Use global variables as default values

Question

I have a function which takes a lot of parameters, and since I dont want to remember their positions I decided to use named arguments

def f(a=None, b=None, c=None):
    print a,b,c
f('test', c=5, b='second param')
>>> test second param 5

now, usually I only change one parameter at a time, so i want to call the function by only typing f(c=3.14) with the intended result being f(a=a, b=b, c=3.14), namely every argument that is not explicitely passed should be read from the local scope.

But of course it doesnt work since to use named arguments I have to set a default value, if I use **kwargs instead, it simply ignores the arguments

def f(**kwargs):
    print a,b,c
a=1; b=2; c=3
f(a=2, b=4, c=8)
>>> 1 2 3 # instead of 2 4 8

I used to define a new function for each parameter but I dont like this approach although it has been the most effective so far

def fc(c_new):
    return f(a, b, c_new)

How do I make the function use the variables in its current scope as default values for its named arguments?

All function arguments have names. I think you mean keyword arguments. — martineau
– martineau, Commented Nov 27, 2013 at 12:33

Alexander Zhukov · Accepted Answer · 2018-06-08 10:13:22Z

9

Here is a solution with a decorator:

from functools import wraps
from inspect import getcallargs


def defaults_from_globals(f):
    @wraps(f)
    def new(**kwargs):
        # filter only those vars, that are in the list of function's named args
        from_globals = {arg: globals()[arg] for arg in getcallargs(f)}
        # overwrite them with user supplied kwargs
        from_globals.update(kwargs)
        return f(**from_gobals)

    return new


@defaults_from_globals
def f(a=None, b=None, c=None):
    return a, b, c


a = 1
b = 2
c = 3

print f(a=2, b=4) # 2 4 3

edited Jun 8, 2018 at 10:13

answered Nov 27, 2013 at 11:52

Alexander Zhukov

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5 Comments

Alexander Zhukov Over a year ago

Took me a while to figure out what is the actual goal though).

martineau Over a year ago

It could potentially result in a lot less overhead to only retrieve globals with the same name as keyword arguments instead of the other way around as you're doing. i.e. globals_ = globals(), from_globals = {k: globals_[k] for k in getcallargs(f) if k in globals_}. Good answer regardless. +1

DenDenDo Over a year ago

That does exactly what i meant and even more, since it can be quickly applied to any function (although I dont feel comfortable using the secret arts of decorators, they are just, ...weird).

Alexander Zhukov Over a year ago

Ones you've understand them (decorators), you'll get an enormously powerful tool.

martineau Over a year ago

The way you incorporated my suggestion into your answer could fail if there's no matching global identifier for a keyword argument (which is why I put that if at the end of the dictionary comprehension in my comment). Yours also potentially calls globals() for each key word argument, which is wasteful.

BartoszKP · Accepted Answer · 2013-11-28 13:00:07Z

1

For the sake of completeness here is one more solution I just found, it's more of a dirty hack, because the use of exec is frowned upon in most cases* but it's more straightforward to understand.

*) in this case the user has control over the code and it is not in his interest to intentionally screw up his own work

def f(x=None, y=None, z=None):
    for  key,val in locals().items():
        if val==None: exec("%s = globals()[key]"%key)
    print x,y,z

edited Nov 28, 2013 at 13:00

BartoszKP

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answered Nov 27, 2013 at 15:46

DenDenDo

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Comments

Foo Bar User · Accepted Answer · 2013-11-27 12:00:04Z

0

i think you should use decorators

import functools

_last_args = {}
def foo(f):
    @functools.wraps(f)
    def wrapper( *args, **kwargs):
        _last_args.update(kwargs)
        ret =  f(*args, **_last_args)
        return ret
    return wrapper

@foo
def bar(a=None,b=None,c=None):
    print(a,b,c)

bar(a=1,b=2,c=3)
bar(a=10,b=20)
bar(a=100)

prints:

1 2 3
10 20 3
100 20 3

answered Nov 27, 2013 at 12:00

Foo Bar User

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2 Comments

BartoszKP Over a year ago

Good call on using a decorator, but that's not what the OP wants

Foo Bar User Over a year ago

i think when he says scope he means to get the previous ones because if it was only default values in the scope he would save them as default in the function instead of None. Maybe wrong

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