Why is Python 3.x's super() magic?
In Python 3.x,
super() can be called without arguments:
class A(object): def x(self): print("Hey now") class B(A): def x(self): super().x()
>>> B().x() Hey now
In order to make this work, some compile-time magic is performed, one consequence of which is that the following code (which rebinds
super_ = super class A(object): def x(self): print("No flipping") class B(A): def x(self): super_().x()
>>> B().x() Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 3, in x RuntimeError: super(): __class__ cell not found
super() unable to resolve the superclass at runtime without assistance from the compiler? Are there practical situations in which this behaviour, or the underlying reason for it, could bite an unwary programmer?
... and, as a side question: are there any other examples in Python of functions, methods etc. which can be broken by rebinding them to a different name?
The new magic
super() behaviour was added to avoid violating the D.R.Y. (Don't Repeat Yourself) principle, see PEP 3135. Having to explicitly name the class by referencing it as a global is also prone to the same rebinding issues you discovered with
class Foo(Bar): def baz(self): return super(Foo, self).baz() + 42 Spam = Foo Foo = something_else() Spam().baz() # liable to blow up
The same applies to using class decorators where the decorator returns a new object, which rebinds the class name:
@class_decorator_returning_new_class class Foo(Bar): def baz(self): # Now `Foo` is a *different class* return super(Foo, self).baz() + 42
__class__ cell sidesteps these issues nicely by giving you access to the original class object.
The PEP was kicked off by Guido, who initially envisioned
super becoming a keyword, and the idea of using a cell to look up the current class was also his. Certainly, the idea to make it a keyword was part of the first draft of the PEP.
However, it was in fact Guido himself who then stepped away from the keyword idea as 'too magical', proposing the current implementation instead. He anticipated that using a different name for
super() could be a problem:
My patch uses an intermediate solution: it assumes you need
__class__whenever you use a variable named
'super'. Thus, if you (globally) rename
super, it won't work without arguments (but it will still work if you pass it either
__class__or the actual class object); if you have an unrelated variable named
super, things will work but the method will use the slightly slower call path used for cell variables.
So, in the end, it was Guido himself that proclaimed that using a
super keyword did not feel right, and that providing a magic
__class__ cell was an acceptable compromise.
I agree that the magic, implicit behaviour of the implementation is somewhat surprising, but
super() is one of the most mis-applied functions in the language. Just take a look at all the misapplied
super(type(self), self) or
super(self.__class__, self) invocations found on the Internet; if any of that code was ever called from a derived class you'd end up with an infinite recursion exception. At the very least the simplified
super() call, without arguments, avoids that problem.
As for the renamed
super_; just reference
__class__ in your method as well and it'll work again. The cell is created if you reference either the
__class__ names in your method:
>>> super_ = super >>> class A(object): ... def x(self): ... print("No flipping") ... >>> class B(A): ... def x(self): ... __class__ # just referencing it is enough ... super_().x() ... >>> B().x() No flipping