Pythonic way to initialise function parameters

Question

Which is more pythonic?

A

def my_function(arg, p=0):
    while arg:
        # do something
         value = arg.pop()
        # save value with a name which uses p for indexing
    p+=1

or B

def my_function(arg):
    p = 0
    while arg:
        # do something
        value = arg.pop()
        # save value with a name which uses p for indexing
    p+=1

A part of me think its silly to include p as an argument to the function incase someone sets it to a weird a value. But at the same time I don't like having p=0 clutter up a function which is already very complicated.

Maybe not relevent to your question but "which is already very complicated" is a red flag if you want to do things in the pythonic way as your question suggests. — Asics
– Asics, Commented Jul 24, 2014 at 14:14
Some more detail would be useful. Looking at this code, it's not clear that p needs to be initialized to 0 at all before the loop runs; you could just initialize it to 1 after the loop. — chepner
– chepner, Commented Jul 24, 2014 at 14:39
If you're supposed to be indexing the items in arg, you can do for p, value in enumerate(arg). — jonrsharpe
– jonrsharpe, Commented Jul 24, 2014 at 14:42

Martijn Pieters · Accepted Answer · 2014-07-24 14:11:35Z

5

Don't clutter up function parameters with locals. If it is not a parameter the caller should use, don't add it to your function signature.

In other words, you don't want anyone using help(my_function) and be surprised at what p might be for.

This isn't a hard and fast rule, of course; some critical-path functions can be made faster by using locals, so sometimes you'll see something like:

some_global = expensive_function()

def foo(bar, baz, _some_local=some_global):
    # code using _some_local instead of some_global

to make use of faster local name lookups. The _ at the start of the argument then tells you that the name is really an internal implementation detail you should not rely on.

answered Jul 24, 2014 at 14:11

Martijn Pieters

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Comments

TheSoundDefense · Accepted Answer · 2014-07-24 14:11:34Z

2

It depends on if p always has to start at 0 or not. If it does, then definitely go with option B. Don't give users an opportunity to mess with your code if it's not necessary.

answered Jul 24, 2014 at 14:11

TheSoundDefense

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