Python Functions

A Python function is a block of statements with a function name.

  • Does a certain function
  • Can accept data (parameters)
  • A function is called by its name
  • Functions can return data

Not to be confused with the mathematical definition of a function:

y = f(x), y is a function of x and x is an independent variable.

Related course: Complete Python Programming Course & Exercises

Why functions?

The role of the function is:

  • Structured programming is the most basic encapsulation of code, generally organizing a piece of code by function
  • The purpose of packaging is to reuse and reduce redundant code
  • The code is simple and easy to read and understand

Classification of functions:

  • Built-in functions such as max(), min(), sum(), etc
  • library functions, such as math.ceil()
  • Custom functions, defined using the def keyword

How to Define a Function in Python?

You can define a Python function like this:

def function name (list of arguments).
    Function body (code block)
    [return function value]

Some facts about functions:

  • Function names are identifiers, same naming requirements
  • Statement blocks must be indented, 4 spaces
  • Python functions that do not have a return statement will return a None value
  • The list of parameters in a definition is called a formal parameter

    function definition

    def add(x,y): return x+y

    function call


How to Call a Function in Python?

The definition of a function, which declares only one function, is not executed and needs to be called in order to be executed. Call:


The parameters written at the time of call are the actual parameters, abbreviated as real parameters.

function_name(parameter_1, parameter_2)

Parameters of the function

The function is defined to agree on the formal parameters and to provide enough actual parameters when called, in general, the number of formal and actual Method of transmission


Keyword pass-through


The location parameter is required to be passed in before the keyword is passed in, and the location parameter corresponds to the location.

python functions and calls

Can a Function have default parameter value?

The default value, also known as the default value, can be added to a function at the time of function definition.

The default value of a parameter can be set to the default value for a parameter without a given real parameter when not enough real participants are

When there are many parameters, you don't need to enter all the parameters every time, simplifying parameter calls

def add(x=4,y=5):
    return x+y


Can def add(x, y=5) or def add(x=4, y) be defined this way?

def add(x=4, y)

SyntaxError: non-default argument follows default argument

The #position parameter must be passed in before the keyword parameter

Python Function Variable Arguments

The use of * in front of the parameter indicates that the parameter is a variable position parameter and can accept more than one real parameter. It encapsulates the collected real participants into tuple groups.

Variable position parameters cannot be passed with keywords.

def fn(x,*args):
fn(3,args=4) #error

What is Function scope?

The visible range of an identifier (variable).

Each function opens up a scope, and the variables within the function cannot be outside the scope of the function. Classification of scope:

global scope

  • Visible throughout the program's operating environment.
  • Variables in a global scope are called global variables

local scope

  • visible within functions, classes, etc.
  • A variable in a local scope is called a local variable and cannot be used more than the local scope in which it is located

basic principles

1 In general, global scope variables are visible in all the program (and functions) 2 local variables cannot be seen or used outside the function.

python function scope of variables, local and global

Scopes of default values

def foo(x=1):
    x += 1


Then try this:

def bar(x=[]): #reference type, address of x-reference []


Can we have multiple return statements inside a Function?

A function can have multiple return statements, but only one is executed. The example below has multiple returns:

>>> def zero_or_one(x):
...     if x == 0:
...         return "zero"
...     else:
...         return "one"
>>> zero_or_one(0)
>>> zero_or_one(1)

Can you destroy functions?

Destruction of functions:

  • Defining a function is generating a function object to which the function name points.
  • You can use the del statement to delete the function so that its reference count is minus 1
  • The original definition can be overwritten with an identifier of the same name, essentially reducing its reference count by 1
  • Python program ends with all objects destroyed
  • A function is also an object, no exception, whether to destroy or not, or whether to reduce the reference count to 0

Python Recursive Function

A recursive function is a special function:

  • A recursive function will call itself
  • It has a stopping condition.

An example of a recursive function is:

def fact(n):
    if n==1:
        return 1
    else :
        return n * fact(n-1)

When called, it returns n * fact(n-1). That means, it will keep calling itself until the stopping condtion n==1 is True. An example run:

>>> def fact(n):
...     if n == 1:
...         return 1
...     else:
...         return n * fact(n-1)
>>> fact(5)
>>> fact(1)
>>> fact(2)
>>> fact(3)

Data Type of Python Function

The data type of a Python function is function. Every function is an instance of the function class. You can test this with the type() function.

>>> def bar():
...     pass
>>> type(bar)
<class 'function'>

Python Function vs Method

So what's the difference between a function and a method?

  • A function is part of a Python script, and a collection of statements
  • A method is part of a class
  • Functions can be called any time, but methods need object creation
  • They have a different data type

  • Methods are harder to understand (classes, objects etc). Functions are just statements

    class Dummy: ... def foo(self): ... print('I am a method') ... def foo(): ... print('I am a function') ...

    call function

    foo() I am a function

    call method

    obj = Dummy() I am a method

    Which data type?

    print(type(foo)) print(type(

Python Anonymous Function

An anonymous function is a function or subroutine that does not require a name (identifier) to be defined.

Python defines an anonymous function in lambda syntax, using only expressions and not declarations.

The lambda syntax is defined as follows.

lambda [arg1 [,arg2, ... argN]] : expression

The anonymous function has a restriction: there can only be one expression, no need to write return, the return value is the result of that expression.

There is an advantage to using anonymous functions because the function has no name and you don't have to worry about conflicting function names.


# Writable function description
sum = lambda arg1, arg2: arg1 + arg2

# Call the sum function
print("The sum of the values is : ", sum(10, 20))
print("The sum of the values is : ", sum(20, 20))

Above code, output.

The sum of the values is : 30
The summed value is : 40

It can be called directly at the time of definition, for example.

print((lambda x, y: x-y)(3, 4))

Above code, output.