Python dynamic type

In Python, the type is automatically determined during the run, not through the declaration of the Code. ---is very important.

>>> a=666

1. Create an object to represent the value 666 (the concept of a type is present in the object rather than in the variable name)
   

2. Create a variable A (create a variable when assigning)

3. Connecting a variable to a new object 3

Variable name and object, after running a=666, variable a becomes a reference to object 666, internally, the variable is actually a pointer to the object's memory space (created by running the constant expression 666).

The connection from a variable to an object in Python is called a reference. A reference is a relationship that is implemented in an in-memory pointer form.

A variable is an element of a system table that has a connection to an object

object is a piece of memory allocated, with enough space to store the value they represent

A reference is a pointer to a variable to an object

An object has two standard header information, a type marker to identify the object's type, and a reference counter to determine whether the object can be recycled.

Garbage collection:

If the object does not have a variable name reference, the Python garbage collection mechanism reclaims the memory. The reference counter of this object is used to determine if there is a reference.

Shared references:

Multiple variable names refer to the same object, called a shared reference ;

>>> a=666

>>> B=a

>>> A

666

>>> b

666

>>> a=666

>>> B=a

>>> b

666

>>> a= ' Fuck '

>>> b

666

>>> when the fuck is finished, a new object (memory space) is created by the constant expression fuck, but the variable B still references the original object 666, which changes the reference to variable a instead of the object 3. Thus variable B has not changed. ( not replace the original object )

Note: Assigning a new value to a variable, instead of replacing the original object, allows the variable to refer to an entirely different object.

Share references and modify in place, the following is the list

>>> l1=[1,2,3,4]

>>> l2=L1

>>> L2

[1, 2, 3, 4]

>>> L1[0]=666

>>> L1

[666, 2, 3, 4]

>>> L2

[666, 2, 3, 4]

>>>

Shared references and equality

>>> a=1111

>>> b=1111

>>> A is B

False

>>> a==b

True

>>> x=999

>>> y=x

>>> x is y

True

>>> x==y

True

where "= =" compares the values of the two referenced objects, the IS operator checks the identity of the object, and two variables point to the same object.

Note: examples

>>> x=11

>>> x= ' Fuck '

Has object 11 been reclaimed by Python garbage collection immediately? Not at all. Python caches and takes small integers and small strings. Such as:

>>> m=1111

>>> n=1111

>>> N is M

False

>>> c=11

>>> d=11

>>> C is D

True #小的整数和字符串被缓存, multiplexing

>>> g=1000

>>> f=1000

>>> G is f

False

How many times the query object is referenced, using the SYS module's Getrefcount (object)

>>> Import Sys

>>> Sys.getrefcount (11)

20

>>> sys.getrefcount (' Fuck ')

4

Python dynamic type