python: passing a variable to mapped function

Question

I am mapping one array to another using a function called filter_json as such:

filtered_json = map(filter_json, data_json)

def filter_json(data_json):
    """Filter data_json to data that is needed"""
    ...
    # return mapped dictionary

Now I want to the filter_json function to accept another parameter. Something like this (but this obviously doesn't work):

filtered_json = map(filter_json, data_json, var)

def filter_json(data_json, var):
    """Filter data_json to data that is needed"""
    ... # do something with var
    # return mapped dictionary

How do I pass in more variables into filter_json?

Two approaches: (1) look into `functools.partial`; (2) consider using a list comprehension instead of `map` — jez, Apr 20 '17 at 17:31

score 3 · Answer 1 · answered Apr 20 '17 at 17:34

Two potential approaches:

use functools.partial

import functools
f = functools.partial(filter_json, var=var)
filtered_json = map(f, data_json)

... or use a list comprehension instead of map:
```
filtered_json = [ filter_json(x, var) for x in data_json ]
```
If you want an lazily-evaluated iterator rather than a list, use round brackets instead of square brackets—then it's called a generator expression rather than a list comprehension.

I generally prefer option 2, on grounds of readability, and the flexibility to include if conditionals and nested loops in the iteration.

score 2 · Answer 2 · answered Apr 20 '17 at 17:33

You can use a lambda to call the function with the variable you want

filtered_json = map(lambda data: filter_json(data, var), data_json)

def filter_json(data_json, var):
    """Filter data_json to data that is needed"""
    ... # do something with var
    # return mapped dictionary

python: passing a variable to mapped function

2 Answers2