Introduction to Python basic functions

The most important purpose of function 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 have todefine 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 informs python: I am defining a function. square_sum is the function name.

a and b in brackets are the parameters of the function and are 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 represent affiliations 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 reaches 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 call and parameter transfer

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

print square_sum(3,4 )

Python knows through the position that 3 corresponds to the first parameter a in the function definition, and 4 corresponds to the second parameter b, and then passes the parameters 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. This 25 is printed out by print.

Let’s look at the following two examples again

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 integervariableto the function, and the function performs operations on it operation, 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 the table, the table is passed to the function as a pointer. The pointer points to the location of the sequence in memory. The operation of the table in the function will be in the original memory. proceed, thus affecting the original variables. (We call this pointer passing)

Summary

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

The purpose of the function: To 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

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