add a utility function for building nice messages from lists of strings

Signed-off-by: Doug Hellmann <[email protected]>

Author: dhellmann

src/pip/_internal/operations/install/wheel.py

@@ -408,7 +408,7 @@ def _report_file_owner_conflicts(
         # we don't need to scan the source.
         return
 
-    for dir, subdirs, files in os.walk(source_dir):
+    for dir, _subdirs, files in os.walk(source_dir):
         basedir = dir[len(source_dir):].lstrip(os.path.sep)
         for f in files:
             partial_src = os.path.join(basedir, f)

src/pip/_internal/utils/messages.py

@@ -0,0 +1,29 @@
+"""
+Utility functions for building messages.
+"""
+
+from pip._internal.utils.typing import MYPY_CHECK_RUNNING
+
+if MYPY_CHECK_RUNNING:
+    from typing import List
+
+
+def oxford_comma_join(
+        values,  # type: List[str]
+        conjunction='and',  # type: str
+):
+    # type: (...) -> str
+    "Join a list of strings for output in a message."
+    if not values:
+        return ''
+    if len(values) == 1:
+        return values[0]
+    comma = ''
+    if len(values) > 2:
+        comma = ','
+    return '{}{} {} {}'.format(
+        ', '.join(values[:-1]),
+        comma,
+        conjunction,
+        values[-1],
+    )

tests/unit/test_messages.py

@@ -0,0 +1,22 @@
+"""Tests for pip._internal.messages
+"""
+
+import pytest
+
+from pip._internal.utils.messages import oxford_comma_join
+
+
+@pytest.mark.parametrize("test_input,expected",
+                         [('', []),
+                          ('a', ['a']),
+                          ('a and b', ['a', 'b']),
+                          ('a, b, and c', ['a', 'b', 'c'])])
+def test_oxford_comma_join_implicit_conjunction(test_input, expected):
+    assert expected == oxford_comma_join(test_input)
+
+
+@pytest.mark.parametrize("test_input,conjunction,expected",
+                         [('a and b', 'and', ['a', 'b']),
+                          ('a or b', 'or', ['a', 'b'])])
+def test_oxford_comma_join_explicit_conjunction(test_input, conjunction, expected):
+    assert expected == oxford_comma_join(test_input, conjunction)