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Links

Want to study Python functions? see here
How to name functions? see here
Alternative ways to pass parameters? see here

I. Functions Without Parameters

Structure

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def <function_name>(): # Mandatory
    <function_body> # Optional
    return <return_value> # Optional

Example Program

Hello world program using functions:

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# prints 'Hello World\nDone'
# Author: GoodCoder666
def getHelloWorldString():
    return 'Hello World'
def print_hello_world():
    print(getHelloWorldString())
print_hello_world()
print('Done')
# Output:
# Hello World
# Done

Program Flow

  flowchat
st=>start: Start: Call print_hello_world()
e=>end: End
get_str_call=>operation: print_hello_world() calls getHelloWorldString()
return_str=>operation: getHelloWorldString() returns 'Hello World'
print_hi=>inputoutput: Return to print_hello_world(), output Hello World
done=>inputoutput: Return to main program, output Done

st->get_str_call->return_str->print_hi->done->e

II. Functions With Parameters

Prerequisite Knowledge

Parameters: Values passed to the function, equivalent to variables within the function:

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def param_test(a, b):
    a, b = b, a
    print(a, b)
param_test(5, 6) # Output: 6 5

This program is equivalent to:

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def param_test():
    a, b = 5, 6
    #-----Same below-----#
    a, b = b, a
    print(a, b)
param_test() # Output: 6 5

Structure

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def <function_name>(<parameter_list>): # Mandatory
    <function_body> # Optional
    return <return_value> # Optional

The parameter list uses commas to separate parameters, e.g., a, b or c (write directly if only one parameter).

Example Program

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# prints 'Hi, {name}!'
# Author: GoodCoder666
def get_greet_str(name):
    return 'Hi, ' + name + '!'

def greet(name):
    print(get_greet_str(name))

greet(input("What's your name? "))
print('Done')
# Input: GoodCoder666
# Output:
# Hi, GoodCoder666!
# Done

Program Flow

  flowchat
st=>start: Start: Call input("What's your name?")
e=>end: End
call_greet=>operation: input returns username, call greet(<username>)
call_get_greet_str=>operation: greet calls get_greet_str(<username>)
print_hi=>inputoutput: Return to get_greet_str, output its return value
done=>inputoutput: Return to main program, output Done

st->call_greet->call_get_greet_str->print_hi->done->e

Functions Are Objects

A function is also an object.

Proof

Define two functions first:

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def a(a, b):
    return a + b
def b():
    return a(1, 2)