Some tasks need to be performed multiple times within a program. Rather than rewrite the same code in multiple places, a function may be defined using the
def keyword. Function definitions may include parameters, providing data input to the function.
Functions may return a value using the
return keyword followed by the value to return.
# Define a function my_function() with parameter xdef my_function(x):return x + 1# Invoke the functionprint(my_function(2)) # Output: 3print(my_function(3 + 5)) # Output: 9
Python functions can be defined with named arguments which may have default values provided. When function arguments are passed using their names, they are referred to as keyword arguments. The use of keyword arguments when calling a function allows the arguments to be passed in any order — not just the order that they were defined in the function. If the function is invoked without a value for a specific argument, the default value will be used.
def findvolume(length=1, width=1, depth=1):print("Length = " + str(length))print("Width = " + str(width))print("Depth = " + str(depth))return length * width * depth;findvolume(1, 2, 3)findvolume(length=5, depth=2, width=4)findvolume(2, depth=3, width=4)
Sometimes functions require input to provide data for their code. This input is defined using parameters.
Parameters are variables that are defined in the function definition. They are assigned the values which were passed as arguments when the function was called, elsewhere in the code.
For example, the function definition defines parameters for a character, a setting, and a skill, which are used as inputs to write the first sentence of a book.
def write_a_book(character, setting, special_skill):print(character + " is in " +setting + " practicing her " +special_skill)
Parameters in python are variables — placeholders for the actual values the function needs. When the function is called, these values are passed in as arguments.
For example, the arguments passed into the function
.sales() are the “The Farmer’s Market”, “toothpaste”, and “$1” which correspond to the parameters
def sales(grocery_store, item_on_sale, cost):print(grocery_store + " is selling " + item_on_sale + " for " + cost)sales("The Farmer’s Market", "toothpaste", "$1")
return keyword is used to return a value from a Python function. The value returned from a function can be assigned to a variable which can then be used in the program.
In the example, the function
check_leap_year returns a string which indicates if the passed parameter is a leap year or not.
def check_leap_year(year):if year % 4 == 0:return str(year) + " is a leap year."else:return str(year) + " is not a leap year."year_to_check = 2018returned_value = check_leap_year(year_to_check)print(returned_value) # 2018 is not a leap year.
Python functions can have multiple parameters. Just as you wouldn’t go to school without both a backpack and a pencil case, functions may also need more than one input to carry out their operations.
To define a function with multiple parameters, parameter names are placed one after another, separated by commas, within the parentheses of the function definition.
def ready_for_school(backpack, pencil_case):if (backpack == 'full' and pencil_case == 'full'):print ("I'm ready for school!")
Python uses simple syntax to use, invoke, or call a preexisting function. A function can be called by writing the name of it, followed by parentheses.
For example, the code provided would call the
In Python, a recursive function accepts an argument and includes a condition to check whether it matches the base case. A recursive function has:
def countdown(value):if value <= 0: #base caseprint("done")else:print(value)countdown(value-1) #recursive case
A recursive function should have a recursive step which calls the recursive function with some input that brings it closer to its base case. In the example, the recursive step is the call to
countdown() with a decremented value.
def countdown(value):if value <= 0:print("done")else:print(value)countdown(value-1) #recursive step
A recursive function should have a base case with a condition that stops the function from recursing indefinitely. In the example, the base case is a condition evaluating a negative or zero value to be true.
function countdown(value)if value is negative or zeroprint "done"otherwise if value is greater than zeroprint valuecall countdown with (value-1)
A lambda function in Python is a simple, anonymous function that is defined without a name. Lambda functions are useful when we want to write a quick function in one line that can be combined with other built-in functions such as
This is the syntax to define lambda functions:
lambda argument(s): expression
Python uses indentation to identify blocks of code. Code within the same block should be indented at the same level. A Python function is one type of code block. All code under a function declaration should be indented to identify it as part of the function. There can be additional indentation within a function to handle other statements such as
if so long as the lines are not indented less than the first line of the function code.
# Indentation is used to identify code blocksdef testfunction(number):# This code is part of testfunctionprint("Inside the testfunction")sum = 0for x in range(number):# More indentation because 'for' has a code block# but still part of he functionsum += xreturn sumprint("This is not part of testfunction")
Function parameters behave identically to a function’s local variables. They are initialized with the values passed into the function when it was called.
Like local variables, parameters cannot be referenced from outside the scope of the function.
In the example, the parameter
value is defined as part of the definition of
my_function, and therefore can only be accessed within
my_function. Attempting to print the contents of
value from outside the function causes an error.
def my_function(value):print(value)# Pass the value 7 into the functionmy_function(7)# Causes an error as `value` no longer existsprint(value)