How to pass a function to another function in python

Question

Ok first bit of a background.

So.. right now, I have a code say( loop.py) which is nothing but a big for loop... which takes in input for stdin and do some manipulation to that string and then i write the output.

so something like

#loop.py
from clean import *
for line in sys.stdin:
     clean_line = clean(line)
     print clean_line

And I run this as

cat input.txt | python loop.py

So, clean.py is certain cleaning logic which user writes.

Now, here we have a "clean" function.. for some it is certain extraction logic..

so you may have

#loop.py
from clean import *
for line in sys.stdin:
     extract_line = extract(line)
     print extract_line

Now, this loop.py hasnt changed.. and it will not change..

So.. maybe i can spell out the experience and someone can help me figure out how to implement that..

What I want is user to write those custom functions..So user writes...

my_custom_func.py

import run_loop
def my_own_logic(string):
    # my logic goes in here

run_loop.run(my_own_logic)

and what this does is automatically execute the loop.py and this my_own_logic function is pushed in that loop?

I hope I am making any sense... Am i?

Hari Menon · Accepted Answer · 2013-10-01 08:50:07Z

2

This is fairly straightforward:

def run(func):
    for line in sys.stdin:
        print func(line)

answered Oct 1, 2013 at 8:50

Hari Menon

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Some programmer dude · Accepted Answer · 2013-10-01 08:50:19Z

1

Python have first-class functions, meaning they can be passed around to other functions without problems.

Example:

def foo(s):
    print(s)

def bar(f, s):
    f(s)

bar(foo, 'Hello world')

answered Oct 1, 2013 at 8:50

Some programmer dude

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falsetru · Accepted Answer · 2013-10-01 08:50:21Z

1

run_loop.py

import sys

def run(func):
    for line in sys.stdin:
        processed = func(line)
        print processed

Treat functions as any other objects (int, str, list, ...).

answered Oct 1, 2013 at 8:50

falsetru

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theodojo · Accepted Answer · 2013-10-01 09:23:01Z

1

Rule of thumb: say you have a function like

def f(x): return x+1

Then f refers to the function itself while f() (followed by parenthesis) to the evaluation of the function.

>>> print f
<function f at 0x0000000001FD9DD8>
>>> print f(1)
2

So, if you want to process a list with an arbitrary function, you have a few ways to do it:

def f(x): return x+1
def g(x): return x*2

>>> def procList(fun,l):
...     for x in l:
...         print fun(x)
...
>>> l=[1,2,3]
>>> procList(f,l)
2
3
4
>>> procList(g,l)
2
4
6
>>>

answered Oct 1, 2013 at 9:23

theodojo

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