Getting Started with Python: Variables

In Python, the concept of variables is basically the same as the equation variables in junior high school algebra.

For example, for the equation y=x*x , x is the variable. When x=2, the calculation result is 4, and when x=5, the calculation result is 25.

It’s just that in computer programs, variables can not only be numbers, but also any data type.

In Python programs, variables are represented by a variable name. The variable name must be a combination of uppercase and lowercase English, numbers and underscores (_), and cannot start with a number, for example:

a = 1  变量a是一个整数。

t_007 = 'T007'  变量t_007是一个字符串。

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是整数

print a

a = 'imooc'   # a变为字符串

print a

This kind of language whose variable type is not fixed is called a dynamic language, and the corresponding language is 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 comment ):

int a = 123; // a是整数类型变量

a = "mooc"; // 错误：不能把字符串赋给整型变量

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 expression on the right x + 2. Get the result 12 and assign it to 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 'ABC' in memory String;

2. Create a variable named a in the memory and point 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 Is the content of variable b 'ABC' or 'XYZ'? If you understand it in 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:

Executed a = 'ABC', the interpreter created the string 'ABC' and the variable a, and pointed a to 'ABC':

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

Executiona = 'XYZ', the interpreter creates the string 'XYZ' and changes the pointer of a to 'XYZ', but b does not change:

So, the final result of printing variable b is naturally 'ABC'.

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