TIL: Partial Functions

Today I discovered partial functions in Python. Partial functions are functions which are derived from general functions. The derived function has default parameters of the general function, unless it’s overwritten.

Very simple example:

>>> from functools import partial
>>> def foo(a, b):
...     return a + b
... 
>>> bar = partial(foo, a=5) 
>>> bar(b=5)
10
>>> bar(a=10, b=10)
20
>>> 

Why do they exist and when to use them? The main reason to use them is to improve reusability. I found a real-world example on Stack Overflow when to use them - if we want to use re.search it’s method signature is as follows:

search(pattern, string, flags=0) 

If we use partials, we can have multiple versions of the search function, for example:

is_spaced_apart = partial(re.search, '[a-zA-Z]\s\=') # pattern '[a-zA-Z]\s\=' applied
is_grouped_together = partial(re.search, '[a-zA-Z]\=') # pattern '[a-zA-Z]\=' applied

Later in the code we could use it as:

for text in lines:
    if is_grouped_together(text):
        some_action(text)
    elif is_spaced_apart(text):
        some_other_action(text)
    else:
        some_default_action()

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