August 22, 2017 at 15:45

Some functional programming tricks in Python

    Hello, Habr!
    In this article, I would like to talk about what came to Python from functional programming languages. Interested, I ask for cat.


    List generators


    It is easy and simple: instead


    l = []
for x in range(10):
    if x % 2 == 0:
        l.append(x)

    we're writing


    l = [x for x in range(10) if x % 2 == 0]

    Short and clear.


    How it works in functional languages

    In Haskell, the same thing will look like this:


    let l = [x | x <- [0..10], x `mod` 2 == 0]

    Lambdas


    Suppose we are writing a graphical interface and we have a button (** kwargs) function, where valid named arguments are: text for text, width for width, height for height and command for a callback function:


    def callback(event):
    print("Button pressed")
button(text="Press me", width=32, height=16, command=callback)

    Notice how small our callback is, is it really easy to push it with an argument? Can! Lambdas will help us:


    button(text="Press me", width=32, height=16, command=lambda x: print("Button pressed"))

    Clean and easy!


    How it works in functional languages

    In Haskell, passing a function as an argument occurs at every step, for example, the map function takes a function and a list and returns a list for each element of which this function was applied:


    map (\x -> x+1) [1..10]

    In Python, this is:


    map(lambda x: x+1, [x for x in range(1, 10)])

    True, there is no map in Python.
    upd: map is!


    Carring


    Currying is when we pass one or more arguments to an old function to get a new one that takes the rest (thanks AnutaU for a more precise definition). For example: print is a function (I use Python 3), it has a named argument end - the end of the line, by default it is "\ n". I want not to jump to a new line, then I write


    print(str, end="")

    Let's make a printf function that will not jump to a new line


    def printf(*args, **kwargs):
    kwargs["end"] = ""
    print(*args, **kwargs)

    Sloppy, it is possible and easier:


    from functools import partial
printf = partial(print, end = "")

    Here it is, currying - we say what we want exactly the same, but with mother-of-pearl buttonsprint function, but so that end is equal to "". Everything is simple.


    How it works in functional languages

    And again Haskell: we have a + function that takes two arguments, we say:


    let plusTwo = (+2)

    I now have a function that adds 2 to a single argument.


    I have everything, if you know what else is in Python from functionalism - I ask in the comment.
    Questions and feedback there too.

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