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Graphical Python | basic grammar

編輯：Python

author ： Han Xinzi @ShowMeAI
Tutorial address ：http://www.showmeai.tech/tutorials/56
This paper addresses ：http://www.showmeai.tech/article-detail/66
Statement ： copyright , For reprint, please contact the platform and the author and indicate the source

introduction

This series of tutorials will explain Python programing language , Before you learn specific grammar knowledge , Let's learn something first Python As a foundation for .

1. Content abstract

This article will Python Give a brief introduction , By reading this article, you will learn ：

Python Programming model
Python Identifiers and keywords
Python Indents and code blocks
Python Simple input and output
Python Basic code structure

2. know Python Program

（1） Interactive programming

Interactive programming does not need to create script files , It's through Python The interpreter's interaction mode comes in to write code .
You just need to type... On the command line Python Command to start interactive programming , The prompt window is as follows ：

$ python
Python 3.9.5 (default, May 4 2021, 03:33:11)
[Clang 12.0.0 (clang-1200.0.32.29)] on darwin
Type "help", "copyright", "credits" **or** "license" **for** more information.
>>>

stay python Enter the following text message at the prompt , Then press Enter Key to see the running effect ：

>>> print("Hello, ShowMeAI, this is Python!")

In my current Python3.9.5 In the version , The output of the above example is as follows ：

Hello, ShowMeAI, this is Python!

You can also use Previous section Mentioned Anaconda In the environment Jupyter Notebook Interactive Python Programming , start-up Jupyter Notebook And new Notebook as follows , You can go to cell Code writing and interaction in .

（2） Scripting

If the task we need to complete is more complex , We can organize the intermediate processing process into python Script , Then call the interpreter through the script parameters to start executing the script , Until the script is finished . When the script is finished , The interpreter is no longer valid .

Let's write a simple Python Script program . all Python The file will be .py Extension name . Copy the following source code to test.py In file .

print("Hello, ShowMeAI, this is Python!")

Use the following command to run the program ：

$ python test.py

Output results ：

Hello, ShowMeAI, this is Python!

3.Python identifier

Identifiers are allowed as variables （ function 、 Class etc. ） A valid string for the name . among , Part of it is keywords （ The identifier reserved by the language itself ）, It cannot be used as an identifier for other purposes , Otherwise it will cause syntax errors （SyntaxError abnormal ）.Python There is also called built-in Identifier set , Although they are not reserved words , But these special names are not recommended .

Python Is a dynamically typed language , That is, you don't need to declare the type of the variable in advance . The type and value of the variable are initialized at the moment of assignment . Variable assignment is performed by an equal sign .

Python A valid identifier for consists of upper and lower case letters 、 Underline and numbers make up . The number cannot be the first character , The length of the identifier is unlimited ,Python Identifiers are case sensitive .

In programming languages , There are two common ways to name variables ：

Hump body ：
- DateOfBirth
- AgeOfBoy
- ShowMeAI
Underline ：
- date_of_birth
- age_of_boy
- show_me_ai

4.Python Reserved characters

The following list shows in Python Reserved word in . These reserved words cannot be used as constants or variables , Or any other identifier name .

all Python The keyword of contains only lowercase letters .

andexecnotassertfinallyorbreakforpassclassfromprintcontinueglobalraisedefifreturndelimporttryelifinwhileelseiswithexceptlambdayield

5. Lines and indents

Study Python The biggest difference from other languages is ,Python The code block of does not use braces {} To control the class , Functions and other logical judgments .python The most characteristic is to use indentation to write modules .

Indent can use tab Or spaces, etc , The number of blanks is variable , But all code block statements must contain the same amount of indented white space .

The following example is indented with four spaces :

if True:
print("True")
else:
print("False")

The following code will execute in error ：

#!/usr/bin/python
# -*- coding: UTF-8 -*-
# file name ：test.py
if True:
print("ShowMeAI")
print("Awesome")
else:
print("Test")
# There is no strict indentation , Error will be reported during execution
print("False")

Execute the above code , The following error alerts will appear ：

 File "<stdin>", line 11
print("False")
^
IndentationError: unindent does not match any outer indentation level

Common alignment errors are 2 Kind of ：

IndentationError: unindent does not match any outer indentation level
- Error indication , You use different indents , There are plenty of them tab Key indent , There's space indentation , Change to consistent
IndentationError: unexpected indent
- The format in the file is wrong , May be tab It's not aligned with the space

therefore , stay Python You must use the same number of indented spaces at the beginning of lines in the code block of .

I suggest you in actual programming , Use... For each indentation level Single tab or Two spaces or Four spaces , Remember not to mix

6. Multi line statement

Python In a statement, a new line is usually used as the end of the statement .

But we can use slashes （ \） Divide a line of statements into multiple lines to display , As shown below ：

total = item_one + \
item_two + \
item_three

The statement contains [], {} or () Brackets do not need to use multiline connectors . The following example ：

days = ['Monday', 'Tuesday', 'Wednesday',
'Thursday', 'Friday']

7.Python Strings and Quotes

Python You can use quotation marks ( ' )、 Double quotes ( " )、 Three quotes ( ''' or """ ) To represent a string , The beginning and end of a quotation mark must be of the same type .（ More detailed python For string knowledge, see python String and operation ）

Three quotation marks can be made up of multiple lines , Write fast syntax for multiline text , Commonly used for document strings , At the specific location of the document , As a comment .

word = 'word'
sentence = " This is a ShowMeAI A tutorial for ."
paragraph = """ This is a multi line statement .
One line contains ShowMeAI"""

8.Python notes

python Middle and single line notes use # start .

#!/usr/bin/python
# -*- coding: UTF-8 -*-
# file name ：test.py
# The first note
print("Hello, ShowMeAI, this is Python!") # The second note

Output results ：

Hello, ShowMeAI, this is Python!

Comments can be at the end of a statement or expression line ：

name = "ShowMeAI" # This is a comment

python Use more than three quotes in a single line (''') Or three double quotes (""").

#!/usr/bin/python
# -*- coding: UTF-8 -*-
# file name ：test.py
'''
This is a multiline comment , Use single quotes .
This is a multiline comment , Use single quotes .
This is a multiline comment , Use single quotes .
'''
"""
This is a multiline comment , Use double quotes .
This is a multiline comment , Use double quotes .
This is a multiline comment , Use double quotes .
"""

9.Python Blank line

Functions or methods of classes are separated by empty lines , Represents the beginning of a new piece of code . Classes and function entries are also separated by a blank line , To highlight the beginning of the function entry .

Blank lines are not the same as code indentation , Air travel is not Python Part of grammar . Don't insert blank lines when writing ,Python The interpreter will not run wrong . But blank lines are used to separate two pieces of code with different functions or meanings , Easy to maintain or refactor the code in the future .

10. User input

The following program will wait for user input after execution , Press enter and exit ：

#!/usr/bin/python
# -*- coding: UTF-8 -*-
input(" Press down enter Key to exit , Any other key displays ...\n")

In the above code ,\n Implement line break . Once the user presses enter( enter ) Key to exit , Other key display .

11. Show multiple statements on the same line

Python You can use multiple statements on the same line , Use semicolons... Between statements (;) Division , Here is a simple example ：

#!/usr/bin/python
import sys; x = 'ShowMeAI'; sys.stdout.write(x + '\n')

Execute the above code , The input result is ：

$ python test.py
ShowMeAI

12.print Output

python3 in print The default output is line feed , If you want to implement no line wrapping, you need to add... At the end of the variable 「, end=''」.

#!/usr/bin/python
# -*- coding: UTF-8 -*-
x="a"
y="b"
# Line feed output
print(x)
print(y)
print('---------')
# Don't wrap output
print(x, end='')
print(y, end='')
# Don't wrap output
print(x, y, end='')

The execution result of the above example is ：

a
b
---------
a b a b

13. Code block / Code group

Indent the same set of statements into a block of code , We call it code group .

image if、while、def and class Such compound statements , The first line starts with keywords , With a colon ( : ) end , One or more lines of code following this line form a code group .

We call the first and subsequent code groups a clause (clause).

The following example ：

if expression :
suite
elif expression :
suite
else :
suite

Data and code download

The code for this tutorial series can be found in ShowMeAI Corresponding github Download , Can be local python Environment is running , Babies who can surf the Internet scientifically can also use google colab One click operation and interactive operation learning Oh ！

This tutorial series covers Python The quick look-up table can be downloaded and obtained at the following address ：