Python high order function use summary!

Datawhale 2020-11-16 16:19:46
python high order function use

author : Super money , Xiamen University ,Datawhale Original author

this paper about 2000 word , Suggest read 6 minute

reviewers : Geng Yuanhao ,Datawhale member , East China Normal University , Open source tutorial 《Joyful-Pandas》 Core contributors .

This article explains in detail with various practical examples Python5 The use of built-in high-order functions , Can help understand Python And improve the efficiency of data processing , this 5 The functions are :

  • map
  • reduce
  • filter
  • sorted/sort
  • zip

One 、map

1.1 grammar

map The basic syntax of a function is map(func, seq), What it means is : Perform the previous function for each element in the following iteratable sequence func The function of , Finally, we get a new sequence . Be careful :

  • Python2 A list is returned directly from
  • Python3 Is an iterator returned in , If you want to go back to the list , have access to list() To deal with
help(map) # View help information

1.2 demo

By way of example map How to use the function

  1. Use Python Built in functions
  1. Use custom functions
  1. Using anonymous functions lambda

When using anonymous functions, you can have multiple parameters

Two 、reduce

2.1 grammar

reduce Definition of function :

reduce(function, sequence [, initial] ) -> value

reduce Successively sequence One of the elements , And the last call function The result of the parameter , Call again function.

“ First call function when , Provided initial Parameters , Will sequence The first element in and initial Call... As a parameter function, Otherwise, it will be in sequence sequence The first number of ”

2.2 Use

Python3 Has been to reduce Function moves to functools Module , You need to import :

from functools import reduce # Import
help(reduce) # View help documents

The above example is illustrated by a graph :

2.3 demo

  1. Use custom functions
  1. Using anonymous functions lambda


  1. A complex example

The specific process is :

1. 1*2+1=3
2. 3*3+1=10 # first 3 For the above results 3, The first 2 Of the raw data 3
3. 10*4+1=41
  1. Examples with initial values

The initialization value and the first value in the sequence are executed func function , Take the result as the next starting value

# The specific process is interpreted as
1. 6+1=7
2. 7+2=9
3. 9+3=12
4. 12+4=16
5. 16+5=21

3、 ... and 、filter

3.1 grammar

filter() Function to filter the sequence , Filter out the elements that don't meet the criteria , Returns the eligible elements to form a new list .

Each element in the sequence is passed as an argument to the function for judgment , return True perhaps False, And then it will return True Is placed in the new list .

filter() The grammar is as follows :

filter(function, iterable) # The former is a function , The latter is the sequence to be executed

3.2 demo

help(filter) # Help document
  1. Use custom functions

return 10 An even number within

  1. Using anonymous functions lambda
  1. Filtering of strings

Select a string that meets the specified requirements

Four 、sorted

4.1 grammar

sorted(iterable, key=None, reverse=False)

4.2 3 Parameters

sorted() Accept 3 Parameters , It returns a sorted list

  • Iteratable object iterable
  • reverse=False, Accept a Boolean value , Choose whether to reverse the sort results , The default is False
  • Accept a callback function key=None, The callback function can only have one parameter , Sort according to the return value of the function

4.3 demo

help(sorted) # Help document
  1. The default is not reversed
  1. Pair tuple 、range object 、 The order of dictionaries

4.4 Result reversal

The meaning of the result reversal is Arrange the results in descending order , Because the original default is ascending , It uses reverse=True

4.5 understand key

key The function of the parameter is to define a function , Then, by applying the elements in the sequence to the function, we sort it

Here we use Absolute value function

4.6 contrast sort()

sort() The method can only be To the original list list Sort , Parameters and sorted It's the same

The result is to modify the original list directly in place , and sorted It's generating a new list , The two are different

5、 ... and 、zip

zip() yes Python One of the most important methods in , It can realize many functions quickly .

5.1 grammar

zip([iterable,...]) # iterable It's one or more iterators
  • The result of function execution is in Python3 It's a zip object , If you need to show it in the form of a list , Use it directly list Method expansion ; The result of the expansion is in the form of nested tuples in the list
  • stay Python2 What is directly returned in is Tuple list form
help(zip) # To view the document

5.2 zip Take a sequence

zip It is acceptable that list 、 Tuples 、 character string The form such as

zip Accept the form of an empty list , The returned list is still empty

5.3 zip Accept multiple sequences

At the same time Combine different types of sequences

5.4 The processing length is different

When multiple sequences exist at the same time , Take the length of the sequence with the smallest length

5.5 zip(*iterables)

We generally think that this method is zip The reverse process of , It's a unzip The process of , Give an example of its use :

5.6 Complex examples

Let's take a more complex example

The explanation for this example is :

  1. [x] It's a list with a list ,x It's a list in itself
  2. [x]*3 The result is [x,x,x], In fact [[4,5,6],[4,5,6],[4,5,6]]
  3. [*[x]*3] The result is [(4,4,4),(5,5,5),(6,6,6)]

5.7 zip Application

Pass below zip To illustrate its application :

  1. Sum list
  1. Data merging
  1. Dictionary key-value transformation

for Cycle to achieve :

Use zip Realization :

This article is from WeChat official account. - Datawhale(Datawhale) , author : Super money

The source and reprint of the original text are detailed in the text , If there is any infringement , Please contact the [email protected] Delete .

Original publication time : 2020-11-07

Participation of this paper Tencent cloud media sharing plan , You are welcome to join us , share .


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