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          


        

Answer 1

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