The super proxy does not seem to forward call to .__str__() when the call occurs via str() function.
It may be an expected behavior, but it looks unexpected to me.

Minimal reproduction (tested on Python 3.6, but I believe may newer versions have similar behavior):

class Parent():
  def __str__(self):
    return "Parent"

class Child(Parent):
  def foo(self):
    s = super(Child, self)
    print(s.__str__())
    print(str(s))


c = Child()
c.foo()

# Output :
### Parent
### <super: <class 'Child'>, <Child object>>