Can I dynamically pass a list of arguments to a python function with predefined arguments? [duplicate]

Question

I have a python function bar(a=None, b=None, c=None, d=None, e=None). I was wondering if it was possible to build the list of arguments I want to pass in, with a list or a string, or something.

I would have a list of attributes to look for and ideally I would simply loop over them and build the argument string.

For example, something like:

dict = {'a' : 5, 'c' : 3, 'd' :9} 
arg_string = ""
for k, v in dict.iteritems():
    arg_string += "{0}={1},".format(k, v)

bar(arg_string)

In this case I don't have any values to pass in for b and e, but the next time the program runs there may be.

Is this possible?

I know the string being built in the for loop will probably be invalid because of the comma at the end. But i didn't think it was necessary to write the handling for that

The comma wouldn't be the problem, rather that you are calling bar("a=5, c=3, d=9") instead of bar(a=5, c=3, d=9). — chepner
– chepner, Commented Feb 22, 2016 at 20:39
And, FWIW, trailing commas in python function calls are actually permissable (though ugly) -- foo(a, b, c,) is a syntactically valid function call. — mgilson
– mgilson, Commented Feb 22, 2016 at 20:41

mgilson · Accepted Answer · 2016-02-22 20:38:44Z

29

The good news is that you don't even need a string. . .

mapping = {'a': 5, 'c': 3, 'd': 9} 
bar(**mapping)

should work. See the Unpacking Argument Lists (and surrounding material) in the official python tutorial.

edited Feb 22, 2016 at 20:38

answered Feb 22, 2016 at 20:37

mgilson

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1 Comment

Michael Over a year ago

Works like a charm with variable values as well, for instance I use it with mongoengine and I don't know the db value to update, so I handle it dynamically via mapping = {dbEntry: value} myDBcollection.update(**mapping) dbEntry can anything like name,price,symbol, etc

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