Traditionally partial evaluation of functions is handled with the partial higher order function from functools. Currying provides syntactic sugar.

>>> double = partial(mul, 2)    # Partial evaluation
>>> doubled = double(2)         # Currying

This syntactic sugar is valuable when developers chain several higher order functions together.

Partial Evaluation

Often when composing smaller functions to form big ones we need partial evaluation. We do this in the word counting example:

>>> def stem(word):
...     """ Stem word to primitive form """
...     return word.lower().rstrip(",.!:;'-\"").lstrip("'\"")

>>> wordcount = compose(frequencies, partial(map, stem), str.split)

Here we want to map the stem function onto each of the words produced by str.split. We want a stem_many function that takes a list of words, stems them, and returns a list back. In full form this would look like the following:

>>> def stem_many(words):
...     return map(stem, words)

The partial function lets us create this function more naturally.

>>> stem_many = partial(map, stem)

In general

>>> def f(x, y, z):
...     # Do stuff with x, y, and z

>>> # partially evaluate f with known values a and b
>>> def g(z):
...     return f(a, b, z)

>>> # partially evaluate f with known values a and b
>>> g = partial(f, a, b)


In this context currying is just syntactic sugar for partial evaluation. A curried function partially evaluates if it does not receive enough arguments to compute a result.

>>> from toolz import curry

>>> @curry              # We can use curry as a decorator
... def mul(x, y):
...     return x * y

>>> double = mul(2)     # mul didn't receive enough arguments to evaluate
...                     # so it holds onto the 2 and waits, returning a
...                     # partially evaluated function, double

>>> double(5)

So if map was curried…

>>> map = curry(map)

Then we could replace the partial with a function evaluation

>>> # wordcount = compose(frequencies, partial(map, stem), str.split)
>>> wordcount = compose(frequencies, map(stem), str.split)

In this particular example it’s probably simpler to stick with partial. Once partial starts occurring several times in your code it may be time to switch to the curried namespace.

The Curried Namespace

All functions present in the toolz namespace are curried in the toolz.curried namespace.

So you can exchange an import line like the following

>>> from toolz import *

For the following

>>> from toolz.curried import *

And all of your favorite toolz functions will curry automatically. We’ve also included curried versions of the standard Python higher order functions like map, filter, reduce so you’ll get them too (whether you like it or not.)