Add a common_prefix helper function in string_utils module

This is a replacement for the os.path.commonprefix function which is deprecated in Python 3.15. Python 3.15 recommends using os.path.commonpath, but that isn't equivalent and yields different results.

Author: tleonhardt

CHANGELOG.md

@@ -81,6 +81,8 @@ prompt is displayed.
     - `cmd2.Cmd.select` has been revamped to use the
       [choice](https://python-prompt-toolkit.readthedocs.io/en/3.0.52/pages/asking_for_a_choice.html)
       function from `prompt-toolkit` when both **stdin** and **stdout** are TTYs
+    - Added `common_prefix` method to `cmd2.string_utils` module as a replacement for
+      `os.path.commonprefix` since that is now deprecated in Python 3.15
 
 ## 3.4.0 (March 3, 2026)
 

cmd2/cmd2.py

@@ -1921,7 +1921,7 @@ def delimiter_complete(
         match_strings = basic_completions.to_strings()
 
         # Calculate what portion of the match we are completing
-        common_prefix = os.path.commonprefix(match_strings)
+        common_prefix = su.common_prefix(match_strings)
         prefix_tokens = common_prefix.split(delimiter)
         display_token_index = len(prefix_tokens) - 1
 

cmd2/string_utils.py

@@ -5,6 +5,10 @@
 full-width characters (like those used in CJK languages).
 """
 
+from collections.abc import (
+    Sequence,
+)
+
 from rich.align import AlignMethod
 from rich.style import StyleType
 from rich.text import Text
@@ -167,3 +171,22 @@ def norm_fold(val: str) -> str:
     import unicodedata
 
     return unicodedata.normalize("NFC", val).casefold()
+
+
+def common_prefix(m: Sequence[str]) -> str:
+    """Return the longest common leading component of a list of strings.
+
+    This is a replacement for os.path.commonprefix which is deprecated in Python 3.15.
+
+    :param m: list of strings
+    :return: common prefix
+    """
+    if not m:
+        return ""
+
+    s1 = min(m)
+    s2 = max(m)
+    for i, c in enumerate(s1):
+        if i >= len(s2) or c != s2[i]:
+            return s1[:i]
+    return s1

tests/test_string_utils.py

@@ -246,3 +246,29 @@ def test_unicode_casefold() -> None:
     micro_cf = micro.casefold()
     assert micro != micro_cf
     assert su.norm_fold(micro) == su.norm_fold(micro_cf)
+
+
+def test_common_prefix() -> None:
+    # Empty list
+    assert su.common_prefix([]) == ""
+
+    # Single item
+    assert su.common_prefix(["abc"]) == "abc"
+
+    # Common prefix exists
+    assert su.common_prefix(["abcdef", "abcde", "abcd"]) == "abcd"
+
+    # No common prefix
+    assert su.common_prefix(["abc", "def"]) == ""
+
+    # One is a prefix of another
+    assert su.common_prefix(["apple", "app"]) == "app"
+
+    # Identical strings
+    assert su.common_prefix(["test", "test"]) == "test"
+
+    # Case sensitivity (matches os.path.commonprefix behavior)
+    assert su.common_prefix(["Apple", "apple"]) == ""
+
+    # Empty string in list
+    assert su.common_prefix(["abc", ""]) == ""