类绑定函数python 2和3

Question

In python 2 I could create a class bound method using:

types.MethodType(func, None, cls)

However in python3 MethodType does not take the 3rd parameter.

How can I achieve the same behaviour in python 3? Preferably in a way that is still valid in python2.7.

Answer 1

It seems that in Python3, you don't need to use MethodType anymore to create class bound methods (though you still need it to assign instance bound methods). Thus if you're going to do this:

class A:
  def __init__(self):
    pass
# end of class definition
# ...
# whoops I forgot a method for A
def foo(self, *args):
  pass

You can later attach foo to A using just A.foo = foo . (In Python2 you had to use A.foo = types.MethodType(foo, None, A) or such.)

If you want to add foo just to a particular instance of A , you would still use MethodType , although (in Python3) only with two arguments:

a = A()
a.foo = types.MethodType(foo, a)

(In Python2, you had to use a.foo = types.MethodType(foo, a, A) or such.)

As far as I can see, if you want a strategy that works with both versions, you'd have to do something like this:

try:
  A.foo = types.MethodType(foo, None, A)
except TypeError:
  # too many arguments to MethodType, we must be in Python3
  A.foo = foo