Automatically setting class member variables in Python

Question

Say, I have the following class in Python

class Foo(object):     a = None     b = None     c = None     def __init__(self, a = None, b = None, c = None):         self.a = a         self.b = b         self.c = c 

Is there any way to simplify this process? Whenever I add a new member to class Foo, I'm forced to modify the constructor.

Answer 1

Please note that

class Foo(object):     a = None 

sets a key-value pair in Foo's dict:

Foo.__dict__['a']=None 

while

def __init__(self, a = None, b = None, c = None):     self.a = a 

sets a key-value pair in the Foo instance object's dict:

foo=Foo() foo.__dict__['a']=a 

So setting the class members at the top of your definition is not directly related to the setting of the instance attributes in the lower half of your definition (inside the __init__.

Also, it is good to be aware that __init__ is Python's initializer. __new__ is the class constructor.

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If you are looking for a way to automatically add some instance attributes based on __init__'s arguments, you could use this:

import inspect import functools  def autoargs(*include,**kwargs):        def _autoargs(func):         attrs,varargs,varkw,defaults=inspect.getargspec(func)         def sieve(attr):             if kwargs and attr in kwargs['exclude']: return False             if not include or attr in include: return True             else: return False                     @functools.wraps(func)         def wrapper(self,*args,**kwargs):             # handle default values             for attr,val in zip(reversed(attrs),reversed(defaults)):                 if sieve(attr): setattr(self, attr, val)             # handle positional arguments             positional_attrs=attrs[1:]                         for attr,val in zip(positional_attrs,args):                 if sieve(attr): setattr(self, attr, val)             # handle varargs             if varargs:                 remaining_args=args[len(positional_attrs):]                 if sieve(varargs): setattr(self, varargs, remaining_args)                             # handle varkw             if kwargs:                 for attr,val in kwargs.iteritems():                     if sieve(attr): setattr(self,attr,val)                         return func(self,*args,**kwargs)         return wrapper     return _autoargs 

So when you say

class Foo(object):     @autoargs()     def __init__(self,x,path,debug=False,*args,**kw):         pass foo=Foo('bar','/tmp',True, 100, 101,verbose=True) 

you automatically get these instance attributes:

print(foo.x) # bar print(foo.path) # /tmp print(foo.debug) # True print(foo.args) # (100, 101) print(foo.verbose) # True 

PS. Although I wrote this (for fun), I don't recommend using autoargs for serious work. Being explicit is simple, clear and infallible. I can't say the same for autoargs.

PPS. Is it just me, or are a lot of buttons broken on Stackoverflow? The editor window has lost all its icons... :( Clearing the browser cache fixed the problem.