Fix astroid.Inference error for undefined-variables with len()

Author: cdce8p

ChangeLog

@@ -48,6 +48,10 @@ Release date: TBA
 
   Closes #4181
 
+* Fix astroid.Inference error for undefined-variables with ``len()```
+
+  Closes #4215
+
 
 What's New in Pylint 2.7.2?
 ===========================

pylint/checkers/refactoring/len_checker.py

@@ -73,7 +73,11 @@ def visit_call(self, node):
             # The node is a generator or comprehension as in len([x for x in ...])
             self.add_message("len-as-condition", node=node)
             return
-        instance = next(len_arg.infer())
+        try:
+            instance = next(len_arg.infer())
+        except astroid.InferenceError:
+            # Probably undefined-varible, abort check
+            return
         mother_classes = self.base_classes_of_node(instance)
         affected_by_pep8 = any(
             t in mother_classes for t in ["str", "tuple", "list", "set"]

tests/functional/l/len_checks.py

@@ -175,3 +175,12 @@ def function_returning_int(r):
     # assert len(function_returning_generator(z))
     # assert len(function_returning_comprehension(z))
     # assert len(function_returning_function(z))
+
+
+def github_issue_4215():
+    # Test undefined variables
+    # https://github.com/PyCQA/pylint/issues/4215
+    if len(undefined_var):  # [undefined-variable]
+        pass
+    if len(undefined_var2[0]):  # [undefined-variable]
+        pass

tests/functional/l/len_checks.txt

@@ -21,3 +21,5 @@ len-as-condition:128:11:github_issue_1879:Do not use `len(SEQUENCE)` without com
 len-as-condition:129:11:github_issue_1879:Do not use `len(SEQUENCE)` without comparison to determine if a sequence is empty
 len-as-condition:130:11:github_issue_1879:Do not use `len(SEQUENCE)` without comparison to determine if a sequence is empty
 len-as-condition:171:11:github_issue_1879:Do not use `len(SEQUENCE)` without comparison to determine if a sequence is empty
+undefined-variable:183:11:github_issue_4215:Undefined variable 'undefined_var'
+undefined-variable:185:11:github_issue_4215:Undefined variable 'undefined_var2'