# Python basic learning DAY2 (9.22)

Keywords: Python Pycharm

1. Chain assignment: used to assign the same object to multiple variables.

a=b=123
print(a) #123
print(b) #123

2. Series unpacking assignment: the series data is assigned to variables corresponding to the same number, and the number must be consistent.

a,b,c=1,2,3
print(a)  #1
print(b)  #2
print(c)   #3
a,b,c=1,2,3
a,b=b,a
print(a) #2
print(b)  #1

3. Constant: Python does not support constant (meaning that it can be changed in fact, not logically.)

4. Introduction to the most basic built-in data types

(1) Integer

(2) Floating point: decimal

(3) Boolean: True, False

(4) String type: "abc", "sxt"

(5) Operators with special points:

/: Floating point division

//: Integer division

%: Surplus

**: power

Use the divmod() function to get the quotient and remainder at the same time

5. Integer: 0b binary, 0o octal, 0x hexadecimal

print(int(3.99))   #3
print(int(True))   #1
print(int(False))  #0

Type conversion with int():

a=3+3.5
print(a)   #6.5


6. Floating point number: decimal float

Type conversion and rounding: float()

round(value) can return a rounded value: the original value will not be changed, but a new value will be generated.

Enhanced assignment operator:

+=: a+=2 is equivalent to a=a+2

Similar are: - =, * =/=

7. Representation of time

import time
a=int(time.time())
print(a)
totleminite=a//60
print(totleminite)
totlehour=tl=totleminite//60
print(totlehour)
totleday=totlehour//24
print(totleday)
totleyear=totleday//365
print(totleyear)  #51

8. Example: draw multi segment discount, connect the known points, and calculate the distance between the first and last points.

import turtle
import math
x1,y1=100,100
x2,y2=100,-100
x3,y3=-100,-100
x4,y4=-100,100
turtle.penup()
turtle.goto(x1,y1)
turtle.pendown()
turtle.goto(x2,y2)
turtle.goto(x3,y3)
turtle.goto(x4,y4)
distance=math.sqrt((x1-x4)**2+(y1-y4)**2)
turtle.write(distance)

9. Boolean values: True(1),False(0)

Comparison operator:

== be equal to       != Not equal to

Logical operators:

or, and, not

10. Same operator: used to compare the storage units of two objects. The actual comparison is the address of the object.

Is: judge whether the two variable reference objects are the same, that is, the address of the comparison object.

==: Used to judge whether the values of reference variables and reference objects are equal.

11. Escape character: use "\ + special character" to achieve some effects that are difficult to be represented by characters. Such as line feed.

Continuation:\

a='aaa\
bbb'
print(a)    #aaabbb

String splicing: + strings on both sides     Or directly put together to achieve splicing

a='aaa'+'bbb'
print(a)  #aaabbb
b='aaa''bbb''ccc'
print(b)  #aaabbbccc

String copy

a='aaa'*3
print(a)   #aaaaaaaaa

Non wrapping printing

print("aaa")
print("bbb")
print("ccc")
'''
aaa
bbb
ccc
'''
print("aaa",end="*")    #Ends with an asterisk and does not wrap
print("bbb",end="\t")
print("ccc")    #aaa*bbb   cccc

Read string from console: use input() to read keyboard input from console.

myname=input("Please enter your first name:")

12. (1) str() implements digital transformation string

print(str(1))  #1
print(str(34.5))  #34.5

(2) Extract characters using []

The essence of a string is a character sequence, which can be extracted by adding an offset specified in [] after the string           Single character of the position. The positive order is numbered from 0 and 1, and the negative order is numbered from - 1 and - 2.

a="abcdefdhigklmnopqrstuvwxyz"
print(a)  #b
print(a[-1])   #z

(3) replace() implements string "change"

The string cannot be changed. Call replace() to generate a new string. The original string has not changed.

a="abcdefdhigklmnopqrstuvwxyz"
print(a.replace('c','pay'))   #ab pay defdhigklmnopqrstuvwxyz
print(a)   #abcdefdhigklmnopqrstuvwxyz the value of the original string does not change
a=a.replace('c','pay')    #a points to the new string
print(a)  #The value of ab defdhigklmnopqrstuvwxyz A is changed

13. (1) slicing string: intercepting string

The standard format is: [start offset: end offset: step size] header without footer

a='abcdefghijklmnopqrstuvwxyz'
print(a[:])  #Entire output
print(a[1:5])  #bcde
print(a[1:5:2])  #bd
print(a[-2::])   #yz step size is positive order 1 by default
print(a[-10:-1:])  #qrstuvwxy uses negative numbers to slice, and arranges the order of starting coordinates according to the positive and negative of the step size
print(a[-1:-10:-1])  #zyxwvutsr

Example 1 statement flashback output

a='to be or not to be'
print(a[::-1])

Example 2 output all x's in the string

a='xstxstxstxstxstxst'
print(a[::3])

14.split() split and join() merge

split() can separate the string into multiple substrings based on the specified separator. If the separator is not specified, the blank character is used by default.

a="to be or not to be"
print(a.split())  #['to', 'be', 'or', 'not', 'to', 'be']
print(a.split("be"))  #['to ', ' or not to ', '']
b="I\nlove\nu"
print(b.split("\n"))  #['I', 'love', 'u']

join() is used to splice a series of substrings. Better than using the form of string addition, string addition requires a newly generated variable. (note the format)

a=["this","is","not","love"]
print("*".join(a))  #this*is*not*love

15. String resident mechanism and string comparison

String resident: a method to save only one copy of the same and immutable string, and different values are stored in the string resident pool.

string comparison

a="abc_d11"
b="abc_d11"
print(a is b)  # True is compares objects (memory)
print(a==b)  #Is the content the same
print("a" in a)  #Does it contain True

16. Summary of common string methods

(1) Common search methods

a='''The birds are in the strong wind
Turn quickly
The boy went to pick it up
A penny
Grapevine fantasy
And extended tentacles
The waves recoiled
And a towering back'''
print(len(a))  #77
print(a.startswith('bird'))  #True
print(a.endswith('Back'))  #True
print(a.find('and'))   #50 location of first appearance
print(a.rfind('and'))  #71 location of last appearance
print(a.count('and'))  #2 and how many times
print("sacd12324".isalnum())  #Is True a string of numbers and letters
print(a[78::])

17. String formatting

format(): format the string by directly mapping the parameter value through {index} / {parameter name}.

a="The name is:{0}，Age is:{1}"
print(a.format('Dream',20))  #The name is Dream and the age is 20
b='The name is{name},Age is{age}'
print(b.format(name='April',age=18))  #His name is April and his age is 18

Use {0} in order. Use names without paying attention to the order.

Number formatting

a="The name is:{0}，Revenue is:{1:.2f}"
print(a.format('datou',15000.324325))

Posted by ozzy on Wed, 22 Sep 2021 08:57:50 -0700