15 Python 3

发布时间：2023-04-15 14:13:26 所属栏目：教程来源：

导读：A function is a block of organized, reusable code that is used to perform a single, related action.

A function is a block of organized, reusable code that is used to perform a single, related actio

A function is a block of organized, reusable code that is used to perform a single, related action.

A function is a block of organized, reusable code that is used to perform a single, related action. Functions provide better modularity for your application and a high degree of code reusing.

As you already kNow, Python gives you many built-in functions like print(), etc. but you can also create your own functions. These functions are called user-defined functions.

Defining a Function
You can define functions to provide the required functionality. Here are simple rules to define a function in Python.

Function blocks begin with the keyword def followed by the function name and parentheses ( ( ) ).

Any input parameters or arguments should be placed within these parentheses. You can also define parameters inside these parentheses.

The first statement of a function can be an optional statement - the documentation string of the function or docstring.

The code block within every function starts with a colon (:) and is indented.

The statement return [expression] exits a function, optionally passing back an expression to the caller. A return statement with no arguments is the same as return None.

Syntax
def functionname( parameters ):
"function_docstring"
function_suite
return [expression]
By default, parameters have a positional behavior and you need to inform them in the same order that they were defined.

Example
The following function takes a string as input parameter and prints it on standard screen.

def printme( str ):
"This prints a passed string into this function"
print (str)
return
Calling a Function
Defining a function gives it a name, specifies the parameters that are to be included in the function and structures the blocks of code.

Once the basic structure of a function is finalized, you can execute it by calling it from another function or directly from the Python prompt. Following is an example to call the printme() function −

Live Demo
#!/usr/bin/python3

# Function deFinition is here
def printme( str ):
"This prints a passed string into this function"
print (str)
return

# Now you can call printme function
printme("This is first call to the user defined function!")
printme("Again second call to the same function")
When the above code is executed, it produces the following result −

This is first call to the user defined function!
Again second call to the same function
Pass by Reference vs Value
All parameters (arguments) in the Python language are passed by reference. It means if you change what a parameter refers to within a function, the change also reflects back in the calling function. For example −

Live Demo
#!/usr/bin/python3

# Function deFinition is here
def changeme( mylist ):
"This changes a passed list into this function"
print ("Values inside the function before change: ", mylist)

mylist[2]=50
print ("Values inside the function after change: ", mylist)
return

# Now you can call changeme function
mylist = [10,20,30]
changeme( mylist )
print ("Values outside the function: ", mylist)
Here, we are maintaining reference of the passed object and appending values in the same object. Therefore, this would produce the following result −

Values inside the function before change: [10, 20, 30]
Values inside the function after change: [10, 20, 50]
Values outside the function: [10, 20, 50]
There is one more example where argument is being passed by reference and the reference is being overwritten inside the called function.

Live Demo
#!/usr/bin/python3

# Function deFinition is here
def changeme( mylist ):
"This changes a passed list into this function"
mylist = [1,2,3,4] # This would assi new reference in mylist
print ("Values inside the function: ", mylist)
return

# Now you can call changeme function
mylist = [10,20,30]
changeme( mylist )
print ("Values outside the function: ", mylist)
The parameter mylist is local to the function changeme. Changing mylist within the function does not affect mylist. The function accomplishes nothing and finally this would produce the following result −

Values inside the function: [1, 2, 3, 4]
Values outside the function: [10, 20, 30]
Function Arguments
You can call a function by using the following types of formal arguments −

required arguments
Keyword arguments
Default arguments
Variable-length arguments
required Arguments
required arguments are the arguments passed to a function in correct positional order. Here, the number of arguments in the function call should match exactly with the function deFinition.

To call the function printme(), you definitely need to pass one argument, otherwise it gives a Syntax error as follows −

Live Demo
#!/usr/bin/python3

# Function deFinition is here
def printme( str ):
"This prints a passed string into this function"
print (str)
return

# Now you can call printme function
printme()
When the above code is executed, it produces the following result −

Traceback (most recent call last):
File "test.py", line 11, in <module>
printme();
TypeError: printme() takes exactly 1 argument (0 given)
Keyword Arguments
Keyword arguments are related to the function calls. When you use keyword arguments in a function call, the caller identifies the arguments by the parameter name.

This allows you to skip arguments or place them out of order because the Python interpreter is able to use the keywords provided to match the values with parameters. You can also make keyword calls to the printme() function in the following ways −

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