Lola vs. python D02: the basics of python entry

02.Getting started with python

Chain assignment and series unpacking assignment

• Chain assignment is used to assign the same object to multiple variables x=y=123
• The series unpacking assignment is assigned to the variables corresponding to the same number (the number must be the same) a,b,c=4,5,6
  [Operation] The series unpacking assignment realizes variable exchange a,b=1,2 a,b= b,a
• Constants, python does not support constants

Built-in data types

Every object has a type. Python’s most basic built-in data types:
1. Integer: integer
2. Floating point: decimal
3. Boolean: true or false, only containing true/false
4. String type: composed of characters Sequence "abc", "little star"

Numerical operators: ±*/(floating point division 8/2=4.0) //(integer division 7///2=3)% (take the remainder 7% 4=3) ** power divmod obtains the quotient and remainder at the same time, 0 cannot be a divisor!

Integer

Integer, 0b secondary system, 0o octal, 0x hexadecimal

• Int realizes type conversion and directly discards the decimal part
• Automatic conversion, integer and floating point operations, automatic conversion to floating point numbers
• Python3 integers have no boundaries and will not overflow, especially suitable for scientific operations

Floating point

Floating point, float, 3.14=314E-2

• float can convert others into floating point numbers
• Mixed operations are automatically converted to floating point numbers
• round(value) can return the rounded value

Enhanced assignment operator

• a=a+1,a+=1

Representation of time

• Base time point: 00:00 on January 1, 1970, calculated in milliseconds (1/1000 second)
• python time.time() gets the current time, first import import time time.time()

Exercise: Define multi-point coordinates _ draw a polyline _ and calculate the distance between the starting point and the ending point.
Answer: day_2:test_1

Error summary
1. Turtle is a module. Turtle is initialized before it is used. It is referenced by import.
2. A=3. A is a variable
. Variables and modules must be initialized before use. 3. Variables and modules are all identifiers. Identifiers: variables, functions, Classes, modules, etc., pay attention to the identifier naming rule
4. Turtle.done() can always retain the running results

Boolean value

Comparison operator

• == is equal to, used for comparison; = used for assignment
• ! = Not equal to

Logical Operators

• And and, x and y, if x is true, y is returned; if x is false, then returns false, a false is false
• Or or, x or y, if x is true, then return true; if x is false, then return y -if true is true
• Not not

Same operator

• Is compares whether the object referenced by the variable is the same address id, compare id
• == Whether the value of the comparison object is equal, compare value
• 例子：a=1000，b=1000，a==b true；a is b false，id（a）id（b）
• Note: integer cache problem (-5, 256)
• is more efficient than ==, it is recommended to use is as much as possible

String

Basic characteristics of strings

• Writing programs pay more attention to logical thinking rather than mathematical ability
• The essence of string: character sequence, string is immutable in python
• String function: Corresponding characters to numbers

String encoding

• Python3 directly supports unicode and can represent characters in any written language in the world
• ASCII code is a subset of Unicode and only supports letters and numbers
• ord() built-in function, which can convert characters into Unicode codes
• chr() can convert decimal to corresponding characters

Quotation marks create string

• You can use single or double quotes to create strings
• Note : pay attention to the difference between inner and outer single and double quotes
• Three consecutive single or double quotes can create a multi-line string

Empty string

• c=“”
• len （c） 0

Escape character

String splicing

• If both sides of + are strings, concatenate
• If there are numbers on both sides of +, add
• Multiple "aa" "bb" == ""aabb"
• “Sct” * 3 “sctsctsct”
• Print without line break end="any string"
  》》》print("aa", end="#")
  》》》print("bb", end="#")
  aabb
• Read the string from the console
  "" "myname = input (" please enter the name ")
  Please enter the name: green onion
  " "" myname
  "green onion"

str() extract characters _replace() to replace

• str() converts other characters into strings
• [] Extract characters
  
• replace()
  

String slice

• Slice slice[], the standard format is: [start offset start: end offset end: step size]
• Note: Baotou does not end
  
• step: how long to take
  
• Common operations
  
• Other operations
  
• The start and end offsets are not in the string range, no error is reported, and the start or end is taken to the end

Assignment:
1. Output the string "to be or not to be" in reverse order
a="to be or not to be"
Error summary

• Only need to adjust the step, no need to think about complexity
• The output in pycharm needs to be called print()