diff --git a/ethpm/backends/base.py b/ethpm/backends/base.py
index 566f4ff..9f309f2 100644
--- a/ethpm/backends/base.py
+++ b/ethpm/backends/base.py
@@ -1,4 +1,5 @@
 from abc import ABC, abstractmethod
+from typing import Union
 
 from ethpm.typing import URI
 
@@ -28,7 +29,7 @@ def can_translate_uri(self, uri: URI) -> bool:
         pass
 
     @abstractmethod
-    def fetch_uri_contents(self, uri: URI) -> bytes:
+    def fetch_uri_contents(self, uri: URI) -> Union[bytes, URI]:
         """
         Fetch the contents stored at a URI.
         """
diff --git a/ethpm/backends/ipfs.py b/ethpm/backends/ipfs.py
index 7e8b094..d7f1b2b 100644
--- a/ethpm/backends/ipfs.py
+++ b/ethpm/backends/ipfs.py
@@ -181,4 +181,9 @@ def get_ipfs_backend(import_path: str = None) -> BaseIPFSBackend:
 def get_ipfs_backend_class(import_path: str = None) -> Type[BaseIPFSBackend]:
     if import_path is None:
         import_path = os.environ.get("ETHPM_IPFS_BACKEND_CLASS", DEFAULT_IPFS_BACKEND)
+        if not import_path:
+            raise CannotHandleURI(
+                "Please provide an import class or set "
+                "`ETHPM_IPFS_BACKEND_CLASS` environment variable."
+            )
     return import_string(import_path)
diff --git a/ethpm/backends/registry.py b/ethpm/backends/registry.py
index c514d7e..88b428d 100644
--- a/ethpm/backends/registry.py
+++ b/ethpm/backends/registry.py
@@ -5,6 +5,7 @@
 
 from ethpm.backends.base import BaseURIBackend
 from ethpm.constants import INFURA_API_KEY
+from ethpm.typing import URI
 from ethpm.utils.registry import fetch_standard_registry_abi
 from ethpm.utils.uri import parse_registry_uri
 from ethpm.validation import is_valid_registry_uri
@@ -31,7 +32,7 @@ def can_translate_uri(self, uri: str) -> bool:
     def can_resolve_uri(self, uri: str) -> bool:
         return False
 
-    def fetch_uri_contents(self, uri: str) -> bytes:
+    def fetch_uri_contents(self, uri: str) -> URI:
         """
         Return content-addressed URI stored at registry URI.
         """
diff --git a/ethpm/contract.py b/ethpm/contract.py
index db0a0eb..8fa684e 100644
--- a/ethpm/contract.py
+++ b/ethpm/contract.py
@@ -1,4 +1,4 @@
-from typing import Any, Dict, List, Tuple, Type  # noqa: F401
+from typing import Any, Dict, List, Optional, Tuple, Type  # noqa: F401
 
 from eth_utils import combomethod, is_canonical_address, to_bytes, to_checksum_address
 from eth_utils.toolz import assoc, curry, pipe
@@ -15,11 +15,11 @@ class LinkableContract(Contract):
     contract factories with link references in their package's manifest.
     """
 
-    unlinked_references: Tuple[Dict[str, Any]] = None
-    linked_references: Tuple[Dict[str, Any]] = None
+    unlinked_references: Optional[Tuple[Dict[str, Any]]] = None
+    linked_references: Optional[Tuple[Dict[str, Any]]] = None
     needs_bytecode_linking = None
 
-    def __init__(self, address: bytes = None, **kwargs: Any) -> None:
+    def __init__(self, address: bytes, **kwargs: Any) -> None:
         if self.needs_bytecode_linking:
             raise BytecodeLinkingError(
                 "Contract cannot be instantiated until its bytecode is linked."
@@ -90,13 +90,9 @@ def validate_attr_dict(self, attr_dict: Dict[str, str]) -> None:
                 "Unable to validate attr dict, this contract has no linked/unlinked references."
             )
 
-        all_link_refs: Tuple[Any, ...]
-        if self.unlinked_references and self.linked_references:
-            all_link_refs = self.unlinked_references + self.linked_references
-        elif not self.unlinked_references:
-            all_link_refs = self.linked_references
-        else:
-            all_link_refs = self.unlinked_references
+        unlinked_refs = self.unlinked_references or ({},)
+        linked_refs = self.linked_references or ({},)
+        all_link_refs = unlinked_refs + linked_refs
 
         all_link_names = [ref["name"] for ref in all_link_refs]
         if set(attr_dict_names) != set(all_link_names):
diff --git a/ethpm/deployments.py b/ethpm/deployments.py
index 192cbe7..141ecaf 100644
--- a/ethpm/deployments.py
+++ b/ethpm/deployments.py
@@ -32,7 +32,7 @@ def __contains__(self, key: str) -> bool:
 
     def get(self, key: str) -> Dict[str, str]:
         self._validate_name_and_references(key)
-        return self.deployment_data.get(key)
+        return self.deployment_data[key]
 
     def items(self) -> ItemsView[str, Dict[str, str]]:
         item_dict = {name: self.get(name) for name in self.deployment_data}
diff --git a/ethpm/package.py b/ethpm/package.py
index 82710ed..b770276 100644
--- a/ethpm/package.py
+++ b/ethpm/package.py
@@ -1,6 +1,6 @@
 import json
 from pathlib import Path
-from typing import Any, Dict, Generator, Tuple, Union
+from typing import Any, Dict, Generator, Optional, Tuple, Union
 
 from eth_utils import to_canonical_address, to_text, to_tuple
 from web3 import Web3
@@ -45,7 +45,9 @@
 
 
 class Package(object):
-    def __init__(self, manifest: Dict[str, Any], w3: Web3, uri: str = None) -> None:
+    def __init__(
+        self, manifest: Dict[str, Any], w3: Web3, uri: Optional[str] = None
+    ) -> None:
         """
         A package should be created using one of the available
         classmethods and a valid w3 instance.
@@ -130,7 +132,7 @@ def manifest_version(self) -> str:
         return self.manifest["manifest_version"]
 
     @property
-    def uri(self) -> str:
+    def uri(self) -> Optional[str]:
         """
         The uri (local file_path / content-addressed URI) of a ``Package``'s manifest.
         """
diff --git a/ethpm/utils/backend.py b/ethpm/utils/backend.py
index c18cec8..e142529 100644
--- a/ethpm/utils/backend.py
+++ b/ethpm/utils/backend.py
@@ -32,7 +32,8 @@ def resolve_uri_contents(uri: URI, fingerprint: bool = None) -> bytes:
     if resolvable_backends:
         for backend in resolvable_backends:
             try:
-                contents = backend().fetch_uri_contents(uri)
+                # ignore type b/c resolvable backends only return uri contents
+                contents: bytes = backend().fetch_uri_contents(uri)  # type: ignore
             except CannotHandleURI:
                 continue
             return contents
@@ -44,7 +45,7 @@ def resolve_uri_contents(uri: URI, fingerprint: bool = None) -> bytes:
                 "Registry URIs must point to a resolvable content-addressed URI."
             )
         package_id = RegistryURIBackend().fetch_uri_contents(uri)
-        return resolve_uri_contents(package_id, True)  # type: ignore
+        return resolve_uri_contents(package_id, True)
 
     raise CannotHandleURI(
         f"URI: {uri} cannot be resolved by any of the available backends."
diff --git a/ethpm/utils/deployments.py b/ethpm/utils/deployments.py
index 2e4b1ca..547579a 100644
--- a/ethpm/utils/deployments.py
+++ b/ethpm/utils/deployments.py
@@ -39,8 +39,8 @@ def validate_linked_references(
     for idx, offset in enumerate(offsets):
         value = values[idx]
         # https://github.com/python/mypy/issues/4975
-        offset_value = int(offset)  # type: ignore
-        dep_length = len(value)  # type: ignore
+        offset_value = int(offset)
+        dep_length = len(value)
         end_of_bytes = offset_value + dep_length
         # Ignore b/c whitespace around ':' conflict b/w black & flake8
         actual_bytes = bytecode[offset_value:end_of_bytes]  # noqa: E203
diff --git a/ethpm/utils/ipfs.py b/ethpm/utils/ipfs.py
index 46fd75f..420fb20 100644
--- a/ethpm/utils/ipfs.py
+++ b/ethpm/utils/ipfs.py
@@ -3,6 +3,8 @@
 from typing import Dict
 from urllib import parse
 
+from google.protobuf.descriptor import Descriptor
+
 from ethpm.pb.ipfs_file_pb2 import Data, PBNode
 from ethpm.utils.base58 import b58encode
 
@@ -69,11 +71,14 @@ def multihash(value: bytes) -> bytes:
     return multihash_bytes
 
 
-def serialize_bytes(file_bytes: bytes) -> PBNode:
+def serialize_bytes(file_bytes: bytes) -> Descriptor:
     file_size = len(file_bytes)
 
     data_protobuf = Data(
-        Type=Data.DataType.Value("File"), Data=file_bytes, filesize=file_size
+        # type ignored b/c DataType is manually attached in ipfs_file_pb2.py
+        Type=Data.DataType.Value("File"),  # type: ignore
+        Data=file_bytes,
+        filesize=file_size,
     )
     data_protobuf_bytes = data_protobuf.SerializeToString()
 
@@ -83,7 +88,8 @@ def serialize_bytes(file_bytes: bytes) -> PBNode:
 
 
 def generate_file_hash(content_bytes: bytes) -> str:
-    file_protobuf = serialize_bytes(content_bytes)
-    file_protobuf_bytes = file_protobuf.SerializeToString()
+    file_protobuf: Descriptor = serialize_bytes(content_bytes)
+    # type ignored b/c SerializeToString is manually attached in ipfs_file_pb2.py
+    file_protobuf_bytes = file_protobuf.SerializeToString()  # type: ignore
     file_multihash = multihash(file_protobuf_bytes)
     return b58encode(file_multihash)
diff --git a/ethpm/utils/manifest_validation.py b/ethpm/utils/manifest_validation.py
index cf92618..788b030 100644
--- a/ethpm/utils/manifest_validation.py
+++ b/ethpm/utils/manifest_validation.py
@@ -112,7 +112,7 @@ def validate_manifest_exists(manifest_id: str) -> None:
         )
 
 
-def format_manifest(manifest: Manifest, *, prettify: bool) -> str:
+def format_manifest(manifest: Manifest, *, prettify: bool = None) -> str:
     if prettify:
         return json.dumps(manifest, sort_keys=True, indent=4)
     return json.dumps(manifest, sort_keys=True, separators=(",", ":"))
diff --git a/setup.py b/setup.py
index da0e3a9..81810d2 100644
--- a/setup.py
+++ b/setup.py
@@ -20,10 +20,10 @@
         'tox>=1.8.0,<2',
     ],
     'lint': [
-        'black>=18.6b4,<19',
-        'isort>=4.2.15,<5',   
-        'flake8>=3.5.0,<4',
-        'mypy<0.600',
+        'black>=19.3b0,<20',
+        'isort>=4.3.17,<5',   
+        'flake8>=3.7.0,<4',
+        'mypy<0.800',
     ],
     'doc': [
         'Sphinx>=1.5.5,<2',
diff --git a/tox.ini b/tox.ini
index 3025e4d..871a390 100644
--- a/tox.ini
+++ b/tox.ini
@@ -7,8 +7,8 @@ envlist=
 [isort]
 combine_as_imports=True
 force_sort_within_sections=True
-include_trailing_comma=True
 skip=__init__.py
+include_trailing_comma=True
 known_third_party=pytest,requests_mock
 known_first_party=ethpm
 line_length=88
@@ -43,6 +43,6 @@ deps=flake8
 extras=lint
 commands=
     flake8 {toxinidir}/tests {toxinidir}/ethpm
-    mypy --follow-imports=silent --ignore-missing-imports --check-untyped-defs --disallow-incomplete-defs --disallow-untyped-defs --disallow-any-generics -p ethpm
+    mypy --follow-imports=silent --ignore-missing-imports --check-untyped-defs --disallow-incomplete-defs --disallow-untyped-defs --disallow-any-generics --warn-unused-ignore -p ethpm
     black --check --diff {toxinidir}/ethpm/ --check --diff {toxinidir}/tests/
     isort --check-only --recursive --diff {toxinidir}/ethpm/ {toxinidir}/tests/