Drop a superfluous and wrong callcontext when inferring the result of…

… a context manager

The callcontext was containing the instance of the class (self) as the sole argument,
but this was tripping the inference of unknown arguments coming down into an
instance, such as the one from the commit's example.
By dropping the superfluous callcontext, the inference can no longer assume
that the first argument is the instance of the class, leading to wrongly
infer `self.client` as the first argument of the instantiation call.

Close pylint-dev/pylint#2859

ChangeLog

@@ -6,6 +6,10 @@ What's New in astroid 2.3.0?
 ============================
 Release Date: TBA
 
+* Drop a superfluous and wrong callcontext when inferring the result of a context manager
+
+  Close PyCQA/pylint#2859
+
 * ``igetattr`` raises ``InferenceError`` on re-inference of the same object
 
   This prevents ``StopIteration`` from leaking when we encounter the same

astroid/protocols.py

@@ -494,8 +494,6 @@ def _infer_context_manager(self, mgr, context):
             raise exceptions.InferenceError(node=inferred)
         if not isinstance(enter, bases.BoundMethod):
             raise exceptions.InferenceError(node=enter)
-        if not context.callcontext:
-            context.callcontext = contextmod.CallContext(args=[inferred])
         yield from enter.infer_call_result(self, context)
     else:
         raise exceptions.InferenceError(node=mgr)

astroid/tests/unittest_inference.py

@@ -5213,5 +5213,38 @@ def __call__(self):
     assert next(node.infer()) is util.Uninferable
 
 
+def test_infer_context_manager_with_unknown_args():
+    code = """
+    class client_log(object):
+        def __init__(self, client):
+            self.client = client
+        def __enter__(self):
+            return self.client
+        def __exit__(self, exc_type, exc_value, traceback):
+            pass
+
+    with client_log(None) as c:
+        c #@
+    """
+    node = extract_node(code)
+    assert next(node.infer()) is util.Uninferable
+
+    # But if we know the argument, then it is easy
+    code = """
+    class client_log(object):
+        def __init__(self, client=24):
+            self.client = client
+        def __enter__(self):
+            return self.client
+        def __exit__(self, exc_type, exc_value, traceback):
+            pass
+
+    with client_log(None) as c:
+        c #@
+    """
+    node = extract_node(code)
+    assert isinstance(next(node.infer()), nodes.Const)
+
+
 if __name__ == "__main__":
     unittest.main()