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Python Functions

Function Parameters

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 " +

Multiple Parameters

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!")


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 x
def my_function(x):
return x + 1
# Invoke the function
print(my_function(2)) # Output: 3
print(my_function(3 + 5)) # Output: 9

Function Indentation

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 for and if so long as the lines are not indented less than the first line of the function code.

# Indentation is used to identify code blocks
def testfunction(number):
# This code is part of testfunction
print("Inside the testfunction")
sum = 0
for x in range(number):
# More indentation because 'for' has a code block
# but still part of he function
sum += x
return sum
print("This is not part of testfunction")

Calling Functions

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 doHomework() method.


Function Arguments

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 grocery_store, item_on_sale, and cost.

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")

Function Keyword Arguments

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)

Returning Multiple Values

Python functions are able to return multiple values using one return statement. All values that should be returned are listed after the return keyword and are separated by commas.

In the example, the function square_point() returns x_squared, y_squared, and z_squared.

def square_point(x, y, z):
x_squared = x * x
y_squared = y * y
z_squared = z * z
# Return all three values:
return x_squared, y_squared, z_squared
three_squared, four_squared, five_squared = square_point(3, 4, 5)

The Scope of Variables

In Python, a variable defined inside a function is called a local variable. It cannot be used outside of the scope of the function, and attempting to do so without defining the variable outside of the function will cause an error.

In the example, the variable a is defined both inside and outside of the function. When the function f1() is implemented, a is printed as 2 because it is locally defined to be so. However, when printing a outside of the function, a is printed as 5 because it is implemented outside of the scope of the function.

a = 5
def f1():
a = 2
print(a) # Will print 5
f1() # Will print 2

Returning Value from Function

A 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."
return str(year) + " is not a leap year."
year_to_check = 2018
returned_value = check_leap_year(year_to_check)
print(returned_value) # 2018 is not a leap year.

Global Variables

A variable that is defined outside of a function is called a global variable. It can be accessed inside the body of a function.

In the example, the variable a is a global variable because it is defined outside of the function prints_a. It is therefore accessible to prints_a, which will print the value of a.

a = "Hello"
def prints_a():
# will print "Hello"

Parameters as Local Variables

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):
# Pass the value 7 into the function
# Causes an error as `value` no longer exists