python basis - variables and constants

constant

A memory unit for storing a fixed value, the value of a constant can not be changed in the program; Python was not named constant , that is not as constant as the C language to a name.
Constant python comprising: a number, string, Boolean, null ;

>>> 'python'
'python'
>>> 3
3
>>> True
True
>>> []
[]

variable

Python is no need to declare variables. Each variable must be assigned before use , variable assignment after the variable will be created.
In Python, a variable is a variable, it is not the type we call "type" is the type of objects in memory within the meaning of the variables.
Equal sign (=) is used to assign values to variables.
The left side of the equal sign (=) operator is a variable name, an equal sign (=) operator is the right value stored in the variable

# Variables must "define" before use (ie to the variable a value), or an error:
 >>> the n-    # try to access an undefined variable
 Traceback (MOST recent Results Last Call): 
File "<stdin>", . 1 Line, in <Module1> 
NameError: name 'n-' IS Not defined


# Variable assignment 
>>> counter = 100           # integer variable 
>>> = 1000.0 Miles        # float variable 
>>> name = " runoob "      # string 
>>> Print (counter)
 100 
>>> Print (Miles)
 1000.0 
>>> Print (name) 
runoob 

# variable value exchange
 >>> A =. 1 
>>> B = 2 
>>> A, B = B, A
 >>> Print (A, B) 
2. 1 


plurality of variable # assignment

>>> a = b = c = 1
>>> print(a,b,c)
1 1 1
>>> a, b, c = 1, 2, "python"
>>> print(a,b,c)
1 2 python

id (identity)

A =. 1 >>> 
>>> ID (A) indicates a value #id location in memory, it may determine whether the two values are the same object
 498 232 336 
>>> ID (A) represented by two values are the same values #id in memory the same place, the same thing
 498 232 336 
>>> B = 300 
>>> ID (B)
 59,417,712 
>>> ID (30 )
 498 233 264

Note: number between 1-256 in memory in python present in the same location

Example:

>>> a=1001
>>> a is 1001
False
>>> b=1
>>> b is 1
True

# Assignment by reference (the same memory address)

>>> a=1000
>>> id(a)
57192176
>>> b=a #=引用赋值
>>> id(b)
57192176

Exercise: Write a program to find the id number is not the same, from a number of traversing back, first find a different number

algorithm:

1. Generate a 300 number for

2. Take a few contrast, it is determined whether the same id declare a constant value, after each comparison +1

3. Stop the cycle after the find and print

>>> num =0
>>> for i in range(300):
...     if id(i) != id(num):
...         print(i)
...         break
...     num +=1
...
257

python basis - variables and constants

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