialize an instance of NormHistComparer.
 
         :param func_hist_collector: histogram collection function
@@ -349,7 +349,9 @@ def __init__(
 class RollingNormHistComparer(NormHistComparer):
     """Compare histogram to previous rolling normalized histograms"""
 
-    def __init__(self, read_key, store_key, window, shift=1, hist_col="histogram"):
+    def __init__(
+        self, read_key, store_key, window, shift: int = 1, hist_col: str = "histogram"
+    ) -> None:
         """Initialize an instance of RollingNormHistComparer.
 
         :param str read_key: key of input data to read from data store
@@ -383,7 +385,9 @@ def transform(self, datastore):
 class ExpandingNormHistComparer(NormHistComparer):
     """Compare histogram to previous expanding normalized histograms"""
 
-    def __init__(self, read_key, store_key, shift=1, hist_col="histogram"):
+    def __init__(
+        self, read_key, store_key, shift: int = 1, hist_col: str = "histogram"
+    ) -> None:
         """Initialize an instance of ExpandingNormHistComparer.
 
         :param str read_key: key of input data to read from data store
@@ -412,7 +416,9 @@ def transform(self, datastore):
 class ReferenceNormHistComparer(NormHistComparer):
     """Compare histogram to reference normalized histograms"""
 
-    def __init__(self, reference_key, assign_to_key, store_key, hist_col="histogram"):
+    def __init__(
+        self, reference_key, assign_to_key, store_key, hist_col: str = "histogram"
+    ) -> None:
         """Initialize an instance of ReferenceNormHistComparer.
 
         :param str reference_key: key of input data to read from data store
diff --git a/popmon/analysis/functions.py b/popmon/analysis/functions.py
index e0370b44..6d28f75c 100644
--- a/popmon/analysis/functions.py
+++ b/popmon/analysis/functions.py
@@ -34,7 +34,7 @@
 from popmon.stats.numpy import probability_distribution_mean_covariance
 
 
-def pull(row, suffix_mean="_mean", suffix_std="_std", cols=None):
+def pull(row, suffix_mean: str = "_mean", suffix_std: str = "_std", cols=None):
     """Calculate normalized residual (pull) for list of cols
 
     Function can be used by ApplyFunc module.
@@ -74,7 +74,7 @@ def pull(row, suffix_mean="_mean", suffix_std="_std", cols=None):
     return pd.Series(x)
 
 
-def expanding_mean(df, shift=1):
+def expanding_mean(df, shift: int = 1):
     """Calculate expanding mean of all numeric columns of a pandas dataframe
 
     Function can be used by ApplyFunc module.
@@ -86,7 +86,7 @@ def expanding_mean(df, shift=1):
     return df.shift(shift).expanding().mean()
 
 
-def expanding_std(df, shift=1):
+def expanding_std(df, shift: int = 1):
     """Calculate expanding std of all numeric columns of a pandas dataframe
 
     Function can be used by ApplyFunc module.
@@ -98,7 +98,7 @@ def expanding_std(df, shift=1):
     return df.shift(shift).expanding().std()
 
 
-def expanding_apply(df, func, shift=1, *args, **kwargs):
+def expanding_apply(df, func, shift: int = 1, *args, **kwargs):
     """Calculate expanding apply() to all columns of a pandas dataframe
 
     Function can be used by ApplyFunc module.
@@ -113,7 +113,7 @@ def expanding_apply(df, func, shift=1, *args, **kwargs):
     return df.shift(shift).expanding().apply(func, args=args, **kwargs)
 
 
-def rolling_std(df, window, shift=1):
+def rolling_std(df, window, shift: int = 1):
     """Calculate rolling std of all numeric columns of a pandas dataframe
 
     Function can be used by ApplyFunc module.
@@ -126,7 +126,7 @@ def rolling_std(df, window, shift=1):
     return df.shift(shift).rolling(window).std()
 
 
-def rolling_mean(df, window, shift=1):
+def rolling_mean(df, window, shift: int = 1):
     """Calculate rolling mean of all numeric columns of a pandas dataframe
 
     Function can be used by ApplyFunc module.
@@ -139,7 +139,7 @@ def rolling_mean(df, window, shift=1):
     return df.shift(shift).rolling(window).mean()
 
 
-def rolling_apply(df, window, func, shift=1, *args, **kwargs):
+def rolling_apply(df, window, func, shift: int = 1, *args, **kwargs):
     """Calculate rolling apply() to all columns of a pandas dataframe
 
     Function can be used by ApplyFunc module.
@@ -156,7 +156,7 @@ def rolling_apply(df, window, func, shift=1, *args, **kwargs):
     return df.shift(shift).rolling(window).apply(func, raw=False, args=args, **kwargs)
 
 
-def rolling_lr(df, window, index=0, shift=0):
+def rolling_lr(df, window, index: int = 0, shift: int = 0):
     """Calculate rolling scipy lin_regress() to all columns of a pandas dataframe
 
     Function can be used by ApplyFunc module.
@@ -175,7 +175,7 @@ def rolling_lr(df, window, index=0, shift=0):
     )
 
 
-def rolling_lr_zscore(df, window, shift=0):
+def rolling_lr_zscore(df, window, shift: int = 0):
     """Calculate rolling z-score of scipy lin_regress() to all columns of a pandas dataframe
 
     Function can be used by ApplyFunc module.
@@ -202,7 +202,7 @@ def func(x):
     return roll(df, window=window, shift=shift).apply(func, axis=1)
 
 
-def roll(df, window, shift=1):
+def roll(df, window, shift: int = 1):
     """Implementation of rolling window that can handle non-numerical columns such as histograms
 
     :param pd.DataFrame df: input dataframe to apply rolling function to.
@@ -250,7 +250,7 @@ def reshape(vs, i):
     return rolled_df
 
 
-def expand(df, shift=1):
+def expand(df, shift: int = 1):
     """Implementation of expanding window that can handle non-numerical values such as histograms
 
     Split up input array into expanding sub-arrays
@@ -283,7 +283,7 @@ def reshape(vs, i):
     return expanded_df
 
 
-def expanding_hist(df, shift=1, *args, **kwargs):
+def expanding_hist(df, shift: int = 1, *args, **kwargs):
     """Apply expanding histogram sum
 
     Function can be used by ApplyFunc module.
@@ -297,7 +297,7 @@ def expanding_hist(df, shift=1, *args, **kwargs):
     return expand(df, shift=shift).apply(hist_sum, axis=1, args=args, **kwargs)
 
 
-def rolling_hist(df, window, shift=1, *args, **kwargs):
+def rolling_hist(df, window, shift: int = 1, *args, **kwargs):
     """Apply rolling histogram sum
 
     Function can be used by ApplyFunc module.
@@ -314,7 +314,7 @@ def rolling_hist(df, window, shift=1, *args, **kwargs):
     )
 
 
-def hist_sum(x, hist_name=""):
+def hist_sum(x, hist_name: str = ""):
     """Return sum of histograms
 
     Usage: df['hists'].apply(hist_sum) ; series.apply(hist_sum)
@@ -351,7 +351,7 @@ def hist_sum(x, hist_name=""):
     return pd.Series(o)
 
 
-def roll_norm_hist_mean_cov(df, window, shift=1, *args, **kwargs):
+def roll_norm_hist_mean_cov(df, window, shift: int = 1, *args, **kwargs):
     """Apply rolling normalized_hist_mean_cov function
 
     Function can be used by ApplyFunc module.
@@ -368,7 +368,7 @@ def roll_norm_hist_mean_cov(df, window, shift=1, *args, **kwargs):
     )
 
 
-def expand_norm_hist_mean_cov(df, shift=1, *args, **kwargs):
+def expand_norm_hist_mean_cov(df, shift: int = 1, *args, **kwargs):
     """Apply expanding normalized_hist_mean_cov function
 
     Function can be used by ApplyFunc module.
@@ -384,7 +384,7 @@ def expand_norm_hist_mean_cov(df, shift=1, *args, **kwargs):
     )
 
 
-def normalized_hist_mean_cov(x, hist_name=""):
+def normalized_hist_mean_cov(x, hist_name: str = ""):
     """Mean normalized histogram and its covariance of list of input histograms
 
     Usage: df['hists'].apply(normalized_hist_mean_cov) ; series.apply(normalized_hist_mean_cov)
@@ -446,10 +446,10 @@ def normalized_hist_mean_cov(x, hist_name=""):
 
 def relative_chi_squared(
     row,
-    hist_name="histogram",
-    suffix_mean="_mean",
-    suffix_cov="_cov",
-    suffix_binning="_binning",
+    hist_name: str = "histogram",
+    suffix_mean: str = "_mean",
+    suffix_cov: str = "_cov",
+    suffix_binning: str = "_binning",
 ):
     """Calculate chi squared of normalized histogram with pre-calculated mean normalized histogram
 
diff --git a/popmon/analysis/hist_numpy.py b/popmon/analysis/hist_numpy.py
index abf2c201..bbe3a1c0 100644
--- a/popmon/analysis/hist_numpy.py
+++ b/popmon/analysis/hist_numpy.py
@@ -19,6 +19,7 @@
 
 
 import warnings
+from typing import List
 
 import histogrammar
 import numpy as np
@@ -102,7 +103,7 @@ def set_ndgrid(hist, keys, n_dim):
         )
         return grid
 
-    def flatten(histogram, keys, grid, dim=0, prefix=None):
+    def flatten(histogram, keys, grid, dim: int = 0, prefix=None):
         if prefix is None:
             prefix = []
 
@@ -137,7 +138,7 @@ def set_2dgrid(hist, keys):
     return set_ndgrid(hist, keys, n_dim=2)
 
 
-def get_ndgrid(hist, get_bin_labels=False, n_dim=2):
+def get_ndgrid(hist, get_bin_labels: bool = False, n_dim: int = 2):
     """Get filled n-d grid of first n dimensions of input histogram
 
     :param hist: input histogrammar histogram
@@ -158,7 +159,7 @@ def get_ndgrid(hist, get_bin_labels=False, n_dim=2):
     return grid
 
 
-def get_2dgrid(hist, get_bin_labels=False):
+def get_2dgrid(hist, get_bin_labels: bool = False):
     """Get filled x,y grid of first two dimensions of input histogram
 
     :param hist: input histogrammar histogram
@@ -167,7 +168,9 @@ def get_2dgrid(hist, get_bin_labels=False):
     return get_ndgrid(hist, get_bin_labels, n_dim=2)
 
 
-def get_consistent_numpy_ndgrids(hist_list=None, get_bin_labels=False, dim=3):
+def get_consistent_numpy_ndgrids(
+    hist_list=None, get_bin_labels: bool = False, dim: int = 3
+):
     """Get list of consistent x,y grids of first n dimensions of (sparse) input histograms
 
     :param list hist_list: list of input histogrammar histograms
@@ -184,22 +187,22 @@ def get_consistent_numpy_ndgrids(hist_list=None, get_bin_labels=False, dim=3):
         )
     assert_similar_hists(hist_list)
 
-    keys = [set() for _ in range(dim)]
+    keys: List[set] = [set() for _ in range(dim)]
     for hist in hist_list:
         hist_keys = prepare_ndgrid(hist, n_dim=dim)
         for i, h_keys in enumerate(hist_keys):
             keys[i] |= set(h_keys)
-    keys = [sorted(k) for k in keys]
+    sorted_keys = [sorted(k) for k in keys]
 
-    gridnd_list = [set_ndgrid(hist, keys, n_dim=dim) for hist in hist_list]
+    gridnd_list = [set_ndgrid(hist, sorted_keys, n_dim=dim) for hist in hist_list]
 
     if get_bin_labels:
-        return gridnd_list, keys
+        return gridnd_list, sorted_keys
 
     return gridnd_list
 
 
-def get_consistent_numpy_2dgrids(hist_list=None, get_bin_labels=False):
+def get_consistent_numpy_2dgrids(hist_list=None, get_bin_labels: bool = False):
     """Get list of consistent x,y grids of first two dimensions of (sparse) input histograms
 
     :param list hist_list: list of input histogrammar histograms
@@ -211,7 +214,9 @@ def get_consistent_numpy_2dgrids(hist_list=None, get_bin_labels=False):
     return get_consistent_numpy_ndgrids(hist_list, get_bin_labels, dim=2)
 
 
-def get_consistent_numpy_1dhists(hist_list, get_bin_labels=False, crop_range=False):
+def get_consistent_numpy_1dhists(
+    hist_list, get_bin_labels: bool = False, crop_range: bool = False
+):
     """Get list of consistent numpy hists for list of sparse (or bin) input histograms
 
     Works for sparse and bin histograms.
@@ -294,7 +299,7 @@ def get_consistent_numpy_1dhists(hist_list, get_bin_labels=False, crop_range=Fal
         return nphist_list
 
 
-def get_consistent_numpy_entries(hist_list, get_bin_labels=False):
+def get_consistent_numpy_entries(hist_list, get_bin_labels: bool = False):
     """Get list of consistent numpy bin_entries for list of 1d input histograms
 
     Works for categorize, sparse and bin histograms.
@@ -345,7 +350,7 @@ def get_consistent_numpy_entries(hist_list, get_bin_labels=False):
         return entries_list
 
 
-def get_contentType(hist):
+def get_contentType(hist) -> str:
     """Get content type of bins of histogram
 
     :param hist: input histogram
@@ -362,7 +367,9 @@ def get_contentType(hist):
     return "Count"
 
 
-def check_similar_hists(hist_list, check_type=True, assert_type=used_hist_types):
+def check_similar_hists(
+    hist_list, check_type: bool = True, assert_type=used_hist_types
+) -> bool:
     """Check consistent list of input histograms
 
     Check that type and dimension of all histograms in input list are the same.
@@ -478,7 +485,9 @@ def check_similar_hists(hist_list, check_type=True, assert_type=used_hist_types)
     return True
 
 
-def assert_similar_hists(hist_list, check_type=True, assert_type=used_hist_types):
+def assert_similar_hists(
+    hist_list, check_type: bool = True, assert_type=used_hist_types
+):
     """Assert consistent list of input histograms
 
     Assert that type and dimension of all histograms in input list are the same.
diff --git a/popmon/analysis/merge_statistics.py b/popmon/analysis/merge_statistics.py
index 0cc95d2a..83a01c82 100644
--- a/popmon/analysis/merge_statistics.py
+++ b/popmon/analysis/merge_statistics.py
@@ -30,7 +30,7 @@ class MergeStatistics(Module):
     _input_keys = ("read_keys",)
     _output_keys = ("store_key",)
 
-    def __init__(self, read_keys: list[str], store_key: str):
+    def __init__(self, read_keys: list[str], store_key: str) -> None:
         """Initialize an instance of MergeStatistics.
 
         :param list read_keys: list of keys of input data to read from the datastore
diff --git a/popmon/analysis/profiling/hist_profiler.py b/popmon/analysis/profiling/hist_profiler.py
index 63f78496..b067bdd8 100644
--- a/popmon/analysis/profiling/hist_profiler.py
+++ b/popmon/analysis/profiling/hist_profiler.py
@@ -56,10 +56,10 @@ def __init__(
         features=None,
         ignore_features=None,
         var_timestamp=None,
-        hist_col="histogram",
-        index_col="date",
+        hist_col: str = "histogram",
+        index_col: str = "date",
         stats_functions=None,
-    ):
+    ) -> None:
         super().__init__()
         self.read_key = read_key
         self.store_key = store_key
diff --git a/popmon/analysis/profiling/pull_calculator.py b/popmon/analysis/profiling/pull_calculator.py
index 6795559e..b1d0e0a6 100644
--- a/popmon/analysis/profiling/pull_calculator.py
+++ b/popmon/analysis/profiling/pull_calculator.py
@@ -52,13 +52,13 @@ def __init__(
         apply_to_key,
         assign_to_key=None,
         store_key=None,
-        suffix_mean="_mean",
-        suffix_std="_std",
-        suffix_pull="_pull",
+        suffix_mean: str = "_mean",
+        suffix_std: str = "_std",
+        suffix_pull: str = "_pull",
         features=None,
         *args,
         **kwargs,
-    ):
+    ) -> None:
         """Initialize an instance of HistComparer.
 
         :param str func_mean: applied-function to calculate mean of profiled statistics
@@ -109,15 +109,15 @@ def __init__(
         self,
         read_key,
         window,
-        shift=1,
+        shift: int = 1,
         features=None,
         store_key=None,
-        suffix_mean="_roll_mean",
-        suffix_std="_roll_std",
-        suffix_pull="_roll_pull",
+        suffix_mean: str = "_roll_mean",
+        suffix_std: str = "_roll_std",
+        suffix_pull: str = "_roll_pull",
         *args,
         **kwargs,
-    ):
+    ) -> None:
         """Initialize an instance of HistComparer.
 
         :param str read_key: key of input data to read from data store
@@ -161,15 +161,15 @@ class ExpandingPullCalculator(PullCalculator):
     def __init__(
         self,
         read_key,
-        shift=1,
+        shift: int = 1,
         features=None,
         store_key=None,
-        suffix_mean="_exp_mean",
-        suffix_std="_exp_std",
-        suffix_pull="_exp_pull",
+        suffix_mean: str = "_exp_mean",
+        suffix_std: str = "_exp_std",
+        suffix_pull: str = "_exp_pull",
         *args,
         **kwargs,
-    ):
+    ) -> None:
         """Initialize an instance of HistComparer.
 
         :param str read_key: key of input data to read from data store
@@ -214,12 +214,12 @@ def __init__(
         assign_to_key,
         store_key=None,
         features=None,
-        suffix_mean="_ref_mean",
-        suffix_std="_ref_std",
-        suffix_pull="_ref_pull",
+        suffix_mean: str = "_ref_mean",
+        suffix_std: str = "_ref_std",
+        suffix_pull: str = "_ref_pull",
         *args,
         **kwargs,
-    ):
+    ) -> None:
         """Initialize an instance of HistComparer.
 
         :param str reference_key: key of input data to read from data store
@@ -264,12 +264,12 @@ def __init__(
         assign_to_key,
         store_key=None,
         features=None,
-        suffix_mean="_ref_mean",
-        suffix_std="_ref_std",
-        suffix_pull="_ref_pull",
+        suffix_mean: str = "_ref_mean",
+        suffix_std: str = "_ref_std",
+        suffix_pull: str = "_ref_pull",
         *args,
         **kwargs,
-    ):
+    ) -> None:
         """Initialize an instance of HistComparer.
 
         :param str reference_key: key of input data to read from data store
diff --git a/popmon/base/module.py b/popmon/base/module.py
index 3c27d415..97fe39d8 100644
--- a/popmon/base/module.py
+++ b/popmon/base/module.py
@@ -21,6 +21,7 @@
 import logging
 from abc import ABCMeta
 from functools import wraps
+from typing import Optional, Tuple
 
 
 def datastore_helper(func):
@@ -88,10 +89,10 @@ def combine_classes(*args):
 class Module(metaclass=combine_classes(ABCMeta, ModuleMetaClass)):
     """Abstract base class used for modules in a pipeline."""
 
-    _input_keys = None
-    _output_keys = None
+    _input_keys: Optional[Tuple[str, ...]] = None
+    _output_keys: Optional[Tuple[str, ...]] = None
 
-    def __init__(self):
+    def __init__(self) -> None:
         """Module initialization"""
         self.logger = logging.getLogger()
         self.features = []
@@ -113,10 +114,10 @@ def get_inputs(self):
     def get_outputs(self):
         return self._get_values(self._output_keys)
 
-    def get_description(self):
+    def get_description(self) -> str:
         return ""
 
-    def set_logger(self, logger):
+    def set_logger(self, logger) -> None:
         """Set logger of module
 
         :param logger: input logger
@@ -171,7 +172,7 @@ def get_features(self, all_features: list) -> list:
         features = [feature for feature in features if feature in all_features]
         return features
 
-    def transform(self, *args):
+    def transform(self, *args, **kwargs):
         """Central function of the module.
 
         Typically transform() takes something from the datastore, does something to it, and puts the results
@@ -183,7 +184,7 @@ def transform(self, *args):
         """
         raise NotImplementedError
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         """String representation for modules when printing a pipeline/list of modules"""
         name = self.__class__.__name__
         input_keys = [f"{v}='{getattr(self, v)}'" for v in self._input_keys]
diff --git a/popmon/base/pipeline.py b/popmon/base/pipeline.py
index f3f753de..b9460933 100644
--- a/popmon/base/pipeline.py
+++ b/popmon/base/pipeline.py
@@ -24,7 +24,7 @@
 class Pipeline:
     """Base class used for to run modules in a pipeline."""
 
-    def __init__(self, modules, logger: logging.Logger | None = None):
+    def __init__(self, modules, logger: logging.Logger | None = None) -> None:
         """Initialization of the pipeline
 
         :param list modules: modules of the pipeline.
@@ -33,7 +33,7 @@ def __init__(self, modules, logger: logging.Logger | None = None):
         self.modules = modules
         self.set_logger(logger)
 
-    def set_logger(self, logger: logging.Logger | None):
+    def set_logger(self, logger: logging.Logger | None) -> None:
         """Set the logger to be used by each module
 
         :param logger: input logger
@@ -42,7 +42,7 @@ def set_logger(self, logger: logging.Logger | None):
         for module in self.modules:
             module.set_logger(self.logger)
 
-    def add_modules(self, modules):
+    def add_modules(self, modules) -> None:
         """Add more modules to existing list of modules.
 
         :param list modules: list of more modules
@@ -67,7 +67,7 @@ def transform(self, datastore):
             datastore = module.transform(datastore)
         return datastore
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         """String representation for pipeline"""
         name = self.__class__.__name__
         ret = f"{name}: [\n"
diff --git a/popmon/base/registry.py b/popmon/base/registry.py
index b38d0627..96f14e09 100644
--- a/popmon/base/registry.py
+++ b/popmon/base/registry.py
@@ -19,17 +19,19 @@
 from __future__ import annotations
 
 from collections import defaultdict
-from typing import Callable
+from typing import Any, Callable, DefaultDict, Dict, List, Optional, Tuple, Union
 
 
 class Registry:
     _properties = ("dim", "htype")
 
-    def __init__(self):
-        self._keys: list[str] = []
-        self._descriptions: dict[str, str] = {}
-        self._properties_to_func = defaultdict(lambda: defaultdict(dict))
-        self._func_name_to_properties = {}
+    def __init__(self) -> None:
+        self._keys: List[str] = []
+        self._descriptions: Dict[str, str] = {}
+        self._properties_to_func: DefaultDict[
+            str, DefaultDict[str, Dict[Any, Any]]
+        ] = defaultdict(lambda: defaultdict(dict))
+        self._func_name_to_properties: Dict[Any, Any] = {}
 
     def register(
         self,
diff --git a/popmon/config.py b/popmon/config.py
index 51169cee..24a921c8 100644
--- a/popmon/config.py
+++ b/popmon/config.py
@@ -355,7 +355,7 @@ class Settings(ValidatedSettings):
     """
 
     # Config utilities
-    def _ensure_features_time_axis(self):
+    def _ensure_features_time_axis(self) -> None:
         self.features = [
             c if c.startswith(self.time_axis) else f"{self.time_axis}:{c}"
             for c in self.features
@@ -375,7 +375,7 @@ def _set_time_axis_dataframe(self, df):
                 f"Found {num} time-axes: {time_axes}. Set *one* time_axis manually!"
             )
 
-    def _set_time_axis_hists(self, hists):
+    def _set_time_axis_hists(self, hists) -> None:
         # auto guess the time_axis: find the most frequent first column name in the histograms list
         first_cols = [k.split(":")[0] for k in list(hists.keys())]
         self.time_axis = max(set(first_cols), key=first_cols.count)
diff --git a/popmon/extensions/extension.py b/popmon/extensions/extension.py
index ad53218d..6149fbe2 100644
--- a/popmon/extensions/extension.py
+++ b/popmon/extensions/extension.py
@@ -22,7 +22,7 @@
 from typing import Callable
 
 
-def is_installed(package):
+def is_installed(package) -> bool:
     is_present = importlib.util.find_spec(package)
     return is_present is not None
 
@@ -33,7 +33,7 @@ class Extension:
     # should also be added to `pyproject.toml` optional-dependencies
     requirements: list[str]
 
-    def check(self):
+    def check(self) -> None:
         if all(is_installed(package) for package in self.requirements):
             func = self.extension
             func = func.__func__
diff --git a/popmon/extensions/profile_diptest.py b/popmon/extensions/profile_diptest.py
index 92118655..e8200f3e 100644
--- a/popmon/extensions/profile_diptest.py
+++ b/popmon/extensions/profile_diptest.py
@@ -34,7 +34,7 @@
 from popmon.extensions.extension import Extension
 
 
-def extension():
+def extension() -> None:
     from diptest import diptest
 
     @Profiles.register(
diff --git a/popmon/extensions/utils.py b/popmon/extensions/utils.py
new file mode 100644
index 00000000..e69de29b
diff --git a/popmon/hist/hist_splitter.py b/popmon/hist/hist_splitter.py
index 168a729f..684dfa91 100644
--- a/popmon/hist/hist_splitter.py
+++ b/popmon/hist/hist_splitter.py
@@ -46,15 +46,15 @@ def __init__(
         store_key,
         features=None,
         ignore_features=None,
-        feature_begins_with="",
-        project_on_axes=True,
-        flatten_output=False,
-        short_keys=True,
+        feature_begins_with: str = "",
+        project_on_axes: bool = True,
+        flatten_output: bool = False,
+        short_keys: bool = True,
         var_timestamp=None,
-        index_col="date",
-        hist_col="histogram",
-        filter_empty_split_hists=True,
-    ):
+        index_col: str = "date",
+        hist_col: str = "histogram",
+        filter_empty_split_hists: bool = True,
+    ) -> None:
         """Initialize an instance.
 
         :param str read_key: key of input histogram-dict to read from data store
@@ -90,7 +90,7 @@ def __init__(
                 "flatten_output requires short_keys attribute to be False."
             )
 
-    def get_description(self):
+    def get_description(self) -> str:
         return ""
 
     def update_divided(self, divided, split, yname):
diff --git a/popmon/hist/hist_utils.py b/popmon/hist/hist_utils.py
index 83967d65..75ffcbba 100644
--- a/popmon/hist/hist_utils.py
+++ b/popmon/hist/hist_utils.py
@@ -45,7 +45,7 @@
 HG_FACTORY = histogrammar.Factory()
 
 
-def sum_entries(hist, default=True):
+def sum_entries(hist, default: bool = True):
     """Recursively get sum of entries of histogram
 
     Sometimes hist.entries gives zero as answer? This function always works though.
@@ -164,7 +164,7 @@ def sum_over_x(hist):
     return h_proj
 
 
-def project_split2dhist_on_axis(splitdict, axis="x"):
+def project_split2dhist_on_axis(splitdict, axis: str = "x"):
     """Project a split 2d-histogram onto one axis
 
     Project a 2d hist that's been split with function split_hist_along_first_dimension
@@ -246,11 +246,11 @@ def get_bin_centers(hist):
 
 def split_hist_along_first_dimension(
     hist,
-    xname="x",
-    yname="y",
-    short_keys=True,
-    convert_time_index=True,
-    filter_empty_split_hists=True,
+    xname: str = "x",
+    yname: str = "y",
+    short_keys: bool = True,
+    convert_time_index: bool = True,
+    filter_empty_split_hists: bool = True,
 ):
     """Split (multi-dimensional) hist into sub-hists along x-axis
 
diff --git a/popmon/io/file_reader.py b/popmon/io/file_reader.py
index 99175382..f33b1308 100644
--- a/popmon/io/file_reader.py
+++ b/popmon/io/file_reader.py
@@ -37,7 +37,7 @@ def __init__(
         file_path: str | Path,
         apply_func: Callable | None = None,
         **kwargs,
-    ):
+    ) -> None:
         """Initialize an instance.
 
         :param str store_key: key of input data to be stored in the datastore
diff --git a/popmon/io/file_writer.py b/popmon/io/file_writer.py
index e80db281..2ead8e52 100644
--- a/popmon/io/file_writer.py
+++ b/popmon/io/file_writer.py
@@ -39,7 +39,7 @@ def __init__(
         file_path: str | Path | None = None,
         apply_func: Callable | None = None,
         **kwargs,
-    ):
+    ) -> None:
         """Initialize an instance.
 
         :param str read_key: key of input histogram-dict to read from data store
diff --git a/popmon/io/json_reader.py b/popmon/io/json_reader.py
index e7f4a3be..811a6317 100644
--- a/popmon/io/json_reader.py
+++ b/popmon/io/json_reader.py
@@ -28,7 +28,7 @@
 class JsonReader(FileReader):
     """Read json file's contents into the datastore."""
 
-    def __init__(self, file_path: str | Path, store_key: str):
+    def __init__(self, file_path: str | Path, store_key: str) -> None:
         """Initialize an instance.
 
         :param str store_key: key of input data to be stored in the datastore
diff --git a/popmon/pipeline/amazing_pipeline.py b/popmon/pipeline/amazing_pipeline.py
index 8c26ab06..52c84715 100644
--- a/popmon/pipeline/amazing_pipeline.py
+++ b/popmon/pipeline/amazing_pipeline.py
@@ -27,7 +27,7 @@
 
 
 class AmazingPipeline(Pipeline):
-    def __init__(self, histogram_path: str, **kwargs):
+    def __init__(self, histogram_path: str, **kwargs) -> None:
         modules = [
             JsonReader(file_path=histogram_path, store_key=kwargs["hists_key"]),
             # Or ExternalReference, RollingReference etc.
@@ -36,7 +36,7 @@ def __init__(self, histogram_path: str, **kwargs):
         super().__init__(modules)
 
 
-def run():
+def run() -> None:
     """Example that run self-reference pipeline and produces monitoring report"""
     logging.basicConfig(
         level=logging.INFO, format="%(asctime)s %(levelname)s [%(module)s]: %(message)s"
diff --git a/popmon/pipeline/metrics.py b/popmon/pipeline/metrics.py
index 19ecb1ca..7e3b0162 100644
--- a/popmon/pipeline/metrics.py
+++ b/popmon/pipeline/metrics.py
@@ -20,6 +20,7 @@
 from __future__ import annotations
 
 import logging
+from typing import Optional
 
 from histogrammar.dfinterface.make_histograms import get_bin_specs, make_histograms
 
@@ -74,9 +75,9 @@ def stability_metrics(
 
 def df_stability_metrics(
     df,
-    settings: Settings = None,
+    settings: Optional[Settings] = None,
     time_width=None,
-    time_offset=0,
+    time_offset: int = 0,
     var_dtype=None,
     reference=None,
     **kwargs,
diff --git a/popmon/pipeline/metrics_pipelines.py b/popmon/pipeline/metrics_pipelines.py
index bee1f270..258b3edf 100644
--- a/popmon/pipeline/metrics_pipelines.py
+++ b/popmon/pipeline/metrics_pipelines.py
@@ -73,7 +73,7 @@ def get_metrics_pipeline_class(reference_type, reference):
 def create_metrics_pipeline(
     settings: Settings,
     reference=None,
-    hists_key="hists",
+    hists_key: str = "hists",
     **kwargs,
 ):
     # configuration and datastore for report pipeline
@@ -214,7 +214,7 @@ def __init__(
         self,
         settings: Settings,
         hists_key,
-    ):
+    ) -> None:
         """Example metrics pipeline for comparing test data with itself (full test set)
 
         :param str hists_key: key to test histograms in datastore. default is 'test_hists'
@@ -269,9 +269,9 @@ class ExternalReferenceMetricsPipeline(Pipeline):
     def __init__(
         self,
         settings: Settings,
-        hists_key="test_hists",
-        ref_hists_key="ref_hists",
-    ):
+        hists_key: str = "test_hists",
+        ref_hists_key: str = "ref_hists",
+    ) -> None:
         """Example metrics pipeline for comparing test data with other (full) external reference set
 
         :param str hists_key: key to test histograms in datastore. default is 'test_hists'
@@ -332,8 +332,8 @@ class RollingReferenceMetricsPipeline(Pipeline):
     def __init__(
         self,
         settings: Settings,
-        hists_key="test_hists",
-    ):
+        hists_key: str = "test_hists",
+    ) -> None:
         """Example metrics pipeline for comparing test data with itself (rolling test set)
 
         :param str hists_key: key to test histograms in datastore. default is 'test_hists'
@@ -391,8 +391,8 @@ class ExpandingReferenceMetricsPipeline(Pipeline):
     def __init__(
         self,
         settings: Settings,
-        hists_key="test_hists",
-    ):
+        hists_key: str = "test_hists",
+    ) -> None:
         """Example metrics pipeline for comparing test data with itself (expanding test set)
 
         :param str hists_key: key to test histograms in datastore. default is 'test_hists'
diff --git a/popmon/pipeline/report.py b/popmon/pipeline/report.py
index 865da325..8ce524eb 100644
--- a/popmon/pipeline/report.py
+++ b/popmon/pipeline/report.py
@@ -80,9 +80,9 @@ def stability_report(
 
 def df_stability_report(
     df,
-    settings: Settings = None,
+    settings: Optional[Settings] = None,
     time_width=None,
-    time_offset=0,
+    time_offset: int = 0,
     var_dtype=None,
     reference=None,
     split=None,
@@ -210,7 +210,7 @@ class StabilityReport:
     as a HTML string, HTML file or Jupyter notebook's cell output.
     """
 
-    def __init__(self, datastore, read_key="html_report"):
+    def __init__(self, datastore, read_key: str = "html_report") -> None:
         """Initialize an instance of StabilityReport.
 
         :param str read_key: key of HTML report data to read from data store. default is html_report.
@@ -232,11 +232,11 @@ def _repr_html_(self):
 
         return display(self.to_notebook_iframe())
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         """Override so that Jupyter Notebook does not print the object."""
         return ""
 
-    def to_html(self, escape=False):
+    def to_html(self, escape: bool = False):
         """HTML code representation of the report (represented as a string).
 
         :param bool escape: escape characters which could conflict with other HTML code. default: False
@@ -249,7 +249,7 @@ def to_html(self, escape=False):
             return html.escape(self.html_report)
         return self.html_report
 
-    def to_file(self, filename):
+    def to_file(self, filename) -> None:
         """Store HTML report in the local file system.
 
         :param str filename: filename for the HTML report
@@ -257,7 +257,7 @@ def to_file(self, filename):
         with open(filename, "w+") as file:
             file.write(self.to_html())
 
-    def to_notebook_iframe(self, width="100%", height="100%"):
+    def to_notebook_iframe(self, width: str = "100%", height: str = "100%"):
         """HTML representation of the class (report) embedded in an iframe.
 
         :param str width: width of the frame to be shown
@@ -280,7 +280,7 @@ def regenerate(
         self,
         store_key: str = "html_report",
         sections_key: str = "report_sections",
-        settings: Settings = None,
+        settings: Optional[Settings] = None,
     ):
         """Regenerate HTML report with different plot settings
         :param str sections_key: key to store sections data in the datastore. default is 'report_sections'.
diff --git a/popmon/pipeline/report_pipelines.py b/popmon/pipeline/report_pipelines.py
index 268507d6..2c8b8c3a 100644
--- a/popmon/pipeline/report_pipelines.py
+++ b/popmon/pipeline/report_pipelines.py
@@ -66,7 +66,7 @@ def __init__(
         self,
         settings: Settings,
         hists_key: str = "test_hists",
-    ):
+    ) -> None:
         """Example pipeline for comparing test data with itself (full test set)
 
         :param str hists_key: key to test histograms in datastore. default is 'test_hists'
@@ -93,7 +93,7 @@ def __init__(
         settings: Settings,
         hists_key: str = "test_hists",
         ref_hists_key: str = "ref_hists",
-    ):
+    ) -> None:
         """Example pipeline for comparing test data with other (full) external reference set
 
         :param str hists_key: key to test histograms in datastore. default is 'test_hists'
@@ -122,7 +122,7 @@ def __init__(
         self,
         settings: Settings,
         hists_key: str = "test_hists",
-    ):
+    ) -> None:
         """Example pipeline for comparing test data with itself (rolling test set)
 
         :param str hists_key: key to test histograms in datastore. default is 'test_hists'
@@ -148,7 +148,7 @@ def __init__(
         self,
         settings: Settings,
         hists_key: str = "test_hists",
-    ):
+    ) -> None:
         """Example pipeline for comparing test data with itself (expanding test set)
 
         :param str hists_key: key to test histograms in datastore. default is 'test_hists'
@@ -177,7 +177,7 @@ def __init__(
         settings: Settings,
         sections_key: str = "report_sections",
         store_key: str = "html_report",
-    ):
+    ) -> None:
         """Initialize an instance of Report.
 
         :param Settings settings: the configuration object
@@ -244,7 +244,7 @@ def __init__(
         ]
         if (
             isinstance(settings.report.report_filepath, (str, Path))
-            and len(settings.report.report_filepath) > 0
+            and len(str(settings.report.report_filepath)) > 0
         ):
             modules.append(
                 FileWriter(store_key, file_path=settings.report.report_filepath)
diff --git a/popmon/pipeline/timing.py b/popmon/pipeline/timing.py
index 95079214..7e6f37c8 100644
--- a/popmon/pipeline/timing.py
+++ b/popmon/pipeline/timing.py
@@ -31,7 +31,7 @@ def __init__(
         self,
         store_key: str,
         **kwargs,
-    ):
+    ) -> None:
         """Initialize an instance.
 
         :param str store_key: key of input data to be stored in the datastore
diff --git a/popmon/stats/numpy.py b/popmon/stats/numpy.py
index cc41eea1..cb994572 100644
--- a/popmon/stats/numpy.py
+++ b/popmon/stats/numpy.py
@@ -22,7 +22,7 @@
 import pandas as pd
 
 
-def mean(a, weights=None, axis=None, dtype=None, keepdims=False, ddof=0):
+def mean(a, weights=None, axis=None, dtype=None, keepdims: bool = False, ddof: int = 0):
     """
     Compute the weighted mean along the specified axis.
 
@@ -47,7 +47,7 @@ def mean(a, weights=None, axis=None, dtype=None, keepdims=False, ddof=0):
         )
 
 
-def std(a, weights=None, axis=None, dtype=None, ddof=0, keepdims=False):
+def std(a, weights=None, axis=None, dtype=None, ddof: int = 0, keepdims: bool = False):
     """
     Compute the weighted standard deviation along the specified axis.
 
@@ -73,7 +73,7 @@ def std(a, weights=None, axis=None, dtype=None, ddof=0, keepdims=False):
         return np.sqrt(v)
 
 
-def median(a, weights=None, axis=None, keepdims=False):
+def median(a, weights=None, axis=None, keepdims: bool = False):
     """
     Compute the weighted median along the specified axis.
 
@@ -224,7 +224,7 @@ def covariance_multinomial_probability_distribution(entries):
     return covariance_matrix
 
 
-def mad(a, c=0.6745, axis=0):
+def mad(a, c=0.6745, axis: int = 0):
     """Median Absolute Deviation along given axis of an array
 
     mad = median(abs(a - median(a)))/c
diff --git a/popmon/stitching/hist_stitcher.py b/popmon/stitching/hist_stitcher.py
index be511a1b..9b677f41 100644
--- a/popmon/stitching/hist_stitcher.py
+++ b/popmon/stitching/hist_stitcher.py
@@ -33,13 +33,13 @@ class HistStitcher(Module):
 
     def __init__(
         self,
-        mode="add",
+        mode: str = "add",
         time_axis=None,
         time_bin_idx=None,
         read_key=None,
         delta_key=None,
         store_key=None,
-    ):
+    ) -> None:
         """Stitching histograms by first axis.
 
         :param str mode: options for histogram stitching: "add" or "replace". default is "add".
@@ -64,7 +64,7 @@ def __init__(
         if self.mode not in self.allowed_modes:
             raise ValueError("mode should be either 'add' or 'replace'")
 
-    def get_description(self):
+    def get_description(self) -> str:
         return f"{self.mode}"
 
     def transform(self, hists_basis: dict, hists_delta: dict) -> dict:
@@ -80,7 +80,7 @@ def stitch_histograms(
         hists_basis=None,
         hists_delta=None,
         hists_list=None,
-        time_axis="",
+        time_axis: str = "",
         time_bin_idx=None,
     ):
         """Stitching histograms by first axis.
diff --git a/popmon/utils.py b/popmon/utils.py
index 7364f643..e2242840 100644
--- a/popmon/utils.py
+++ b/popmon/utils.py
@@ -62,7 +62,7 @@ def filter_metrics(
     return metrics
 
 
-def parallel(func: Callable, args_list, mode="args"):
+def parallel(func: Callable, args_list, mode: str = "args"):
     """
     Routine for parallel processing
     """
diff --git a/popmon/visualization/alert_section_generator.py b/popmon/visualization/alert_section_generator.py
index 9bd7747b..8afa0e9b 100644
--- a/popmon/visualization/alert_section_generator.py
+++ b/popmon/visualization/alert_section_generator.py
@@ -47,10 +47,10 @@ def __init__(
         ignore_features=None,
         static_bounds=None,
         dynamic_bounds=None,
-        prefix="traffic_light_",
+        prefix: str = "traffic_light_",
         suffices=None,
         ignore_stat_endswith=None,
-    ):
+    ) -> None:
         """Initialize an instance of SectionGenerator.
 
         :param str read_key: key of input data to read from the datastore and use for plotting
diff --git a/popmon/visualization/histogram_section.py b/popmon/visualization/histogram_section.py
index d590a2e0..a50b95c9 100644
--- a/popmon/visualization/histogram_section.py
+++ b/popmon/visualization/histogram_section.py
@@ -53,8 +53,8 @@ def __init__(
         features=None,
         ignore_features=None,
         hist_names=None,
-        hist_name_starts_with="histogram",
-    ):
+        hist_name_starts_with: str = "histogram",
+    ) -> None:
         """Initialize an instance of SectionGenerator.
 
         :param str read_key: key of input data to read from the datastore and use for plotting
@@ -212,7 +212,7 @@ def transform(self, data_obj: dict, sections: list | None = None):
         return sections
 
 
-def _plot_histograms(feature, date, hc_list, hist_names, top_n, max_nbins=1000):
+def _plot_histograms(feature, date, hc_list, hist_names, top_n, max_nbins: int = 1000):
     """Split off plot histogram generation to allow for parallel processing
 
     :param str feature: feature
diff --git a/popmon/visualization/overview_section.py b/popmon/visualization/overview_section.py
index 2b02ebcc..af403177 100644
--- a/popmon/visualization/overview_section.py
+++ b/popmon/visualization/overview_section.py
@@ -54,10 +54,10 @@ def __init__(
         ignore_features=None,
         static_bounds=None,
         dynamic_bounds=None,
-        prefix="traffic_light_",
+        prefix: str = "traffic_light_",
         suffices=None,
         ignore_stat_endswith=None,
-    ):
+    ) -> None:
         """Initialize an instance of SectionGenerator.
 
         :param str read_key: key of input data to read from the datastore and use for plotting
diff --git a/popmon/visualization/report_generator.py b/popmon/visualization/report_generator.py
index f9b8c791..9c4ac232 100644
--- a/popmon/visualization/report_generator.py
+++ b/popmon/visualization/report_generator.py
@@ -34,7 +34,7 @@ class ReportGenerator(Module):
     _input_keys = ("read_key",)
     _output_keys = ("store_key",)
 
-    def __init__(self, read_key, store_key, settings: Report):
+    def __init__(self, read_key, store_key, settings: Report) -> None:
         """Initialize an instance of ReportGenerator.
 
         :param str read_key: key of input sections data to read from the datastore
@@ -48,7 +48,7 @@ def __init__(self, read_key, store_key, settings: Report):
         self.online_report = settings.online_report
         self.tl_colors = settings.tl_colors
 
-    def get_description(self):
+    def get_description(self) -> str:
         return "HTML Report"
 
     def transform(self, sections: list) -> str:
diff --git a/popmon/visualization/section_generator.py b/popmon/visualization/section_generator.py
index 7b58b1c7..d204f826 100644
--- a/popmon/visualization/section_generator.py
+++ b/popmon/visualization/section_generator.py
@@ -54,7 +54,7 @@
 }
 
 
-def get_stat_description(name: str):
+def get_stat_description(name: str) -> str:
     """Gets the description of a statistic.
 
     :param str name: the name of the statistic.
@@ -70,8 +70,8 @@ def get_stat_description(name: str):
     if name in "mean_trend10_zscore":
         return "Significance of (rolling) trend in means of features"
 
-    head, *tail = name.split("_")
-    tail = "_".join(tail)
+    head, *tails = name.split("_")
+    tail = "_".join(tails)
 
     if tail in comparisons and head in references:
         return comparisons[tail]
@@ -98,11 +98,11 @@ def __init__(
         ignore_features=None,
         static_bounds=None,
         dynamic_bounds=None,
-        prefix="traffic_light_",
+        prefix: str = "traffic_light_",
         suffices=None,
         ignore_stat_endswith=None,
-        description="",
-    ):
+        description: str = "",
+    ) -> None:
         """Initialize an instance of SectionGenerator.
 
         :param str read_key: key of input data to read from the datastore and use for plotting
diff --git a/popmon/visualization/traffic_light_section_generator.py b/popmon/visualization/traffic_light_section_generator.py
index 53bd505f..3c073a87 100644
--- a/popmon/visualization/traffic_light_section_generator.py
+++ b/popmon/visualization/traffic_light_section_generator.py
@@ -50,10 +50,10 @@ def __init__(
         ignore_features=None,
         static_bounds=None,
         dynamic_bounds=None,
-        prefix="traffic_light_",
+        prefix: str = "traffic_light_",
         suffices=None,
         ignore_stat_endswith=None,
-    ):
+    ) -> None:
         """Initialize an instance of SectionGenerator.
 
         :param str read_key: key of input data to read from the datastore and use for plotting
@@ -173,7 +173,7 @@ def _plot_metrics(
     skip_first_n,
     skip_last_n,
     tl_colors,
-    style="heatmap",
+    style: str = "heatmap",
 ):
     # prune dates and values
     dates = _prune(dates, last_n, skip_first_n, skip_last_n)
diff --git a/popmon/visualization/utils.py b/popmon/visualization/utils.py
index 38aa82a3..01e4394a 100644
--- a/popmon/visualization/utils.py
+++ b/popmon/visualization/utils.py
@@ -25,6 +25,7 @@
 import math
 import warnings
 from collections import defaultdict
+from typing import DefaultDict, Dict, List, Union
 
 import numpy as np
 import pandas as pd
@@ -324,7 +325,7 @@ def plot_bars(
 
 
 def plot_traffic_lights_overview(feature, data, metrics: list[str], labels: list[str]):
-    colors = defaultdict(dict)
+    colors: DefaultDict[str, Dict[str, List[str]]] = defaultdict(dict)
     color_map = ["g", "y", "r"]
     for c1, metric in enumerate(metrics):
         for c2, label in enumerate(labels):
@@ -365,7 +366,7 @@ def plot_traffic_lights_alerts_aggregate(
     yellow = hex_to_rgb(tl_colors["yellow"])
     red = hex_to_rgb(tl_colors["red"])
 
-    colors = defaultdict(dict)
+    colors: DefaultDict[str, Dict[str, List[str]]] = defaultdict(dict)
     for c1, metric in enumerate(metrics):
         row_max = np.max(data[c1])
         for c2, label in enumerate(labels):
@@ -437,11 +438,11 @@ def histogram_basic_checks(plots=None):
 
 def plot_histogram_overlay(
     plots=None,
-    is_num=True,
-    is_ts=False,
-    is_static_reference=True,
-    top=20,
-    n_choices=2,
+    is_num: bool = True,
+    is_ts: bool = False,
+    is_static_reference: bool = True,
+    top: int = 20,
+    n_choices: int = 2,
 ):
     """Create and plot (overlapping/grouped) histogram(s) of column values.
 
@@ -796,7 +797,7 @@ def plot_heatmap(
     }
 
 
-def _prune(values, last_n=0, skip_first_n=0, skip_last_n=0):
+def _prune(values, last_n: int = 0, skip_first_n: int = 0, skip_last_n: int = 0):
     """inline function to select first or last items of input list
 
     :param values: input list to select from
diff --git a/tests/popmon/base/test_module.py b/tests/popmon/base/test_module.py
index ac9bfd19..b7f7dfdb 100644
--- a/tests/popmon/base/test_module.py
+++ b/tests/popmon/base/test_module.py
@@ -8,7 +8,7 @@ class Scaler(Module):
     _input_keys = ("input_key",)
     _output_keys = ("output_key",)
 
-    def __init__(self, input_key, output_key, mean, std):
+    def __init__(self, input_key, output_key, mean, std) -> None:
         super().__init__()
         self.input_key = input_key
         self.output_key = output_key
diff --git a/tests/popmon/base/test_pipeline.py b/tests/popmon/base/test_pipeline.py
index 75453d90..d7e4b38b 100644
--- a/tests/popmon/base/test_pipeline.py
+++ b/tests/popmon/base/test_pipeline.py
@@ -10,7 +10,7 @@ class LogTransformer(Module):
     _input_keys = ("input_key",)
     _output_keys = ("output_key",)
 
-    def __init__(self, input_key, output_key):
+    def __init__(self, input_key, output_key) -> None:
         super().__init__()
         self.input_key = input_key
         self.output_key = output_key
@@ -25,7 +25,7 @@ class PowerTransformer(Module):
     _input_keys = ("input_key",)
     _output_keys = ("output_key",)
 
-    def __init__(self, input_key, output_key, power):
+    def __init__(self, input_key, output_key, power) -> None:
         super().__init__()
         self.input_key = input_key
         self.output_key = output_key
@@ -40,7 +40,7 @@ class SumNormalizer(Module):
     _input_keys = ("input_key",)
     _output_keys = ("output_key",)
 
-    def __init__(self, input_key, output_key):
+    def __init__(self, input_key, output_key) -> None:
         super().__init__()
         self.input_key = input_key
         self.output_key = output_key
@@ -54,7 +54,7 @@ class WeightedSum(Module):
     _input_keys = ("input_key", "weight_key")
     _output_keys = ("output_key",)
 
-    def __init__(self, input_key, weight_key, output_key):
+    def __init__(self, input_key, weight_key, output_key) -> None:
         super().__init__()
         self.input_key = input_key
         self.weight_key = weight_key