Python populating input arguments for a function with *args

Question

If I have a function which supports variable argument number, i.e. uses *args, how can I populate the arguments from a loop/list comprehension? Say the function expects multiple lists as arguments and I have a table/dataframe and want to use each column as an input argument, why does this not work?

funName([df[iCol].values for iCol in df.columns])

Say my dataframe has 5 columns, I would be required to call the function like so:

funName(col1, col2, col3, col4, col5)

But I do not want to manually create variables for each column but rather populate the argument list dynamically. Thanks.

Bahrom · Accepted Answer · 2016-04-26 14:12:04Z

6

Unpack your list when passing it in:

funName(*[df[iCol].values for iCol in df.columns])

List unpacking is required because if you don't, fn will get a single argument, which is a list, say [1, 2, 3], and you want a sequence of arguments 1, 2, 3.

For example:

>>> def fn(*args):
...     print args
...     
>>> fn([1, 2, 3])
([1, 2, 3],)
>>> fn(*[1, 2, 3])
(1, 2, 3)
>>>

edited Apr 26, 2016 at 14:12

answered Apr 26, 2016 at 14:08

Bahrom

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3 Comments

dter Over a year ago

Works perfectly. Thanks! Why is list unpacking required? (will accept the answer in 11 mins).

Amit Gold Over a year ago

@dter That's the syntax

dter Over a year ago

Makes sense. Basically identifying whether the 1,2,3 is 3 different arguments or 1 argument list containing [1,2,3] as elements.

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