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[Python] basic Python syntax

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【Python】python Basic grammar .md

“
C Too many seats , You will know ~ It's strange ~--- Kaikai doesn't cry ~ Kaikai is strong
”

I just finished writing the grammar set ,

Then make up some python Grammar point ~ Just open a third-party library ~~ This is more interesting

Even review the spelling of academic terms , It also makes sense .

Python Some notes

About whether to add the semicolon at the end of the statement

I saw that the semicolon can be added or not , Add no error , Although most of the time play python Not at all Get used to a language and just play it python There is no need to change your habits , Looking for a Formatting tool that will do ~
Python It's indented language ~ Be careful not to leave blank spaces tab
- Report errors unexpected indent
Use # Notation ~ commonly # Followed by a space ~ ( Standard )

Literal literal Variable variable

In fact, it is difficult to understand the literal quantity directly ~ I understand it the other way around

A literal is a value that can be assigned to a variable ~
- The number 1 , 2,3,4
- character string "1" "2"
- ...

python Variables do not need to be declared

It can be used directly
But you can't use variables without assignment , Will you report an error defined
python and js The same is dynamic language , You can assign any type of value ~, It can also be modified at will !

identifier identifier

All can be named by themselves --- Function name , Class name , Variable names belong to identifiers identifier

Identifier naming specification
- Can contain alphanumeric underscores
  - No dollar sign ,js Yes
- The number can't start
- Keyword reserved words cannot be used ~
Naming method
- Underline naming underscore case max_number
- The short bar The actual translation should be " Mutton kebab nomenclature " kabob case max-number
- Little hump Lower Camel Case maxNumber
- Hump Upper Camel Case Also called Pascal Case MaxNumber

data type

The type of variable value ~ That is, the type of literal quantity ~

The number : Integers , Floating point numbers ( decimal ), The plural

stay Python All integers in are int type

python There is no limit on the size of integers in , It can be an infinite integer
- If the number is too large, you can use the underscore as the separator

c = 123_456_789 #  Underline at random

If the input content is not recognized, it will be reported invalid token
Other numbers in base ( I don't know how to use it )
- 0b It starts in decimal
  - 0b10
- 0o It starts with octal
  - 0o10
- 0x It starts with hexadecimal
  - 0x10
Statements and expressions
- Expressions do not produce substantial results , It's like half a sentence
- A sentence is a complete sentence ~
  - c+3 Output c There is no change
  - c = c+3 That's the statement.
  - that c++ Is it a statement or an expression ~ Is the statement
Floating point numbers ( decimal ), stay Python All the decimals in are float type
When calculating floating-point numbers , You may get an imprecise result ( It's interesting )

a1 = 0.1
a2 = 0.2
print(a1+a2)  # 0.30000000000000004

####  character string 

 Text message ~ String is used to represent ' Text information ' Of 

 Wrap in quotation marks 

-    You also need to use quotation marks inside double quotation marks. You need to use single quotation marks ---`  The same quotation marks cannot be nested next to each other `

```python
a1 = ' Hoe standing grain gradually pawning a midday ,\
 Sweat dripping under the grass ,\
 Who knows what's on the plate ,\
 Every grain is hard ,\
'
print(a1)  #  The result line 
a2 = ''' Hoe standing grain gradually pawning a midday ,\
 Sweat dripping under the grass ,\
 Who knows what's on the plate ,\
 Every grain is hard ,\
'''
print(a2)  #  No branches 
a3 = ''' Hoe standing grain gradually pawning a midday ,
 Sweat dripping under the grass ,
 Who knows what's on the plate ,
 Every grain is hard ,
'''
print(a3)  #  branch

From the above, we can find , This backslash acts like a " Soft wrap "

Just tell the interpreter , The following line breaks should not be regarded as line breaks / Connect content

“
It's actually called Line continuation operator ~ It is also a kind of Escape character
”

Escape character

Escape character describe \（ At the end of the line ） Line continuation operator \ The backslash ’ Single quotation marks " Double quotes \b Backspace \n Line break \v Vertical tabs \t Horizontal tabs \r enter \f Change the page

“
There are some inconvenient web pages ~ Because it is not necessarily a single web page ~ For example, format retention . Need to keep writing escape characters
C In this respect, it is even more outrageous
”

Write an interesting \u0040 You can display a @ Symbol

\u After that unicode code 4 position

“
No dice ... The editor doesn't produce results You dare to use it directly in the browser ?
”

String operations

intro = ' Hello '
name = 'Shinkai'
#  The same string type can be added 
print(intro + name)  #  Hello Shinkai
#  The following expression is used in python Not often 
print(intro + name + "!!!!!")  #  Hello Shinkai!!!!!
#  It is usually written like this 
print(intro+name, "!!!!!")  #  Hello Shinkai !!!!!

Place holder

str = 'hello %s' % ' The Monkey King ' # %s  yes  string Abbreviation 
print(str)  # hello  The Monkey King 
str1 = 'hello %s  Hello  %s' % ('monkey', ' The Monkey King ')
print(str1)  # hello monkey  Hello   The Monkey King 
str2 = 'hello %3s' % 'ab'  #  Minimum string length 3
print(str2)  # hello  ab
str3 = 'hello %3.5s' % 'abcdef'  #  The string length is 3-5
print(str3)  # hello abcde

%f It's a floating-point placeholder

%d Is an integer placeholder , Just round off the decimal part

“
Just don't use so fine ~ I just know when I study ~
”

General writing method

res = 'res'
print('res = %s' % res)  # res = res

Formatted string ( Better written in a similar way js)

intro = 'hello'
name = 'Shinkai'
res = f'{intro} {name}'
print(res)  # hello Shinkai

“
I want to play with the flowers. I read the newspaper wrong. OK .. Don't simulate everything
”

To summarize the string splicing methods 4 Kind of ( There are actually two kinds )

String operations
String parameters
String placeholder
String formatting

String copy

“
python Unique , character string *n Will repeat the string n Return times
”

name = 'Shinkai' * 2
print('hello ' + name)  # hello ShinkaiShinkai
print('hello', name)
print('hello %s ' % name)
print(f'hello {name} ')

Boolean value bool

logic

Boolean values are actually integers , True yes 1, False yes 0

print(1 + True)  # 2
print(1 + False)  # 1
print(1 * False)  # 0

None( It doesn't work very well )

None Does not exist Don't follow null It doesn't exist null Type but with None type

Type checking

The contents mentioned so far above, There are strings , The number , Null value

The value contains : integer ( Contains Boolean ), floating-point , The plural

Why type checking

str = '123'
num = 123
print(str)  # 123
print(num)  # 123

You can see Different types of content It's all printed out 123 I don't know the type ~

“
actually pycharm When the pointer is placed on the variable, it shows ~
”

print(type(num))  # <class 'str'>
print(type(str))  # <class 'int'>

All types of checks

#  Type checking 
print(type('123'))  # <class 'str'>
print(type(123))  # <class 'int'>
print(type(123.123))  # <class 'float'>
print(type(True))  # <class 'bool'>
print(type(None))  # <class 'NoneType'>

python It's an object-oriented language ~

“
To put it bluntly, today's are all object-oriented , Object - oriented is called object - oriented as long as its syntax contains declarative expressions .....
”

Python It's an object-oriented language

Everything is the object

“
There are many things to learn again ~ This is the content of the operating system
Scheduling algorithm ~ Although we don't have to ~ But having one doesn't make much difference
This difference has a static data area Static variables are placed here .
”

What is an object

Don't write here ~ There's too much content ....

Structure of objects

Each object will have one id, type, value

“
We downloaded the official version of python, That is, there is no prefix python Interpreter ~ So is Cpython, Cpython Of id Is the memory address of variable storage
”

id Is a unique identifier
type It's the type
- Determines what functions a variable has ( Method )
value It's worth it
- Objects are divided into variable objects and immutable objects
  - The value of a mutable object can be changed ,

#  Object structure 
name = 'Shinkai'
print(id(name))  # 4313468016, 4443786352
print(type(name))  # <class 'str'>
print(name)  # Shinkai

python It's a strong type of , Once the type is created , Do not modify

Objects and variables

Objects don't actually exist directly in variables , What is actually stored is the address of the object in memory

test = 123
test2 = test
test3 = 123
print(test == test2)  # True
print(test == test3)  # True

#  It's the same way to write 

test = {"key": "value"}
test2 = test
test3 = {"key": "value"}
print(test == test2)  # True
print(test == test3)  # True

Type conversion int(),float(),bool(),str()

Python It's a strong type , It cannot affect the declared variables , And can not be implicitly converted ~ But you can assign the content to the original variable after forced conversion ~

Variables can be assigned infinitely ~

“
To explain, for example int(a), Actually a Change the type of int Then copy the rest , Then give this address to a.
”

Integer conversion ,int()

Boolean value
- Ture -> 1 Flase ->0
character string
- A valid numeric string can be converted to an integer
- Illegal ones will report errors
Floating point numbers
- Round directly
None Report errors

Floating point conversion , float()

similar int(), But it's in decimal form

String conversion , str()

Ture->'True'

False->'False'

123->'123'

...

a=True

Turn bull ,bool()

0,None,'' Empty strings are False

Operator

classification

Arithmetic operator Arithmetic operator
Assignment operator assignment operator
Comparison operator ( Relational operator )relational operator
Logical operators logical operator
Conditional operator ( Ternary operator )ternary operator

Arithmetic operator Arithmetic operator

# + The addition operator （ If it's an addition between two strings , String splicing will be performed ）
# - Subtraction operators 
# * Multiplication operators （ If you multiply a string by a number , Then the string will be copied , Repeat the string a specified number of times ）
# /  Division operator , The result will always return a floating point type 
# // to be divisible by , Only the whole digits after calculation will be reserved 
# **  Power operation 
# %  Modulo complementary operation   The result is the remainder

Assignment operator assignment operator

= Simply assign the right value to the left

a+=a -> a = a+a
-=
*=
/=
//=
**=
%=

Relational operator relational operator

There is an interesting point String size comparison

print('2'>'11') # true

In fact, the string comparison is unicode code ~ Everyone 's unicode. The first greater than is true

“
js equally
”

It can be inferred that

'a'>'b' # false
'ab' > 'b' # false

“
It's useless but interesting .
”

Logical operators logical operator

Logic is not not

Logic and and

Logic or or

Short circuit operation is also available ~

Logic and As long as the first one is false So the result is false Will not execute the following , first true, After execution
Logic or As long as the first one is true So the result is true Will not execute the following , first false, After execution

False and print(a)  #  No printing 
True and print(a)  #  Print 
False or print(a)  #  Print 
True or print(a)  #  No printing

Non Boolean logic operation

Convert each value to a Boolean operation

res = 1 and 2 #  The first is true, Go back to the second 
res = 1 and 0 
res =  0 and 1 #  The first is false  A short circuit   Go straight back to false

or equally ~

Conditional operator

grammar : sentence 1 if expression else sentence 2

print('1') if True else print('2')

It's usually used this way

max = a if a > b else b

Namely max yes ab A medium large number

Operator priority

I suggest adding brackets first

Look at the priority table , Don't pay any attention to him Hate him , If you don't understand it diss He

sentence

Conditional statements (if sentence )Conditional statement

if Conditional expression : sentence
When only one statement is executed, it can be executed in : Followed by a statement
If there are multiple statements, you have to open one Code block
python The code block is indented ~ Restart a line without curly braces

a = 2
if a == 1:
    print('1')
    print('2')
print('3') # 3

input() Function can temporarily prevent the program from ending ~ End after entering random characters ~

if- elif -else

A few small exercises

# #  Even and odd numbers 
# num = int(input(' Enter any number '))
# print(' even numbers ' if num % 2 == 0 else ' Odd number ')

# #  Leap year judgment 
# year = int(input(' Enter any year '))
# print(' Leap year ' if ((year % 100 != 0 and year % 4 == 0) or (year % 100 == 0))else ' Non leap year ')

# #  A dog is a man's age 
# dog_age = int(input(' Please enter your dog's age '))
# if dog_age <= 0:
#     print(' Who are you kidding ')
# elif dog_age <= 2:
#     person_age = dog_age * 10.5
#     print(f' A dog is as old as a man {person_age}')
# else:
#     person_age = (dog_age - 2) * 4 + 21
#     print(f' A dog is as old as a man {person_age}')

#  According to the score ,  Give a reward 
# score = int(input(' Please enter your score '))
# if score == 100:
#     print('niubi666')
# elif 80 <= score <= 99:
#     print(' ok ')
# elif 60 <= score <= 79:
#     print(' The butt blossoms ')
# else:
#     print('no  Reward ')
# print(bool(0))
# print(bool('0'))

#  Whether to marry or not ~
# question1 = bool(input('1 be for the purpose of , If it is not filled in, it means No ,  A house ?'))
# question2 = bool(input('1 be for the purpose of , If it is not filled in, it means No ,  A car ?'))
# question3 = bool(input('1 be for the purpose of , If it is not filled in, it means No ,  Handsome ?'))
# if question1 or question2 or question3:
#     if question1 and question2 and question3:
#         print(' Married married ')
#     else:
#         print(' You can live a little ')
# else:
#     print(' You are here to make fun of me ')

All the code

# a1 = ' Hoe standing grain gradually pawning a midday ,\
#  Sweat dripping under the grass ,\
#  Who knows what's on the plate ,\
#  Every grain is hard ,\
# '
# print(a1)  #  The result line 
# a2 = ''' Hoe standing grain gradually pawning a midday ,\
#  Sweat dripping under the grass ,\
#  Who knows what's on the plate ,\
#  Every grain is hard ,\
# '''
# print(a2)  #  No branches 
# a3 = ''' Hoe standing grain gradually pawning a midday ,
#  Sweat dripping under the grass ,
#  Who knows what's on the plate ,
#  Every grain is hard ,
# '''
# print(a3)  #  branch 
# intro = ' Hello '
# name = 'Shinkai'
# #  The same string type can be added 
# print(intro + name)  #  Hello Shinkai
# #  The following expression is used in python Not often 
# print(intro + name + "!!!!!")  #  Hello Shinkai!!!!!
# #  It is usually written like this 
# print(intro+name, "!!!!!")  #  Hello Shinkai !!!!!
# str = 'hello %s' % ' The Monkey King ' # %s  yes  string Abbreviation 
# print(str)  # hello  The Monkey King 
# str1 = 'hello %s  Hello  %s' % ('monkey', ' The Monkey King ')
# print(str1)  # hello monkey  Hello   The Monkey King 
# str2 = 'hello %3s' % 'ab'  #  Minimum string length 3
# print(str2)  # hello  ab
# str3 = 'hello %3.5s' % 'abcdef'  #  The string length is 3-5
# print(str3)  # hello abcde
# res = 'res'
# print('res = %s' % res)  # res = res
# #  A better way to write 
# intro = 'hello'
# name = 'Shinkai'
# res = f'{intro} {name}'
# print(res)  # hello Shinkai
# assignment
# name = 'Shinkai' * 2
# print('hello ' + name)  # hello ShinkaiShinkai
# print('hello', name)
# print('hello %s ' % name)
# print(f'hello {name} ')
# print(1 + True)  # 2
# print(1 + False)  # 1
# print(1 * False)  # 0

# str = '123'
# num = 123
# print(str)  # 123
# print(num)  # 123
# print(type(num))  # <class 'str'>
# print(type(str))  # <class 'int'>

# #  Type checking 
# print(type('123'))  # <class 'str'>
# print(type(123))  # <class 'int'>
# print(type(123.123))  # <class 'float'>
# print(type(True))  # <class 'bool'>
# print(type(None))  # <class 'NoneType'>

# #  Object structure 
# name = 'Shinkai'
# print(id(name))  # 4313468016, 4443786352
# print(type(name))  # <class 'str'>
# print(name)  # Shinkai

# test = {"key": "value"}
# test2 = test
# test3 = {"key": "value"}
# print(test == test2)  # True
# print(test == test3)  # True

# #  Type conversion 
# a = True
# a = int(a)
# print(a)

# a = 2**2
# print(a)

# obj1 = {'a': 123}
# obj2 = {'a': 123}
# print(obj1 is obj2)
# a = 0
# a = not a
# print(a)
# a = 0
# False and print(a)  #  No printing 
# True and print(a)  #  Print 
# False or print(a)  #  Print 
# True or print(a)  #  No printing 
# print('1') if True else print('2')
# if 1 == 1:
#     print('1')
#     print('1')
# a = 2
# if a == 1:
#     print('1')
#     print('2')
# print('3')
# a = input(' Please enter 1-100 A number ')
# if a == '123':
#     print(' Correct answer ')
# else:
#     print(' Wrong answer ,  Your answer is ', a)
# age = int(input(' Please enter your age :'))
# if age > 18:
#     print(' You are an adult ')
# elif age > 10:
#     print(' You went to primary school ')
# elif age <= 3:
#     print(' You are still a child ')
# else:
#     print(' Strange age ')

# #  Even and odd numbers 
# num = int(input(' Enter any number '))
# print(' even numbers ' if num % 2 == 0 else ' Odd number ')
# #  Leap year judgment 
# year = int(input(' Enter any year '))
# print(' Leap year ' if ((year % 100 != 0 and year % 4 == 0) or (year % 100 == 0))else ' Non leap year ')

# dog_age = int(input(' Please enter your dog's age '))
# if dog_age <= 0:
#     print(' Who are you kidding ')
# elif dog_age <= 2:
#     person_age = dog_age * 10.5
#     print(f' A dog is as old as a man {person_age}')
# else:
#     person_age = (dog_age - 2) * 4 + 21
#     print(f' A dog is as old as a man {person_age}')
# score = int(input(' Please enter your score '))
# if score == 100:
#     print('niubi666')
# elif 80 <= score <= 99:
#     print(' ok ')
# elif 60 <= score <= 79:
#     print(' The butt blossoms ')
# else:
#     print('no  Reward ')
# print(bool(0))
# print(bool('0'))
# question1 = bool(input('1 be for the purpose of , If it is not filled in, it means No ,  A house ?'))
# question2 = bool(input('1 be for the purpose of , If it is not filled in, it means No ,  A car ?'))
# question3 = bool(input('1 be for the purpose of , If it is not filled in, it means No ,  Handsome ?'))
# if question1 or question2 or question3:
#     if question1 and question2 and question3:
#         print(' Married married ')
#     else:
#         print(' You can live a little ')
# else:
#     print(' You are here to make fun of me ')

Shortcut record

command + / notes

shift+ enter The cursor jumps to the beginning of the next line

option+command + L formatting code

shift+command+ Up and down All contents of the current line can be moved up and down

and JavaScript The difference between

Indents represent blocks of code at different levels
Different types cannot be calculated
have access to \ Distinguish between different lines , For multi line content ''' '''triple quotation mark
- js Use back quotes backquote
python Strings can * Numbers ( Grammatical sugar )
python Of boolean yes True, False lowercase It's wrong.
- js Of boolean yes ture, false uppercase It's wrong.
python The variable of has no type , The type of the variable is the type of the value assigned

“
Many differences ....js Give up everything you can
js Not at all ' Static resource area ' Or static resources are stored in the stack . See, no one asked
”

python It's a strong type of , Once the type is created , Do not modify
- js Weak type , And there are implicit conversions ~. Out of line

“
js Can't print Memory address , The trouble can only be judged by double and third class
js On the bottom api Many have not been thrown out
”

python There is no third class
- Don't compare addresses ~ After all, you can see the memory address ~
- js The object content is the same , But the memory addresses are different. The comparison will show false
  - js in == Two objects actually compare memory addresses ~so It can't be the same unless it points to the same
  - python use is and is not To compare The memory address is id similar js===
  - It can be used to compare whether it is the same object
python Do not distinguish between reference data types and basic data types , The distinction is between variable and immutable data types
python Illegal string to numeric value , Will report a mistake js There is no error report NaN type ~
python One more integral division operation // And multiply by the string ~
- js Also have ** Power operation
python and js Short circuit operation is also available
- But the logical operators look different not->! or->|| and-> &&
python It can be compared in a chain 1<score<100
- js no way , The result of the operation is wrong
js Of '0' Turn Boolean yes false js Implicit conversions tend to be numeric '0' by 0 0 yes false
python Of '0' Turn Boolean yes true.