Show warnings for username and password

tests/test_auth.py

@@ -1,3 +1,4 @@
+import getpass
 import logging
 
 import pytest
@@ -202,3 +203,26 @@ def test_logs_config_values(config, caplog):
         "username set from config file",
         "password set from config file",
     ]
+
+
+@pytest.mark.parametrize(
+    "password, warning",
+    [
+        ("", "Your password is empty"),
+        ("\x16", "Your password contains control characters"),
+        ("entered\x16pw", "Your password contains control characters"),
+    ],
+)
+def test_warns_for_empty_password(
+    password,
+    warning,
+    monkeypatch,
+    entered_username,
+    config,
+    caplog,
+):
+    monkeypatch.setattr(getpass, "getpass", lambda prompt: password)
+
+    assert auth.Resolver(config, auth.CredentialInput()).password == password
+
+    assert caplog.messages[0].startswith(f"  {warning}")

twine/utils.py

@@ -18,6 +18,7 @@
 import logging
 import os
 import os.path
+import unicodedata
 from typing import Any, Callable, DefaultDict, Dict, Optional, Sequence, Union
 from urllib.parse import urlparse
 from urllib.parse import urlunparse
@@ -241,7 +242,25 @@ def get_userpass_value(
         logger.info(f"{key} set from config file")
         return config[key]
     elif prompt_strategy:
-        return prompt_strategy()
+        warning = ""
+        value = prompt_strategy()
+
+        if not value:
+            warning = f"Your {key} is empty"
+        elif any(unicodedata.category(c).startswith("C") for c in value):
+            # See https://www.unicode.org/reports/tr44/#General_Category_Values
+            # Most common case is "\x16" when pasting in Windows Command Prompt
+            warning = f"Your {key} contains control characters"
+
+        if warning:
+            logger.warning(f"  {warning}. Did you enter it correctly?")
+            # TODO: Link to new entry in Twine docs
+            logger.warning(
+                "  See https://pypi.org/help/#invalid-auth for more information."
+            )
+
+        return value
+
     else:
         return None