fix check for self_arg when class has __len__

Calling `if self` on a class that has
a __len__ method calls this method
(similar to how `if my_list` checks if
the list is empty).
We only want to check if self exists.

Author: Otto-AA

e2e_projects/my_lib/src/my_lib/__init__.py

@@ -74,6 +74,9 @@ def to_origin(self):
     def ignored(self):
         self.foo = 'bar' # pragma: no mutate
 
+    def __len__(self):
+        return 0
+
     @staticmethod
     def from_coords(coords) -> 'Point':
         return Point(coords[0], coords[1])

mutmut/trampoline_templates.py

@@ -63,7 +63,7 @@ def _mutmut_trampoline(orig, mutants, call_args, call_kwargs, self_arg = None):
         result = orig(*call_args, **call_kwargs)
         return result
     mutant_name = mutant_under_test.rpartition('.')[-1]
-    if self_arg:
+    if self_arg is not None:
         # call to a class method where self is not bound
         result = mutants[mutant_name](self_arg, *call_args, **call_kwargs)
     else:

tests/e2e/snapshots/my_lib.json

@@ -56,6 +56,7 @@
     "my_lib.x\u01c1Point\u01c1to_origin__mutmut_2": 1,
     "my_lib.x\u01c1Point\u01c1to_origin__mutmut_3": 0,
     "my_lib.x\u01c1Point\u01c1to_origin__mutmut_4": 0,
+    "my_lib.x\u01c1Point\u01c1__len____mutmut_1": 33,
     "my_lib.x_escape_sequences__mutmut_1": 1,
     "my_lib.x_escape_sequences__mutmut_2": 0,
     "my_lib.x_escape_sequences__mutmut_3": 1,