Detailed explanation and examples of Python variables

In Python, the equal sign = is an assignment statement. Any data type can be assigned to a variable. The same variable can be assigned repeatedly, and it can be a variable of different types, for example:
a = 123 # a is an integer
print a
a = 'imooc' # a becomes a string
print a
This kind of language whose variable type is not fixed is called a dynamic language, and its counterpart is a static language.
Static language must specify the variable type when defining a variable. If the type does not match when assigning a value, an error will be reported. For example, Java is a static language, and the assignment statement is as follows (// represents a comment):
int a = 123; // a is an integer type variable
a = "mooc"; // Error: cannot assign a string to Integer variables
Compared with static languages, dynamic languages ​​are more flexible for this reason.
Please do not equate the equal sign of the assignment statement with the mathematical equal sign. For example, the following code:
x = 10
x = x + 2
If you understand x = x + 2 mathematically, it is not true anyway. In the program, the assignment statement first calculates the right side The expression x + 2 gets the result 12, which is then assigned to the variable x. Since the previous value of x was 10, after reassignment, the value of x becomes 12.
Finally, it is also important to understand how variables are represented in computer memory. When we write: a = 'ABC', the Python interpreter does two things:
1. Creates a string of 'ABC' in memory;
2. Creates a name in memory is a variable and points it to 'ABC'.
You can also assign a variable a to another variable b. This operation actually points variable b to the data pointed to by variable a. For example, the following code:
a = 'ABC'
b = a
a = 'XYZ'
print b
The last line prints out the content of variable b. Is it 'ABC' or 'XYZ'? If you understand it from a mathematical sense, you will mistakenly conclude that b and a are the same and should also be 'XYZ', but in fact the value of b is 'ABC'. Let us execute the code line by line and you can see what happens. What happened:
When a = 'ABC' is executed, the interpreter creates the string 'ABC' and the variable a, and points a to 'ABC':

Execute b = a, the interpreter creates variable b, and points b to the string 'ABC' pointed to by a:

Execute a = 'XYZ', the interpreter creates string 'XYZ', and changed the pointer of a to 'XYZ', but b has not changed:

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