Basically I'm trying to do something like this:
class A:
some_field = None # that is the field which should have 123 value
# what needs to be here?
class B(A(123)):
pass
class C(A(456)):
pass
I want one base class A. And I want children B and C to be able to pass arguments to it like that A(123).
And for example if I do B.some_field I should get 123
How can I do that?
unutbu's answer is correct. I'd just like to point out that you can even do this stuff dynamically...
def make_class(base, value):
class NewClass(base):
some_field = value
return NewClass
class A(object):
some_field = None
B = make_class(A, 123)
C = make_class(A, 456)
And actually, this one is important enough that python actually has a builtin to do it:
class A(object):
some_field = None
B = type('B', (A,), {'some_field': 123})
type takes 3 arguments -- The name of the class, a tuple of the base classes and a dictionary of class attributes.
If you define B and C like this:
class B(A):
some_field = 123
class C(A):
some_field = 123
then:
In [147]: B.some_field
Out[147]: 123
By the way, A(123) would pass 123 to A.__init__. It would not set some_field.
Also, in a class definition,
class B(...)
The stuff in parentheses must be a class or a comma-separated list of classes (or, in Python3, metaclass=SomeMetaClass). It can not be an instance of a class. So
class B(A(123))
is a definite no-no.
In response to the comment: You could use the A.__new__ method to return a class:
class A(object):
some_field = None
def __new__(cls, val):
cls.some_field = val
return cls
class B(A(123)): pass
then
In [161]: B.some_field
Out[161]: 123
But this is a non-standard use of __new__ since usually it is used to return an instance of A. When it does not return an instance of A, then A.__init__ is not called.
Instead of using a class A it would be more understandable to use a class factory:
class B(make_class(123)): ...
as you mentioned in the comment below.
def A(some_field): return make_class(SomeBase, some_field) and then class B(A(123)) would be perfectly valid. I just need some syntactic sugar. Wondered if I somehow could use a class instead of a method.