FUNCTIONS

FUNCTIONS

A function is a block of code which only runs when it is called.

You can pass data, known as parameters, into a function.

A function can return data as a result.

Creating a Function

In Python a function is defined using the def keyword:

Example

def my_function():

print("Hello from a function")

Calling a Function

To call a function, use the function name followed by parenthesis:

Example

def my_function():

print("Hello from a function")

my_function()

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Arguments

Information can be passed into functions as arguments.

Arguments are specified after the function name, inside the parentheses. You can add as many arguments as you want, just separate them with a comma.

The following example has a function with one argument (fname). When the function is called, we pass along a first name, which is used inside the function to print the full name:

Example

def my_function(fname):

print(fname + " Refsnes")

my_function("Emil")

my_function("Tobias")

my_function("Linus")

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Arguments are often shortened to args in Python documentations.

Parameters or Arguments?

The terms parameter and argument can be used for the same thing: information that are passed into a function.

From a function's perspective:

A parameter is the variable listed inside the parentheses in the function definition.

An argument is the value that is sent to the function when it is called.

Number of Arguments

By default, a function must be called with the correct number of arguments. Meaning that if your function expects 2 arguments, you have to call the function with 2 arguments, not more, and not less.

Example

This function expects 2 arguments, and gets 2 arguments:

def my_function(fname, lname):

print(fname + " " + lname)

my_function("Emil", "Refsnes")

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If you try to call the function with 1 or 3 arguments, you will get an error:

Example

This function expects 2 arguments, but gets only 1:

def my_function(fname, lname):

print(fname + " " + lname)

my_function("Emil")

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Arbitrary Arguments, *args

If you do not know how many arguments that will be passed into your function, add a * before the parameter name in the function definition.

This way the function will receive a tuple of arguments, and can access the items accordingly:

Example

If the number of arguments is unknown, add a * before the parameter name:

def my_function(*kids):

print("The youngest child is " + kids[2])

my_function("Emil", "Tobias", "Linus")

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Arbitrary Arguments are often shortened to *args in Python documentations.

Keyword Arguments

You can also send arguments with the key = value syntax.

This way the order of the arguments does not matter.

Example

def my_function(child3, child2, child1):

print("The youngest child is " + child3)

my_function(child1 = "Emil", child2 = "Tobias", child3 = "Linus")

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The phrase Keyword Arguments are often shortened to kwargs in Python documentations.

Arbitrary Keyword Arguments, **kwargs

If you do not know how many keyword arguments that will be passed into your function, add two asterisk: ** before the parameter name in the function definition.

This way the function will receive a dictionary of arguments, and can access the items accordingly:

Example

If the number of keyword arguments is unknown, add a double ** before the parameter name:

def my_function(**kid):

print("His last name is " + kid["lname"])

my_function(fname = "Tobias", lname = "Refsnes")

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Arbitrary Kword Arguments are often shortened to **kwargs in Python documentations.

Default Parameter Value

The following example shows how to use a default parameter value.

If we call the function without argument, it uses the default value:

Example

def my_function(country = "Norway"):

print("I am from " + country)

my_function("Sweden")

my_function("India")

my_function()

my_function("Brazil")

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Passing a List as an Argument

You can send any data types of argument to a function (string, number, list, dictionary etc.), and it will be treated as the same data type inside the function.

E.g. if you send a List as an argument, it will still be a List when it reaches the function:

Example

def my_function(food):

for x in food:

print(x)

fruits = ["apple", "banana", "cherry"]

my_function(fruits)

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Return Values

To let a function return a value, use the return statement:

Example

def my_function(x):

return 5 * x

print(my_function(3))

print(my_function(5))

print(my_function(9))

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The pass Statement

function definitions cannot be empty, but if you for some reason have a function definition with no content, put in the pass statement to avoid getting an error.

Example

def myfunction():

pass

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Recursion

Python also accepts function recursion, which means a defined function can call itself.

Recursion is a common mathematical and programming concept. It means that a function calls itself. This has the benefit of meaning that you can loop through data to reach a result.

The developer should be very careful with recursion as it can be quite easy to slip into writing a function which never terminates, or one that uses excess amounts of memory or processor power. However, when written correctly recursion can be a very efficient and mathematically-elegant approach to programming.

In this example, tri_recursion() is a function that we have defined to call itself ("recurse"). We use the k variable as the data, which decrements (-1) every time we recurse. The recursion ends when the condition is not greater than 0 (i.e. when it is 0).

To a new developer it can take some time to work out how exactly this works, best way to find out is by testing and modifying it.

Example

Recursion Example

def tri_recursion(k):

if(k > 0):

result = k + tri_recursion(k - 1)

print(result)

else:

result = 0

return result

print("\n\nRecursion Example Results")

tri_recursion(6)

SCOPE

A variable is only available from inside the region it is created. This is called scope.

Local Scope

A variable created inside a function belongs to the local scope of that function, and can only be used inside that function.

Example

A variable created inside a function is available inside that function:

def myfunc():

x = 300

print(x)

myfunc()

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Function Inside Function

As explained in the example above, the variable x is not available outside the function, but it is available for any function inside the function:

Example

The local variable can be accessed from a function within the function:

def myfunc():

x = 300

def myinnerfunc():

print(x)

myinnerfunc()

myfunc()

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Global Scope

A variable created in the main body of the Python code is a global variable and belongs to the global scope.

Global variables are available from within any scope, global and local.

Example

A variable created outside of a function is global and can be used by anyone:

x = 300

def myfunc():

print(x)

myfunc()

print(x)

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Naming Variables

If you operate with the same variable name inside and outside of a function, Python will treat them as two separate variables, one available in the global scope (outside the function) and one available in the local scope (inside the function):

Example

The function will print the local x, and then the code will print the global x:

x = 300

def myfunc():

x = 200

print(x)

myfunc()

print(x)

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Global Keyword

If you need to create a global variable, but are stuck in the local scope, you can use the global keyword.

The global keyword makes the variable global.

Example

If you use the global keyword, the variable belongs to the global scope:

def myfunc():

global x

x = 300

myfunc()

print(x)

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Also, use the global keyword if you want to make a change to a global variable inside a function.

Example

To change the value of a global variable inside a function, refer to the variable by using the global keyword:

x = 300

def myfunc():

global x

x = 200

myfunc()

print(x)

LAMBDA

A lambda function is a small anonymous function.

A lambda function can take any number of arguments, but can only have one expression.

Syntax

lambda arguments : expression

The expression is executed and the result is returned:

Example

Add 10 to argument a, and return the result:

x = lambda a : a + 10

print(x(5))

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Lambda functions can take any number of arguments:

Example

Multiply argument a with argument b and return the result:

x = lambda a, b : a * b

print(x(5, 6))

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Example

Summarize argument a, b, and c and return the result:

x = lambda a, b, c : a + b + c

print(x(5, 6, 2))

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Why Use Lambda Functions?

The power of lambda is better shown when you use them as an anonymous function inside another function.

Say you have a function definition that takes one argument, and that argument will be multiplied with an unknown number:

def myfunc(n):

return lambda a : a * n

Use that function definition to make a function that always doubles the number you send in:

Example

def myfunc(n):

return lambda a : a * n

mydoubler = myfunc(2)

print(mydoubler(11))

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Or, use the same function definition to make a function that always triples the number you send in:

Example

def myfunc(n):

return lambda a : a * n

mytripler = myfunc(3)

print(mytripler(11))

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Or, use the same function definition to make both functions, in the same program:

Example

def myfunc(n):

return lambda a : a * n

mydoubler = myfunc(2)

mytripler = myfunc(3)

print(mydoubler(11))

print(mytripler(11))