python class variable not visible in __init__?

Question

This code produces an error message, which I found surprising:

class Foo(object):
    custom = 1
    def __init__(self, custom=Foo.custom):
        self._custom = custom

x = Foo()

Can anyone provide enlightenment?

Answer 1

The class body is executed before the class its self is defined, so default argument values can't reference the class. Just making custom the default (without class qualification) should work.

Answer 2

It's Foo that isn't visible, because you're in the middle of building it. But since you're in the same scope as custom, you can just say custom rather than Foo.custom:

class Foo(object):
    custom = 1
    def __init__(self, mycustom=custom):
        self._custom = mycustom

But note that changing Foo.custom later on won't affect the value of custom that subsequently-created Foos see:

class Foo(object):
    custom = 1
    def __init__(self, mycustom=custom):
        self._custom = mycustom

one = Foo()
Foo.custom = 2
two = Foo()
print (two._custom)  # Prints 1

By using a sentinel default value instead, you can get what you want:

class Foo(object):
    custom = 1
    def __init__(self, mycustom=None):
        if mycustom is None:
            self._custom = Foo.custom
        else:
            self._custom = mycustom

one = Foo()
Foo.custom = 2
two = Foo()
print (two._custom)  # Prints 2

Answer 3

I get the following error:

Traceback (most recent call last):
  Line 1, in <module>
    class Foo(object):
  Line 3, in Foo
    def __init__(self, custom=Foo.custom):
NameError: name 'Foo' is not defined

This is because the name Foo is in the process of being defined as the __init__ function is defined, and is not fully available at that time.

The solution is to avoid using the name Foo in the function definition (I also renamed the custom paramter to acustom to distinguish it from Foo.custom):

class Foo(object):
    custom = 1
    def __init__(self, acustom=custom):
        self._custom = acustom
x = Foo()
print x._custom

Answer 4

What we do instead is the following

class Foo( object ):
    custom = 1
    def __init__( self, arg=None )
        self._custom = self.custom if arg is None else arg

This bypasses the confusing issue of whether or not the name Foo has been defined yet.