Python initialization

Question

I have this code:

def __init__(self, a, b, c, d...):
 self.a = a
 self.b = b
 etc

I'm thinking of replacing it with:

def __init__(self, a, b, c, d...):
 args=locals()
 for key in args:
 self.__dict__[key] = args[key]

Is this a bad idea? Are there any better ways to do this?

• lxop: Might want to be careful about the self argument, but otherwise looks OK
• Hod - Monica's Army: Will the number of inputs change, or is there a set number?
• user e to the power of 2pi: Thanks Ixop, The number of input will not change.
• Bryan Oakley: Can you explain what you hope to accomplish with this change?

Answer 1

From the Zen of Python: Simple is better than complex. and Readability counts.

The explicit assignments are much more readable and simpler than using magic to set the values.

• user e to the power of 2pi: The thing is there are approximately 20 such variables; is explicit assignment still better? and by explicit assignment, you mean "self.a = a", etc?
• ThiefMaster: A method accepting 20 variables sounds bad. HAve you considered accepting **kwargs and doing self.__dict__.update(kwargs)?

Answer 2

Building on @ThiefMaster's comment about **kwargs:

If you are taking in 20 arguments, it might make more sense to require your users to send arguments via keyword instead of position: with 20 arguments, there is a decent chance that someone using your code will get the arguments in the wrong order.

Consider only accepting kwargs while having a predefined list of keys you want to accept and raising a ValueError if you don't receive them. So you could use **kwargs and then check that everything is there. E.g.

INITIAL_ARGS = set(['a','b','c','d','e'...])
def __init__(self, **kwargs):
 if not INITIAL_ARGS.issubset(set(kwargs.iterkeys())):
 raise ValueError("Class <myclass> requires 20 keyword arguments"
 "only given %d" % len(kwargs))
 self.__dict__.update(kwargs)

Not sure whether this is more or less Pythonic than your original, but it seems like it would save a ton of time later on when trying to figure out why someone using your code might be getting strange errors.

• weronika: If the OP expects the same number of args in every call (that was said in a comment), I'd go with == rather than issubset - don't want the user to add other random keyword arguments that don't do anything but pollute the namespace!

Answer 3

Always consider readability over clever design. Is the replacement code easier to read? I would probably just leave it. Remember that simple is better than complex. As the ThiefMaster said, the explicit assignments are more readable.

Answer 4

Another alternative for Python3 users: the init decorator from drastic.

@init
def __init__(self, a, b, c, d, e, f, g, h):
 # nothing to write here!