Using map() function with keyword arguments

Question

Here is the loop I am trying to use the map function on:

volume_ids = [1,2,3,4,5]
ip = '172.12.13.122'
for volume_id in volume_ids:
    my_function(volume_id, ip=ip)

Is there a way I can do this? It would be trivial if it weren't for the ip parameter, but I'm not sure how to deal with that.

score 92 · Accepted Answer · answered Nov 21 '12 at 18:35

Use functools.partial():

from functools import partial

mapfunc = partial(my_function, ip=ip)
map(mapfunc, volume_ids)

partial() creates a new callable, that'll apply any arguments (including keyword arguments) to the wrapped function in addition to whatever is being passed to that new callable.

score 18 · Answer 2 · answered Nov 21 '12 at 18:44

18

Here is a lambda approach (not better, just different)

volume_ids = [1,2,3,4,5]
ip = '172.12.13.122'
map(lambda ids: my_function(ids, ip), volume_ids);

answered Nov 21 '12 at 18:44

Jason Sperske

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4

partials are faster, lambdas support out of order arguments – Jason Sperske Nov 21 '12 at 18:45

shapr · Answer 3 · 2014-06-03T18:07:01.357

11

This can be done easily with a list comprehension.

volume_ids = [1,2,3,4,5]
ip = '172.12.13.122'
results = [my_function(i,ip=ip) for i in volume_ids]

edited Jun 03 '14 at 18:07

answered Mar 05 '14 at 22:17

shapr

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This has side effects. You are assigning a bunch of memory that may not be desired (and wasn't specified by OP). – aydow Oct 05 '20 at 04:31
@aydow Why didn't you complain about the same thing under the other answers? – Kelly Bundy Aug 18 '22 at 09:47

score 3 · Answer 4 · answered Jun 02 '18 at 01:53

In general, one can use map to pass keywords to a function by wrapping that function in something which unpacks a dictionary, and then passing an iterable of dictionaries to map. Example:

from itertools import product

volume_ids = [1,2,3,4,5]
volume_ids = (("volume_id", volume_id) for volume_id in volume_ids)
ips = [("ip", '172.12.13.122')]
kwargs_iterable = map(dict, product(volume_ids, ips))

result = map(lambda kwargs: my_function(**kwargs), kwargs_iterable)

For your special case, however, a simpler solution would be:

map(my_function, volume_ids, [ip]*len(volume_ids))

This is concise and does not rely on any imports. Another possibility could be to combine product and starmap from itertools:

from itertools import product, starmap

ips = [ip]
starmap(my_function, product(volume_ids, ips))

This generalizes nicely to the setting with more than one ip adress, or more than two variables.

score 0 · Answer 5 · answered Nov 21 '12 at 19:02

0

How about this?

results = []
for volume_id in volume_ids:
    results.append(my_function(volume_id, ip=ip))

This is three lines of code instead of one --- it's three lines of clear and obvious code instead of importing some special-case helper from module such-and-such. This argument is probably a matter of taste, but it has a lot of weight depending on who you talk to.

answered Nov 21 '12 at 19:02

Armin Rigo

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The problem is that we want to avoid the for loop – mleger45 May 09 '16 at 18:47
Why? Because it's clearer? Or because of some misconception like "for loops are slow in Python"? – Armin Rigo May 14 '16 at 12:03
Yes, I would say indeed is clearer, more Pythonish, and map function is meant to perform the laziest iteration posible. Besides, we need a list back, so map would perform it better for us than writing more lines coding a for loop – mleger45 May 27 '16 at 13:28

Using map() function with keyword arguments

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