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

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

Author: dhellmann

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 +1,24 @@
+"""Tests for pip._internal.messages
+"""
+
+from pip._internal.utils.messages import oxford_comma_join
+
+
+def test_empty_list():
+    assert '' == oxford_comma_join([])
+
+
+def test_one_item():
+    assert 'a' == oxford_comma_join(['a'])
+
+
+def test_two_items():
+    assert 'a and b' == oxford_comma_join(['a', 'b'])
+
+
+def test_three_items():
+    assert 'a, b, and c' == oxford_comma_join(['a', 'b', 'c'])
+
+
+def test_two_items_or():
+    assert 'a or b' == oxford_comma_join(['a', 'b'], 'or')