Restrict the number of classes affected by len-as-condition

3 files changed, 58 insertions, 3 deletions

diff --git a/pylint/checkers/refactoring/len_checker.py b/pylint/checkers/refactoring/len_checker.py
index 07ccf4ebe..ae4a502d0 100644
--- a/pylint/checkers/refactoring/len_checker.py
+++ b/pylint/checkers/refactoring/len_checker.py
@@ -63,11 +63,20 @@ class LenChecker(checkers.BaseChecker):
             parent = node.parent
             while isinstance(parent, astroid.BoolOp):
                 parent = parent.parent
-
             # we're finally out of any nested boolean operations so check if
             # this len() call is part of a test condition
             if utils.is_test_condition(node, parent):
-                self.add_message("len-as-condition", node=node)
+                instance = next(node.args[0].infer())
+                mother_classes = self.base_classes_of_node(instance)
+                affected_by_pep8 = any(
+                    t in mother_classes for t in ["str", "tuple", "range", "list"]
+                )
+                if affected_by_pep8 and not self.instance_has_bool(instance):
+                    self.add_message("len-as-condition", node=node)
+
+    @staticmethod
+    def instance_has_bool(class_def: astroid.ClassDef) -> bool:
+        return any(hasattr(f, "name") and f.name == "__bool__" for f in class_def.body)
 
     @utils.check_messages("len-as-condition")
     def visit_unaryop(self, node):
diff --git a/tests/functional/l/len_checks.py b/tests/functional/l/len_checks.py
index 216a7e672..98c33424a 100644
--- a/tests/functional/l/len_checks.py
+++ b/tests/functional/l/len_checks.py
@@ -7,7 +7,7 @@ if len('TEST'):  # [len-as-condition]
 if not len('TEST'):  # [len-as-condition]
     pass
 
-z = False
+z = []
 if z and len(['T', 'E', 'S', 'T']):  # [len-as-condition]
     pass
 
@@ -102,3 +102,46 @@ def github_issue_1331_v4(*args):
 
 b = bool(len(z)) # [len-as-condition]
 c = bool(len('TEST') or 42) # [len-as-condition]
+
+
+def github_issue_1879():
+
+    class ClassWithBool(list):
+        def __bool__(self):
+            return True
+
+    class ClassWithoutBool(list):
+        pass
+
+    class ChildClassWithBool(ClassWithBool):
+        pass
+
+    class ChildClassWithoutBool(ClassWithoutBool):
+        pass
+
+    assert len(ClassWithBool())
+    # We could expect to not have a len-as-condition for ChildClassWithBool,
+    # but I don't think the required analysis is worth it.
+    assert len(ChildClassWithBool()) # [len-as-condition]
+    assert len(ClassWithoutBool())  # [len-as-condition]
+    assert len(ChildClassWithoutBool())  # [len-as-condition]
+    # assert len(range(0)) != 0
+
+    # pylint: disable=import-outside-toplevel
+    import numpy
+    numpy_array = numpy.array([0])
+    if len(numpy_array) > 0:
+        print('numpy_array')
+    if len(numpy_array):
+        print('numpy_array')
+    if numpy_array:
+        print('b')
+
+    import pandas as pd
+    pandas_df = pd.DataFrame()
+    if len(pandas_df):
+        print("this works, but pylint tells me not to use len() without comparison")
+    if len(pandas_df) > 0:
+        print("this works and pylint likes it, but it's not the solution intended by PEP-8")
+    if pandas_df:
+        print("this does not work (truth value of dataframe is ambiguous)")
diff --git a/tests/functional/l/len_checks.txt b/tests/functional/l/len_checks.txt
index 23e3f583f..7fbf5e8da 100644
--- a/tests/functional/l/len_checks.txt
+++ b/tests/functional/l/len_checks.txt
@@ -13,3 +13,6 @@ len-as-condition:98:github_issue_1331_v3:Do not use `len(SEQUENCE)` without comp
 len-as-condition:101:github_issue_1331_v4:Do not use `len(SEQUENCE)` without comparison to determine if a sequence is empty
 len-as-condition:103::Do not use `len(SEQUENCE)` without comparison to determine if a sequence is empty
 len-as-condition:104::Do not use `len(SEQUENCE)` without comparison to determine if a sequence is empty
+len-as-condition:125:github_issue_1879:Do not use `len(SEQUENCE)` without comparison to determine if a sequence is empty
+len-as-condition:126:github_issue_1879:Do not use `len(SEQUENCE)` without comparison to determine if a sequence is empty
+len-as-condition:127:github_issue_1879:Do not use `len(SEQUENCE)` without comparison to determine if a sequence is empty