Select best imperative by longest prefix match

src/pydocstyle/checker.py

@@ -13,7 +13,7 @@
 from .parser import (Package, Module, Class, NestedClass, Definition, AllError,
                      Method, Function, NestedFunction, Parser, StringIO,
                      ParseError)
-from .utils import log, is_blank, pairwise
+from .utils import log, is_blank, pairwise, common_prefix_length
 from .wordlists import IMPERATIVE_VERBS, IMPERATIVE_BLACKLIST, stem
 
 
@@ -448,8 +448,12 @@ def check_imperative_mood(self, function, docstring):  # def context
                     return
 
                 if correct_forms and check_word not in correct_forms:
+                    best = max(
+                        correct_forms,
+                        key=lambda f: common_prefix_length(check_word, f)
+                    )
                     return violations.D401(
-                        next(iter(correct_forms)).capitalize(),
+                        best.capitalize(),
                         first_word
                     )
 

src/pydocstyle/utils.py

@@ -25,3 +25,16 @@ def pairwise(
     a, b = tee(iterable)
     _ = next(b, default_value)
     return zip_longest(a, b, fillvalue=default_value)
+
+
+def common_prefix_length(a: str, b: str) -> int:
+    """Return the length of the longest common prefix of a and b.
+
+    >>> common_prefix_length('abcd', 'abce')
+    3
+
+    """
+    for common, (ca, cb) in enumerate(zip(a, b)):
+        if ca != cb:
+            return common
+    return common + 1

src/tests/test_cases/test.py

@@ -387,6 +387,22 @@ def docstring_ignore_some_violations_but_catch_D401():  # noqa: E501,D400,D415
     pass
 
 
+@expect(
+    "D401: First line should be in imperative mood "
+    "('Initiate', not 'Initiates')"
+)
+def docstring_initiates():
+    """Initiates the process."""
+
+
+@expect(
+    "D401: First line should be in imperative mood "
+    "('Initialize', not 'Initializes')"
+)
+def docstring_initializes():
+    """Initializes the process."""
+
+
 @wraps(docstring_bad_ignore_one)
 def bad_decorated_function():
     """Bad (E501) but decorated"""

src/tests/test_utils.py

@@ -0,0 +1,23 @@
+"""Unit test for pydocstyle utils.
+
+Use tox or py.test to run the test suite.
+"""
+from pydocstyle import utils
+
+
+__all__ = ()
+
+
+def test_common_prefix():
+    """We can find the common prefix of two strings."""
+    assert utils.common_prefix_length('abcd', 'abce') == 3
+
+
+def test_no_common_prefix():
+    """If two strings have no common prefix, return the empty string."""
+    assert utils.common_prefix_length('abcd', 'cdef') == 0
+
+
+def test_differ_length():
+    """We can find a common prefix of two strings differing in length."""
+    assert utils.common_prefix_length('abcd', 'ab') == 2