Can't have a function as a class attribute in Python

Question

I want to have a plain old function as a class constant. However, Python "helpfully" turns it into a method for me:

class C(object):
    a = 17
    b = (lambda x : x+1)

print C.a     # Works fine for int attributes
print C.b     # Uh-oh... is a <unbound method C.<lambda>> now
print C.b(1)  # TypeError: unbound method <lambda>() must be called
              #    with C instance as first argument (got int instance instead)

Is there a way to prevent my function from becoming a method?
In any case, what is the best "Pythonic" approach to this problem?

Note, the above code works fine in Python 3, where the concept of unbound methods is gone: python.org/download/releases/3.0/whatsnew . C.b is simply a function, usable as such in the usual manner. — Alex Povel
– Alex Povel, Commented Jun 10, 2022 at 13:38

bluepnume · Accepted Answer · 2011-05-14 03:19:01Z

23

staticmethod:

class C(object):
    a = 17

    @staticmethod
    def b(x):
      return x+1

Or:

class C(object):
    a = 17
    b = staticmethod(lambda x : x+1)

answered May 14, 2011 at 3:19

bluepnume

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Comments

senderle · Accepted Answer · 2011-05-14 03:18:25Z

4

Use staticmethod:

class C(object):
    a = 17
    @staticmethod
    def b(x):
        return x + 1

answered May 14, 2011 at 3:18

senderle

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1 Comment

senderle Over a year ago

But you should consider these caveats.

user142800 · Accepted Answer · 2013-12-13 18:27:43Z

4

Define your function in a module but not class. It's better in your case.

First paramter of normal python method is self, self must be an instance object.

you should use staticmethod:

class C(object):
    a = 17
    b = staticmethod(lambda x: x+1)
print C.b(1)
# output: 2

or add self parameter, and make an instance of C:

class C(object):
    a = 17
    b = lambda self, x: x+1
c = C()
c.b(1)
# output: 2

another choice is using classmethod (just show you can do this):

class C(object):
    a = 17
    b = classmethod(lambda cls, x: x+1)
C.b(1)
# output: 2

edited Dec 13, 2013 at 18:27

answered Dec 13, 2013 at 18:15

user142800

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1 Comment

Danny Staple Over a year ago

Interesting - lambdas need to be wrapped in staticmethod/classmethod too. I wasn't aware of this before.

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