functools in Python

partial, reduce and cached_property: three tools worth reaching for.

from functools import cached_property, partial, reduce


def scale(factor, value):
    return factor * value


double = partial(scale, 2)
print(double(21), [double(n) for n in (1, 2, 3)])
print(reduce(lambda a, b: a * b, [1, 2, 3, 4], 1))


class Corpus:
    def __init__(self, words):
        self.words = words

    @cached_property
    def vocabulary(self):
        print("computing once")
        return sorted(set(self.words))


corpus = Corpus(["a", "b", "a"])
print(corpus.vocabulary, corpus.vocabulary)

How it works

1. partial freezes arguments and returns a callable.
2. cached_property computes once per instance, then stores.
3. reduce folds a sequence when there is no built-in for it.

Keywords and builtins used here

• Corpus
• class
• def
• for
• lambda
• print
• return
• scale
• self
• set
• sorted
• vocabulary

The run, in numbers

Lines

24

Characters to type

470

Tokens

145

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 83 seconds.

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Step 1 of 3 in functools & operator, step 38 of 53 in Pythonic Python.

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