Split out a custom call to assert_diff for a .startswith equivalent.

tests/comparators/test_python.py

@@ -21,7 +21,7 @@ import sys
 
 from diffoscope.comparators.python import PycFile
 
-from ..utils.data import assert_diff, load_fixture
+from ..utils.data import assert_diff_startswith, load_fixture
 
 
 pyc1 = load_fixture("test1.pyc-renamed")
@@ -47,8 +47,7 @@ def differences(pyc1, pyc2):
 
 
 def test_diff(differences):
-    assert_diff(
+    assert_diff_startswith(
         differences[0],
         "pyc_expected_diff",
-        lambda haystack, needle: haystack.startswith(needle),
     )

tests/utils/data.py

@@ -56,12 +56,18 @@ def get_data(filename):
         return f.read()
 
 
-def assert_diff(difference, filename, cmp=lambda x, y: x == y):
+def assert_diff(difference, filename):
     # Assign seen and expected values to local variables to improve contextual
     # information in failed tests.
     seen = difference.unified_diff
     expected = get_data(filename)
-    assert cmp(seen, expected)
+    assert seen == expected
+
+
+def assert_diff_startswith(difference, filename):
+    haystack = difference.unified_diff
+    needle = get_data(filename)
+    assert needle.startswith(haystack)
 
 
 # https://code.activestate.com/recipes/576620-changedirectory-context-manager/#c3