Assign a modular method to a class variable or instance variable

Question

In a.py module

def task(): print "task called" a = task class A: func = task # this show error unbound method #func = task.__call__ # if i replace with this work def __init__(self): self.func_1 = task def test_1(self): self.func_1() @classmethod def test(cls): cls.func() a() A().test_1() A.test() 

Output:

 task called task called Traceback (most recent call last): File "a.py", line 26, in <module> A.test() File "a.py", line 21, in test cls.func() TypeError: unbound method task() must be called with A instance as first argument (got nothing instead) 

In a module, I can easily assign a function to a variable. When inside the class, try assigning the module level function of the class variable func = task , it shows an error, to remove this error, I have to replace it with func = task .__ call __ But when I assign the instance variable my work self.func_1 = task .

My question is: why can't I assign a module level function to a class variable without __ calling __ and when does the same function work that I can assign to an instance variable.

Answer 1

Since you displayed the function as an unrelated method A , so when you call cls.func , you first set something equal to getattr(cls, 'func') , which returns the <unbound method A.task> BUT, you need to call this unbound method with the class as the first argument.

Since cls.func in this particular case means “gives me the class attribute func of cls ”, it cannot mean “ func class method” at the same time - it means Python does not translate cls.func() to func(cls) .

But at the same time, since func is the <unbound method A.task> (bound to A.task ), it needs to be called as func(cls) to work.

Mark this as follows:

 @classmethod def test(cls): print getattr(cls, 'func') # <unbound method A.task> 

You can fix this with something like:

 def task(cls=None): if cls is None: print 'task()' else: print 'A.foo({})'.format(cls) a = task class A: func = task # this show error unbound method def __init__(self): self.func_1 = task def test_1(self): self.func_1() @classmethod def test(cls): cls.func(cls()) a() A().test_1() A.test() 

Output:

 task() task() A.foo(<__main__.A instance at 0x7fd0310a46c8>) 

Note that python3 removes unrelated methods, this only works with python2.x