Issue #21883: os.path.join() and os.path.relpath() now raise a TypeError with

more helpful error message for unsupported or mismatched types of arguments.

Lib/genericpath.py

@@ -130,3 +130,16 @@ def _splitext(p, sep, altsep, extsep):
             filenameIndex += 1
 
     return p, p[:0]
+
+def _check_arg_types(funcname, *args):
+    hasstr = hasbytes = False
+    for s in args:
+        if isinstance(s, str):
+            hasstr = True
+        elif isinstance(s, bytes):
+            hasbytes = True
+        else:
+            raise TypeError('%s() argument must be str or bytes, not %r' %
+                            (funcname, s.__class__.__name__)) from None
+    if hasstr and hasbytes:
+        raise TypeError("Can't mix strings and bytes in path components") from None

Lib/macpath.py

@@ -50,6 +50,7 @@ def isabs(s):
 
 
 def join(s, *p):
+    try:
         colon = _get_colon(s)
         path = s
         for t in p:
@@ -64,6 +65,9 @@ def join(s, *p):
                 path = path + colon
             path = path + t
         return path
+    except (TypeError, AttributeError, BytesWarning):
+        genericpath._check_arg_types('join', s, *p)
+        raise
 
 
 def split(s):

Lib/ntpath.py

@@ -80,6 +80,7 @@ def join(path, *paths):
         sep = '\\'
         seps = '\\/'
         colon = ':'
+    try:
         result_drive, result_path = splitdrive(path)
         for p in paths:
             p_drive, p_path = splitdrive(p)
@@ -106,6 +107,9 @@ def join(path, *paths):
             result_drive and result_drive[-1:] != colon):
             return result_drive + sep + result_path
         return result_drive + result_path
+    except (TypeError, AttributeError, BytesWarning):
+        genericpath._check_arg_types('join', path, *paths)
+        raise
 
 
 # Split a path in a drive specification (a drive letter followed by a
@@ -558,6 +562,7 @@ def relpath(path, start=None):
     if not path:
         raise ValueError("no path specified")
 
+    try:
         start_abs = abspath(normpath(start))
         path_abs = abspath(normpath(path))
         start_drive, start_rest = splitdrive(start_abs)
@@ -579,6 +584,9 @@ def relpath(path, start=None):
         if not rel_list:
             return curdir
         return join(*rel_list)
+    except (TypeError, ValueError, AttributeError, BytesWarning):
+        genericpath._check_arg_types('relpath', path, start)
+        raise
 
 
 # determine if two files are in fact the same file

Lib/posixpath.py

@@ -82,13 +82,9 @@ def join(a, *p):
                 path += b
             else:
                 path += sep + b
-    except (TypeError, AttributeError):
-        for s in (a,) + p:
-            if not isinstance(s, (str, bytes)):
-                raise TypeError('join() argument must be str or bytes, not %r' %
-                                s.__class__.__name__) from None
-        # Must have a mixture of text and binary data
-        raise TypeError("Can't mix strings and bytes in path components") from None
+    except (TypeError, AttributeError, BytesWarning):
+        genericpath._check_arg_types('join', a, *p)
+        raise
     return path
 
 
@@ -446,9 +442,9 @@ def relpath(path, start=None):
     if start is None:
         start = curdir
 
+    try:
         start_list = [x for x in abspath(start).split(sep) if x]
         path_list = [x for x in abspath(path).split(sep) if x]
-
         # Work out how much of the filepath is shared by start and path.
         i = len(commonprefix([start_list, path_list]))
 
@@ -456,3 +452,6 @@ def relpath(path, start=None):
         if not rel_list:
             return curdir
         return join(*rel_list)
+    except (TypeError, AttributeError, BytesWarning):
+        genericpath._check_arg_types('relpath', path, start)
+        raise

Lib/test/test_genericpath.py

@@ -434,6 +434,39 @@ class CommonTest(GenericTest):
             with support.temp_cwd(name):
                 self.test_abspath()
 
+    def test_join_errors(self):
+        # Check join() raises friendly TypeErrors.
+        with support.check_warnings(('', BytesWarning), quiet=True):
+            errmsg = "Can't mix strings and bytes in path components"
+            with self.assertRaisesRegex(TypeError, errmsg):
+                self.pathmodule.join(b'bytes', 'str')
+            with self.assertRaisesRegex(TypeError, errmsg):
+                self.pathmodule.join('str', b'bytes')
+            # regression, see #15377
+            errmsg = r'join\(\) argument must be str or bytes, not %r'
+            with self.assertRaisesRegex(TypeError, errmsg % 'int'):
+                self.pathmodule.join(42, 'str')
+            with self.assertRaisesRegex(TypeError, errmsg % 'int'):
+                self.pathmodule.join('str', 42)
+            with self.assertRaisesRegex(TypeError, errmsg % 'bytearray'):
+                self.pathmodule.join(bytearray(b'foo'), bytearray(b'bar'))
+
+    def test_relpath_errors(self):
+        # Check relpath() raises friendly TypeErrors.
+        with support.check_warnings(('', BytesWarning), quiet=True):
+            errmsg = "Can't mix strings and bytes in path components"
+            with self.assertRaisesRegex(TypeError, errmsg):
+                self.pathmodule.relpath(b'bytes', 'str')
+            with self.assertRaisesRegex(TypeError, errmsg):
+                self.pathmodule.relpath('str', b'bytes')
+            errmsg = r'relpath\(\) argument must be str or bytes, not %r'
+            with self.assertRaisesRegex(TypeError, errmsg % 'int'):
+                self.pathmodule.relpath(42, 'str')
+            with self.assertRaisesRegex(TypeError, errmsg % 'int'):
+                self.pathmodule.relpath('str', 42)
+            with self.assertRaisesRegex(TypeError, errmsg % 'bytearray'):
+                self.pathmodule.relpath(bytearray(b'foo'), bytearray(b'bar'))
+
 
 if __name__=="__main__":
     unittest.main()

Lib/test/test_macpath.py

@@ -142,6 +142,8 @@ class MacPathTestCase(unittest.TestCase):
 class MacCommonTest(test_genericpath.CommonTest, unittest.TestCase):
     pathmodule = macpath
 
+    test_relpath_errors = None
+
 
 if __name__ == "__main__":
     unittest.main()

Lib/test/test_posixpath.py

@@ -57,22 +57,6 @@ class PosixPathTest(unittest.TestCase):
         self.assertEqual(posixpath.join(b"/foo/", b"bar/", b"baz/"),
                          b"/foo/bar/baz/")
 
-    def test_join_errors(self):
-        # Check posixpath.join raises friendly TypeErrors.
-        errmsg = "Can't mix strings and bytes in path components"
-        with self.assertRaisesRegex(TypeError, errmsg):
-            posixpath.join(b'bytes', 'str')
-        with self.assertRaisesRegex(TypeError, errmsg):
-            posixpath.join('str', b'bytes')
-        # regression, see #15377
-        errmsg = r'join\(\) argument must be str or bytes, not %r'
-        with self.assertRaisesRegex(TypeError, errmsg % 'NoneType'):
-            posixpath.join(None, 'str')
-        with self.assertRaisesRegex(TypeError, errmsg % 'NoneType'):
-            posixpath.join('str', None)
-        with self.assertRaisesRegex(TypeError, errmsg % 'bytearray'):
-            posixpath.join(bytearray(b'foo'), bytearray(b'bar'))
-
     def test_split(self):
         self.assertEqual(posixpath.split("/foo/bar"), ("/foo", "bar"))
         self.assertEqual(posixpath.split("/"), ("/", ""))

Misc/NEWS

@@ -162,6 +162,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #21883: os.path.join() and os.path.relpath() now raise a TypeError with
+  more helpful error message for unsupported or mismatched types of arguments.
+
 - Issue #22219: The zipfile module CLI now adds entries for directories
   (including empty directories) in ZIP file.