Fix crash when providing None to mode arg of open() (#5732)

Author: jacobtylerwalls

ChangeLog

@@ -50,6 +50,11 @@ Release date: TBA
 
   Closes #3781
 
+* Fixed a crash in ``unspecified-encoding`` checker when providing ``None``
+  to the ``mode`` argument of an ``open()`` call.
+
+  Closes #5731
+
 * Added ``lru-cache-decorating-method`` checker with checks for the use of ``functools.lru_cache``
   on class methods. This is unrecommended as it creates memory leaks by never letting the instance
   getting garbage collected.

doc/whatsnew/2.13.rst

@@ -156,6 +156,11 @@ Other Changes
 
   Closes #5479
 
+* Fixed a crash in ``unspecified-encoding`` checker when providing ``None``
+  to the ``mode`` argument of an ``open()`` call.
+
+  Closes #5731
+
 * Fix false negative for ``consider-iterating-dictionary`` during membership checks encapsulated in iterables
   or ``not in`` checks
 

pylint/checkers/stdlib.py

@@ -671,7 +671,7 @@ def _check_open_encoded(self, node: nodes.Call, open_module: str) -> None:
         if (
             not mode_arg
             or isinstance(mode_arg, nodes.Const)
-            and "b" not in mode_arg.value
+            and (not mode_arg.value or "b" not in mode_arg.value)
         ):
             encoding_arg = None
             try:

tests/functional/u/unspecified_encoding_py38.py

@@ -156,3 +156,6 @@ class IOArgs:
 
 Path(FILENAME).write_text("string", "utf-8")
 Path(FILENAME).write_text("string")  # [unspecified-encoding]
+
+# Test for crash reported in https://github.com/PyCQA/pylint/issues/5731
+open(FILENAME, mode=None)  # [bad-open-mode, unspecified-encoding]

tests/functional/u/unspecified_encoding_py38.txt

@@ -27,3 +27,5 @@ unspecified-encoding:149:0:149:23::Using open without explicitly specifying an e
 unspecified-encoding:152:0:152:28::Using open without explicitly specifying an encoding:UNDEFINED
 unspecified-encoding:155:0:155:26::Using open without explicitly specifying an encoding:UNDEFINED
 unspecified-encoding:158:0:158:35::Using open without explicitly specifying an encoding:UNDEFINED
+bad-open-mode:161:0:161:25::"""%s"" is not a valid mode for open.":UNDEFINED
+unspecified-encoding:161:0:161:25::Using open without explicitly specifying an encoding:UNDEFINED