Question

Possible Duplicate:
Assign function arguments to `self`

Often I have constructors that look like the following:

class Foo(object):
  def __init__(self,a,b,c):
    self.a = a
    self.b = b
    self.c = c

I was wondering if there was a convenient way to encode constructors like this. Perhaps something that looks like:

class Foo(object):
  SOME_MACRO_LIKE_THINGY_THAT_SPECIFIES_THE_CONSTRUCTOR(a,b,c)

that would behave exactly like the original code above.

What bugs me with the original version is that I have to write each instance variable of Foo three times (once as an argument, once as self.a and once again as the value to assign to self.a).

I suppose it’s not that big a deal, but I feel like the code would look neater with less repetition.

What is the Pythonic way to handle this situation?

Answer 1

It’s weird, but you could use __slots__ to achieve this:

class Base(object):
    def __init__(self, *args, **kw):
        for k,v in zip(self.__slots__, args):
            setattr(self, k, v)

        for k,v in kw.items():
            setattr(self, k, v)

class ChildA(Base):
    __slots__ = 'a', 'b', 'c'

class ChildB(Base):
    __slots__ = 'x', 'y', 'z'

You could also use the following technique to save yourself some typing when initializing classes with lots of arguments:

def autoargs(l):
    self = l.pop('self')
    for k,v in l.items():
        setattr(self, k, v)

class Base(object):
    def __init__(self, a, b, c):
        autoargs(locals())

I hope I’m understanding your question correctly.