Python Basics Tutorial - Functions

The most important purpose of functions is to facilitate us to reuse the same program.

Affiliate some operations to a function. When you want to implement the same operation in the future, you only need to call the function name without repeating all the statements.

Definition of function

First, we need to define a function to illustrate the function of this function.

def square_sum(a,b):
    c = a**2 + b**2
    return c

The function of this function is to find the sum of the squares of two numbers.

First of all, def, this keyword notifies python: I am defining a function. square_sum is the function name.

a and b in parentheses are the parameters of the function and the input to the function. There can be multiple parameters or none at all (but the parentheses must be retained).

We have already seen colons and indentations to indicate affiliation in loops and selections.

c = a**2 + b**2        # 这一句是函数内部进行的运算
return c               # 返回c的值，也就是输出的功能。Python的函数允许不返回值，也就是不用return。return可以返回多个值，以逗号分隔。相当于返回一个tuple(定值表)。
return a,b,c          # 相当于 return (a,b,c)

In Python, when the program executes return, the program will stop executing the remaining statements in the function. Return is not necessary. When there is no return, or there is no return value after return, the function will automatically return None. None is a special data type in Python, used to represent nothing, equivalent to NULL in C. None is mostly used as the default value for keyword argument passing.

Function calling and parameter passing

After defining the function, you can use this function in subsequent programs

print square_sum(3,4)

Python knows that 3 corresponds to the first position in the function definition through the position The first parameter a, 4 corresponds to the second parameter b, and then the parameter is passed to the function square_sum.

(Python has rich parameter transfer methods, including keyword transfer, table transfer, dictionary transfer, etc. The basic tutorial will only cover positional transfer)

The function returns a value of 25 after operation, and this 25 is printed out by print.

Let’s look at the following two examples

a = 1
def change_integer(a):
    a = a + 1
    return a
print change_integer(a)
print a
#===(Python中 "#" 后面跟的内容是注释，不执行 )
b = [1,2,3]
def change_list(b):
    b[0] = b[0] + 1
    return b
print change_list(b)
print b

In the first example, we pass an integer variable to the function, and the function operates on it, but the original integer variable a does not change.

In the second example, we pass a table to the function, the function operates, and the original table b changes.

For variables of basic data types, after the variable is passed to the function, the function will copy a new variable in the memory, thus not affecting the original variable. (We call this value transfer)

But for tables, what the table passes to the function is a pointer, which points to the location of the sequence in memory. Operations on the table in the function will be performed in the original memory, thus affecting original variable. (We call this pointer passing)

Summary

def function_name(a,b,c):
    statement
    return something  # return不是必须的

Purpose of function: Improve the repeatability of the program.

return None

Pass parameters by position.

Parameters of basic data types: passed by value

Table as parameter: passed by pointer