Issue #23780: Improved error message in os.path.join() with single argument.

Idea by R. David Murray.

Lib/macpath.py

@@ -53,6 +53,8 @@ def join(s, *p):
     try:
         colon = _get_colon(s)
         path = s
+        if not p:
+            path[:0] + colon  #23780: Ensure compatible data type even if p is null.
         for t in p:
             if (not path) or isabs(t):
                 path = t

Lib/ntpath.py

@@ -81,6 +81,8 @@ def join(path, *paths):
         seps = '\\/'
         colon = ':'
     try:
+        if not paths:
+            path[:0] + sep  #23780: Ensure compatible data type even if p is null.
         result_drive, result_path = splitdrive(path)
         for p in paths:
             p_drive, p_path = splitdrive(p)

Lib/posixpath.py

@@ -76,6 +76,8 @@ def join(a, *p):
     sep = _get_sep(a)
     path = a
     try:
+        if not p:
+            path[:0] + sep  #23780: Ensure compatible data type even if p is null.
         for b in p:
             if b.startswith(sep):
                 path = b

Lib/test/test_genericpath.py

@@ -448,6 +448,10 @@ class CommonTest(GenericTest):
                 self.pathmodule.join(42, 'str')
             with self.assertRaisesRegex(TypeError, errmsg % 'int'):
                 self.pathmodule.join('str', 42)
+            with self.assertRaisesRegex(TypeError, errmsg % 'int'):
+                self.pathmodule.join(42)
+            with self.assertRaisesRegex(TypeError, errmsg % 'list'):
+                self.pathmodule.join([])
             with self.assertRaisesRegex(TypeError, errmsg % 'bytearray'):
                 self.pathmodule.join(bytearray(b'foo'), bytearray(b'bar'))
 

Misc/NEWS

@@ -49,6 +49,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #23780: Improved error message in os.path.join() with single argument.
+
 - Issue #6598: Increased time precision and random number range in
   email.utils.make_msgid() to strengthen the uniqueness of the message ID.