Emit using-constant-test when testing truth value of a variable holding a generator

doc/whatsnew/2/2.15/index.rst

@@ -40,6 +40,10 @@ False negatives fixed
 
   Closes #6909
 
+* Emit ``using-constant-test`` when testing the truth value of a variable holding a generator.
+
+  Refs #6909
+
 
 Other bug fixes
 ===============

pylint/checkers/base/basic_checker.py

@@ -315,11 +315,37 @@ def _check_using_constant_test(
         )
         inferred = None
         emit = isinstance(test, (nodes.Const,) + structs + const_nodes)
+        maybe_generator_call = None
         if not isinstance(test, except_nodes):
             inferred = utils.safe_infer(test)
+            if inferred is astroid.Uninferable and isinstance(test, nodes.Name):
+                lookup_result = test.frame(future=True).lookup(test.name)
+                if lookup_result:
+                    maybe_generator_assigned = (
+                        isinstance(assign_name.parent.value, nodes.GeneratorExp)
+                        for assign_name in lookup_result[1]
+                        if isinstance(assign_name.parent, nodes.Assign)
+                    )
+                    first_item = next(maybe_generator_assigned, None)
+                    if first_item is not None:
+                        # Emit if this variable is certain to hold a generator
+                        if all(
+                            itertools.chain((first_item,), maybe_generator_assigned)
+                        ):
+                            emit = True
+                        # If this variable holds the result of a call, save it for next test
+                        elif (
+                            len(lookup_result[1]) == 1
+                            and isinstance(lookup_result[1][0].parent, nodes.Assign)
+                            and isinstance(lookup_result[1][0].parent.value, nodes.Call)
+                        ):
+                            maybe_generator_call = lookup_result[1][0].parent.value
+
         # Emit if calling a function that only returns GeneratorExp (always tests True)
         elif isinstance(test, nodes.Call):
-            inferred_call = utils.safe_infer(test.func)
+            maybe_generator_call = test
+        if maybe_generator_call:
+            inferred_call = utils.safe_infer(maybe_generator_call.func)
             if isinstance(inferred_call, nodes.FunctionDef):
                 # Can't use all(x) or not any(not x) for this condition, because it
                 # will return True for empty generators, which is not what we want.

tests/functional/u/using_constant_test.py

@@ -163,3 +163,16 @@ def maybe_get_generator(arg):
 
 if maybe_get_generator(None):
     pass
+
+y = (a for a in range(10))
+if y:  # [using-constant-test]
+    pass
+
+z = (a for a in range(10))
+z = "red herring"
+if z:
+    pass
+
+gen = get_generator()
+if gen:  # [using-constant-test]
+    pass

tests/functional/u/using_constant_test.txt

@@ -26,3 +26,5 @@ using-constant-test:89:3:89:15::Using a conditional statement with a constant va
 using-constant-test:93:3:93:18::Using a conditional statement with a constant value:INFERENCE
 comparison-of-constants:117:3:117:8::"Comparison between constants: '2 < 3' has a constant value":HIGH
 using-constant-test:156:0:157:8::Using a conditional statement with a constant value:INFERENCE
+using-constant-test:168:3:168:4::Using a conditional statement with a constant value:UNDEFINED
+using-constant-test:177:0:178:8::Using a conditional statement with a constant value:INFERENCE