Python: basic commands, functions and data structures

Smoke tower like flowers 2021-01-23 03:58:19
python basic commands functions data



Reading guide : This paper deals with Python Make a brief introduction to the basic use of . Limited to space , It is impossible to explain in detail Python Use , Just for the data mining cases involved in this book to use the code for basic explanation . If the reader is in initial contact Python, And use Python The goal is data mining , So I believe that the introduction of this article is quite sufficient for you .

author : Zhang Liangjun Tan Liyun Liu Mingjun Jiang Jianming

source : Huazhang technology

 Zero Basics Python: Basic commands 、 function 、 data structure

01 Operation mode


The sample code in this article uses Python Version is Python 3.6. function Python There are two ways to code :
  • One way is to start Python, Then directly enter the corresponding command in the command window ;
  • Another way is to write the complete code as .py Script , Such as hello.py, Then go through... In the corresponding path python hello.py perform .

hello.py The code in the script is as follows :
# hello.pyprint('Hello World!')

The execution result of the script is shown in the figure .

 Zero Basics Python: Basic commands 、 function 、 data structure

▲Hello.py Script execution results


When writing scripts , You can add appropriate comments . In every line , You can use the well number “#” To add comments , The way to add a single line comment is as follows :
a = 2 + 3 #  The meaning of this order is to 2+3 The result of is assigned to a

If the comment has more than one line , Can be in two “'''”( Three English status single quotes ) Add comments between , The way to add multiline comments is as follows :
a = 2 + 3''' Here is Python Multiline comments for . Here is Python Multiline comments for .'''

If the script has Chinese ( Chinese comment or Chinese string , Chinese string should be preceded by u), Then you need to indicate the code in the document header , And save the script as utf-8 Coding format , The coding method is as follows :

02 Basic commands


1. Basic operation

A preliminary understanding Python when , Think of it as a handy calculator . Readers can open Python, Try entering the code list 1 The order shown .
  • Code list 1:Python Basic operation
a = 2a * 2a ** 2

Code list 1 The order shown is Python A few basic operations , The first command is assignment , The second command is multiplication , The last command is power operation ( namely a2), These are basically common to all programming languages . however Python Support multiple assignments , The method is as follows :
a, b, c = 2, 3, 4

This multiple assignment command is equivalent to the following command :
a = 2b = 3c = 4

Python Support the flexible operation of string , Such as code list 2 Shown .
  • Code list 2:Python String manipulation
s = 'I like python's + ' very much'  #  take s And ' very much' Splicing , obtain 'I like python very much's.split(' ') #  take s Split by space , Get the list ['I', 'like', 'python']

2. Judgment and circulation

Judgment and loop are the basic commands of all programming languages ,Python The format of the judgment statement is as follows :
if  Conditions 1:     sentence 2elif  Conditions 3:     sentence 4else:     sentence 5

It is important to note that ,Python There is no curly bracket {}, either end sentence , It uses indent alignment as a hierarchical marker for statements . Indents at the same level should correspond one by one , Otherwise, an error will be reported . Here's an example of a bad indentation , Such as code list 3 Shown .
  • Code list 3: Wrong indent
if a==1:    print(a)#  Indent two spaces else:        print('a It's not equal to 1')#  Indent three spaces 

No matter what language , Proper indentation is an elegant programming habit .

Accordingly ,Python The cycle of while Circulation and for loop ,while Loop like code list 4 Shown .
  • Code list 4:while loop
s,k = 0,0while k < 101:#  The cycle is to ask for 1+2+3+...+100    k = k + 1    s = s + kprint(s)

for Loop like code list 5 Shown .
  • Code list 5:for loop
s = 0for k in range(101): #  This cycle is also a process of seeking 1+2+3+...+100    s = s + kprint(s)

Here we see in and range grammar .in It's a very convenient and intuitive grammar , Used to determine whether an element is in the list / Tuple ;range Used to generate a continuous sequence , The general grammar is range(a, b, c), Said to a The first item 、c For tolerances not exceeding b-1 Equal difference sequence of , Such as code list 6 Shown .
  • Code list 6: Use range To generate a sequence of equal differences
s = 0if s in range(4):    print('s stay 0, 1, 2, 3 in ')if s not in range(1, 4, 1):    print('s be not in 1, 2, 3 in ')

3. function

Python use def From defining functions , Such as code list 7 Shown .
  • Code list 7: Custom function
def add2(x):    return x+2print(add2(1)) #  The output is 3

Different from the general programming language is ,Python The function return value of can be in various forms , You can go back to the list , Even return multiple values , Such as code list 8 Shown .
  • Code list 8: Custom functions that return lists and multiple values
def add2(x = 0, y = 0):  #  Defined function , At the same time, define the default value of the parameter     return [x+2, y+2]  #  The return value is a list def add3(x, y):    return x+3, y+3  #  Double return a, b = add3(1,2) #  here a=4,b=5

occasionally , Like definition add2() This kind of simple function , use def To write a formal name 、 It's a little bit cumbersome to calculate and return ,Python Support with lambda For simple function definition “ Inline functions ”, It's a bit like MATLAB Medium “ Anonymous functions ”, Such as code list 9 Shown .
  • Code list 9: Use lambda Defined function
f = lambda x : x + 2  #  Defined function f(x)=x+2g = lambda x, y: x + y #  Defined function g(x,y)=x+y

03 data structure


Python Yes 4 Built in data structures —List( list )、Tuple( Tuples )、Dictionary( Dictionaries ) as well as Set( aggregate ), They can be collectively referred to as containers (Container), Because they are actually some “ thing ” A composite structure , And these “ thing ” It could be a number 、 character 、 A list or some combination of them .

Generally speaking , Anything in the container will do , And the type of elements inside the container is not required to be the same .

1. list / Tuples

Lists and tuples are sequence structures , They are very similar in themselves , But there are some different places .

From the aspect of appearance , There are some differences between a list and a tuple . The list is marked with square brackets , Such as a = [1, 2, 3], Tuples are marked with parentheses , Such as b = (4, 5, 6), The way to access elements in lists and tuples is the same , Such as a[0] be equal to 1,b[2] be equal to 6, wait . I just talked about , Anything in the container will do , therefore , The following definition also holds :
c = [1, 'abc', [1, 2]]'''c It's a list , The first element of the list is an integer 1, The second is the string 'abc', The third is the list [1, 2]'''

functionally , The difference between a list and a tuple is : The list can be modified , And tuples cannot . such as , about a = [1, 2, 3], So the statement a[0] = 0, And I'll put the list a It is amended as follows [0, 2, 3], And for tuples b = (4, 5, 6), sentence b[0] = 1 You're going to report a mistake .

It should be noted that , If you already have a list a, At the same time, I want to copy a, And name it variable b, that b = a It's invalid , Now b just a Another name for ( Or quote ), modify b It will also modify a. The right way to copy should be b = a[:].

The functions associated with lists are list, The functions related to tuples are tuple, Their usage and function are almost the same , It's all about converting an object to a list / Tuples , Such as list('ab') The result is ['a', 'b'],tuple([1, 2]) The result is (1, 2). Some common with list / Tuple related functions are as follows .
  • cmp(a, b): Compare two lists / Elements of tuples
  • len(a): list / Number of tuple elements
  • max(a): Returns a list of / Tuple element maximum
  • min(a): Returns a list of / Tuple element minimum
  • sum(a): Will list / Sum the elements in a tuple
  • sorted(a): Sort the elements of the list in ascending order

Besides , As an object , The list itself comes with many practical methods ( Tuples do not allow modification , So there are few ways ), As shown below .
  • a.append(1): take 1 Add to list a At the end of
  • a.count(1): Statistical list a Medium element 1 Number of occurrences
  • a.extend([1, 2]): Will list [1, 2] The content of is appended to the list a At the end of
  • a.index(1): From the list a Find the first 1 Index position of
  • a.insert(2, 1): take 1 Insert list a The index for 2 The location of
  • a.pop(1): Remove list a The index for 1 The elements of

Last , I can't help but mention “ List of analytical ” This function , It can simplify the code that we operate on the elements in the list one by one . Use append Function to operate on list elements , Such as code list 10 Shown .
  • Code list 10: Use append Function to operate on list elements
a = [1, 2, 3]b = []for i in a:    b.append(i + 2)

Use list parsing for simplicity , Such as code list 11 Shown .
  • Code list 11: Use list parsing for simplicity
a = [1, 2, 3]b = [i+2 for i in a]

Such grammar is not only convenient , And intuitive . This fully embodies Python The humanization of grammar . In this book , We're going to use a lot of this simple code .

2. Dictionaries

Python Introduced “ Make up by yourself ” The concept of convenience . Mathematically speaking , It's actually a mapping . Popular speaking , It's also a list , However, its “ Subscript ” No longer 0 Number at the beginning , It's what you define “ key ”(Key).

The basic way to create a dictionary is as follows :
d = {'today':20, 'tomorrow':30}

there today、tomorrow It's the dictionary “ key ”, It must be the only one in the dictionary , and 20、30 Namely “ key ” Corresponding value . The way to access elements in the dictionary is also intuitive , Such as code list 12 Shown .
  • Code list 12: Visit the elements in the dictionary
d['today']    #  The value is 20d['tomorrow'] #  The value is 30

To create a dictionary , There are other convenient methods , Such as through dict() Function conversion , Or by dict.fromkeys To create , Such as code list 13 Shown .
  • Code list 13: adopt dict perhaps dict.fromkeys Create a dictionary
dict([['today', 20], ['tomorrow', 30]])  #  Is equivalent to {'today':20, 'tomorrow':30}dict.fromkeys(['today', 'tomorrow'], 20) #  amount to {'today':20, 'tomorrow':20}

Many dictionary related functions and methods are the same as the list , I won't repeat it here .

3. aggregate

Python Built in the data structure of collection , This concept is basically consistent with the concept of set in Mathematics , The difference between it and the list is :① Its elements are not repeated , And it's out of order ;② It does not support indexing . Usually we use curly braces {} perhaps set() Function to create a collection , Such as code list 14 Shown .
  • Code list 14: Create set
s = {1, 2, 2, 3}  #  Be careful 2 It will automatically go heavy , obtain {1, 2, 3}s = set([1, 2, 2, 3]) #  similarly , It converts a list to a collection , obtain {1, 2, 3}

Set has certain particularity ( Especially the disorder ), So the set has some special operations , Such as code list 15 Shown .
  • Code list 15: Set operations
a = t | s  # t and s Union b = t & s  # t and s Intersection c = t – s  #  Difference set ( Item in t in , But not here. s in )d = t ^ s  #  Symmetric difference set ( Item in t or s in , But not both )

4. Functional programming

Functional programming (Functional programming) Or functional programming or functional programming , It's a programming paradigm , It treats computer operations as mathematical function calculations , And avoid using program state and mutable objects .

simply , Functional programming is a kind of “ dependent ” Programming , Usually in combination with the above lambda Define functions for scientific calculation , It will be simple and convenient .

stay Python in , Functional programming mainly consists of the use of several functions :lambda、map、reduce、filter, among lambda I've already introduced , Mainly used to customize “ Inline functions ”, So now let's introduce the later one by one 3 individual .

(1)map function

Suppose there is a list a = [1, 2, 3], Add... To each element in the list 2 Get a new list , Use the list parsing I mentioned earlier , We can write this way , Such as code list 16 Shown .
  • Code list 16: Use list parsing to manipulate list elements
b = [i+2 for i in a]

And the use of map We can write functions like this , Such as code list 17 Shown .
  • Code list 17: Use map Function operation list elements
b = map(lambda x: x+2, a)b = list(b)  #  The result is [3, 4, 5]

in other words , Let's first define a function , And then use map Command to apply functions one by one to (map) Every element in the list , Finally, an array is returned .map The command also accepts functions with multiple arguments , Such as map(lambda x,y: x*y, a, b) It means that you will a、b The elements of two lists are multiplied , Return the results to the new list .

Maybe some readers have questions : With list parsing , Why do we have to map Orders ? In fact, the list parsing code is short , But in essence for command , and Python Of for Command efficiency is not high , and map Function to achieve the same function , And more efficient , In principle , Its circular command is C Language speed .

(2)reduce function

reduce It's kind of like map, but map Used to traverse one by one , and reduce For recursive calculation . stay Python 3.x in ,reduce The function has been moved out of the global namespace , Placed in fuctools In the library , Use through from fuctools import reduce introduce reduce. Here's an example , This example can work out n The factorial , Such as code list 18 Shown .
  • Code list 18: Use reduce Calculation n The factorial
from fuctools import reduce#  Import reduce function reduce(lambda x,y: x*y, range(1, n+1))

among range(1, n+1) It's like giving a list , The element is 1~n this n It's an integer .lambda x,y: x*y We construct a binary function , Returns the product of two parameters .

reduce The command first operates on the first two elements of the list as arguments to the function , Then the result of the operation and the third number are used as parameters of the function , And then we take the result and the fourth number as the parameters of the function …… According to this post , Until the end of the list , Return the final result . If you use the loop command , Then write a code list 19 The form shown .
  • Code list 19: Use the loop command to calculate n The factorial
s = 1for i in range(1, n+1):    s = s * i

(3)filter

seeing the name of a thing one thinks of its function , It's a filter , Used to filter eligible elements in the list , Such as code list 20 Shown .
  • Code list 20: Use filter Filter list elements
b = filter(lambda x: x > 5 and x < 8, range(10))b = list(b)  #  The result is [6, 7]

Use filter First you need a return value of bool Function of type , If above “lambda x: x > 5 and x < 8” Defines a function , Judge x Is it greater than 5 And less than 8, And then apply this function to range(10) In every element of , If True, be “ Pick out ” That element , Finally, a list of all the elements that meet the conditions is formed to return .

Of course , Above filter sentence , You can also use list parsing , Such as code list 21 Shown .
  • Code list 21: Use list parsing filter
b = [i for i in range(10) if i > 5 and i < 8]

It's no better than filter Complex sentences . But be careful , We use map、reduce or filter, The ultimate goal is to be concise and efficient , because map、reduce or filter The cycle speed ratio of Python Built in for Cycle or while The cycle is much faster .

04 Import and add of Library


We've already talked about Python Construction and use of basic platform , However, only by default it doesn't load all the features . We need to put more libraries ( Or called a module 、 Bag, etc ) Loading in , You even need to install third-party extension Libraries , To enrich Python The function of , To achieve our purpose .

1. Library import

Python Built in a lot of powerful libraries , Like math math library , It can provide us with more abundant and complex mathematical operations , Such as code list 22 Shown .
  • Code list 22: Use math Libraries do math
import mathmath.sin(1)  #  Calculate the sine math.exp(1)  #  Calculate the index math.pi  #  Built in PI constant 

How to import a library , In addition to direct “import Library name ” outside , You can also create an alias for the library , Such as code list 23 Shown .
  • Code list 23: Use alias to import library
import math as mm.sin(1) #  Calculate the sine 

Besides , If you don't need to import all the functions in the library , You can specifically specify the name of the import function , Such as code list 24 Shown .
  • Code list 24: Import the specified function by name
from math import exp as e #  Import only math In the library exp function , And name it ee(1)  #  Calculate the index sin(1)  #  here sin(1) and math.sin(1) They all make mistakes , Because it wasn't imported 

If all functions in the library are imported directly , Such as code list 25 Shown .
  • Code list 25: Import all functions in the library
#  Direct import math library , That is to get rid of math., But if we introduce a third library like this in large quantities , It is easy to cause naming conflicts from math import *exp(1)sin(1)

We can go through help('modules') Command to get all the module names that have been installed .

2. Import future features (For 2.x)

Python 2.x And Python 3.x The difference is not just in the kernel , It's also part of the implementation of the code . such as , stay Python 2.x in ,print It appears as a statement , Usage for print a; But in Python 3.x in , It appears as a function , Usage for print(a).

To ensure compatibility , The basic code of this article is based on Python 3.x The grammar of , While using Python 2.x Readers of , By introducing future Features in a way compatible with code , Such as code list 26 Shown .
  • Code list 26: Import future features
#  take print In the form of functions , The box print(a) Format output from __future__ import print_function# 3.x Of 3/2=1.5,3//2 Is equal to 1;2.x in 3/2=1from __future__ import division

3. Add third party Library

Python Brought a lot of Libraries , But not necessarily to meet our needs . In terms of data analysis and data mining , We also need to add some third-party libraries to expand its functions . Here is a brief introduction to the third-party library installation .

There are many ways to install third-party libraries , As shown below .
  • Download the source code and install it yourself : Flexible installation , But we need to solve the problem of superior dependence by ourselves
  • use pip Command to install : It's more convenient , Automatically solve the problem of superior dependence
  • use easy_install Command to install : It's more convenient , Automatically solve the problem of superior dependence , Than pip Slightly weak
  • Download the compiled package : It's usually Windows The system provides out of the box executable packages
  • Installation mode of the system :Linux System or Mac The software manager of the system comes with the installation method of some libraries

These installation methods have been used in 《 Dry goods collection ! Read a text 8 Common use Python Library from installation to application 》 In detail .

About author : Zhang Liangjun , Senior big data mining and Analysis Expert 、 Pattern recognition experts 、AI technician . Yes 10 Years of big data mining and analysis experience , Good at Python、R、Hadoop、Matlab And so on , Machine learning, etc AI Technology driven data analysis also has in-depth research .

This article is excerpted from 《Python Data analysis and mining practice 》( The first 2 edition ), Issued under the authority of the publisher .

 Zero Basics Python: Basic commands 、 function 、 data structure

Extended reading 《Python Data analysis and mining practice 》


Recommended language : The best seller is upgraded , The first 1 The sales of the edition exceed 10 Ten thousand volumes , By domestic 100 Other institutions of higher learning use it as teaching materials , At the same time, it is regarded as a classic by the data scientists , Is the accepted standard of fact in this field . The author is in the field of big data mining and analysis 10 Years of engineering practice 、 Experience in teaching and starting a business , Not only master the latest technology and practice methods of the industry , And understand the needs and pain points of students and teachers .
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