选题: Getting started with functional programming in Python using the toolz library

sources/tech/20181023 Getting started with functional programming in Python using the toolz library.md

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+Getting started with functional programming in Python using the toolz library
+======
+
+The toolz library allows you to manipulate functions, making it easier to understand and test code.
+
+![](https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/programming-code-keyboard-laptop-music-headphones.png?itok=EQZ2WKzy)
+
+In the second of a two-part series, we continue to explore how we can import ideas from functional programming methodology into Python to have the best of both worlds.
+
+In the previous post, we covered [immutable data structures][1]. Those allow us to write "pure" functions, or functions that have no side effects, merely accepting some arguments and returning a result while maintaining decent performance.
+
+In this post, we build on that using the `toolz` library. This library has functions that manipulate such functions, and they work especially well with pure functions. In the functional programming world, these are often referred to as "higher-order functions" since they take functions as arguments and return functions as results.
+
+Let's start with this:
+
+```
+def add_one_word(words, word):
+    return words.set(words.get(word, 0) + 1)
+```
+
+This function assumes that its first argument is an immutable dict-like object, and it returns a new dict-like object with the relevant place incremented: It's a simple frequency counter.
+
+However, it is useful only if we apply it to a stream of words and reduce. We have access to a reducer in the built-in module `functools`. `functools.reduce(function, stream, initializer)`.
+
+We want a function that, applied to a stream, will return a frequency count.
+
+We start by using `toolz.curry`:
+
+```
+add_all_words = curry(functools.reduce, add_one_word)
+```
+
+With this version, we will need to supply the initializer. However, we can't just add `pyrsistent.m` to the `curry`; it is in the wrong order.
+
+```
+add_all_words_flipped = flip(add_all_words)
+```
+
+The `flip` higher-level function returns a function that calls the original, with arguments flipped.
+
+```
+get_all_words = add_all_words_flipped(pyrsistent.m())
+```
+
+We take advantage of the fact that `flip` auto-curries its argument to give it a starting value: an empty dictionary.
+
+Now we can do `get_all_words(word_stream)` and get a frequency dictionary. However, how do we get a word stream? Python files are by line streams.
+
+```
+def to_words(lines):
+    for line in lines:
+        yield from line.split()
+```
+
+After testing each function by itself, we can combine them:
+
+```
+words_from_file = toolz.compose(get_all_words, to_words)
+```
+
+In this case, the composition being of just being two functions was straightforward to read: Apply `to_words` first, then apply `get_all_words` to the result. The prose, it seems, is in the inverse of the code.
+
+This matters when we start taking composability seriously. It is sometimes possible to write the code as a sequence of units, test each individually, and finally, compose them all. If there are several elements, the ordering of compose can get tricky to understand.
+
+The `toolz` library borrows from the Unix command line and uses `pipe` as a function that does the same, but in the reverse order.
+
+```
+words_from_file = toolz.pipe(to_words, get_all_words)
+```
+
+Now it reads more intuitively: Pipe the input into `to_words`, and pipe the results into `get_all_words`. On a command line, the equivalent would look like this:
+
+```
+$ cat files | to_words | get_all_words
+```
+
+The `toolz` library allows us to manipulate functions, slicing, dicing, and composing them to make our code easier to understand and to test.
+
+--------------------------------------------------------------------------------
+
+via: https://opensource.com/article/18/10/functional-programming-python-toolz
+
+作者：[Moshe Zadka][a]
+选题：[lujun9972][b]
+译者：[译者ID](https://github.com/译者ID)
+校对：[校对者ID](https://github.com/校对者ID)
+
+本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
+
+[a]: https://opensource.com/users/moshez
+[b]: https://github.com/lujun9972
+[1]: https://opensource.com/article/18/10/functional-programming-python-immutable-data-structures