astrojuanlu astrojuanlu - 5 months ago 14
Python Question

Assign external function to class variable in Python

I am trying to assign a function defined elsewhere to a class variable so I can later call it in one of the methods of the instance, like this:

from module import my_func

class Bar(object):
func = my_func
def run(self):
self.func() # Runs my function


The problem is that this fails because when doing
self.func()
, then the instance is passed as the first parameter.

I've come up with a hack but seems ugly to me, anybody has an alternative?

In [1]: class Foo(object):
...: func = lambda *args: args
...: def __init__(self):
...: print(self.func())
...:

In [2]: class Foo2(object):
...: funcs = [lambda *args: args]
...: def __init__(self):
...: print(self.funcs[0]())
...:

In [3]: f = Foo()
(<__main__.Foo object at 0x00000000044BFB70>,)

In [4]: f2 = Foo2()
()


Edit: The behavior is different with builtin functions!

In [13]: from math import pow

In [14]: def pow_(a, b):
....: return pow(a, b)
....:

In [15]: class Foo3(object):
....: func = pow_
....: def __init__(self):
....: print(self.func(2, 3))
....:

In [16]: f3 = Foo3()
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-16-c27c8778655e> in <module>()
----> 1 f3 = Foo3()

<ipython-input-15-efeb6adb211c> in __init__(self)
2 func = pow_
3 def __init__(self):
----> 4 print(self.func(2, 3))
5

TypeError: pow_() takes exactly 2 arguments (3 given)

In [17]: class Foo4(object):
....: func = pow
....: def __init__(self):
....: print(self.func(2, 3))
....:

In [18]: f4 = Foo4()
8.0

Answer

Functions are descriptor objects, and when attributes on a class accessing them an instance causes them to be bound as methods.

If you want to prevent this, use the staticmethod function to wrap the function in a different descriptor that doesn't bind to the instance:

class Bar(object):
    func = staticmethod(my_func)
    def run(self):
        self.func()

Alternatively, access the unbound function via the __func__ attribute on the method:

def run(self):
    self.func.__func__()

or go directly to the class __dict__ attribute to bypass the descriptor protocol altogether:

def run(self):
    Bar.__dict__['func']()
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