Python Learning note Eight--dynamic type

One. Variables, objects, references:

1. Variables:

2. Object: Contains a header information, the following two parts of the content

A. The data type of the object,

B. Reference counter: Records the number of current references to the object, and once the counter is zeroed, the object's memory space is reclaimed.

3. Reference: A pointer between an associative variable and an object,

A=3

Two. Shared references, newly created objects:

1. A=3

B=a

A= ' spam '

A changes, B is also equal to 3, because the object that B points to does not change, and B's pointer does not change.

2. A=3

B=a

A=a+2

b What's the situation?

Python assigns a new value to a variable, does not change the original object, is to recreate an object, and then point the new object pointer to the variable, the old object's counter will remove a reference

Three. To share the app, modify the object in place:

1. l1=[2,3,4]

L2=l1

L1[0]=24

L1 and L2 are changed to [24,3,4]

2. l1=[2,3,4]

l2=l1[:]

L1[0]=24

L1 is [24,3,4]l2=[2,3,4], because l1[:] is a copy-only object and does not create a reference.

Four. Shared references and equality:

1. ' = = ': Determine whether the value of two objects is the same

2. ' Is ': Determine the identity of two objects, i.e. whether two variables point to the same object

Python Learning note Eight--dynamic type