Functions

If you have a piece of code that does a particular job and you need to reuse it multiple times it is useful to define it as a function.

To define a function you use the def keyword followed by the name you want to give the function, followed by brackets containing any arguments that the function takes. The definition line should be finished with a colon :. The function contents then follow, with indents at the start of the line defining code that is within the function. E.g.,

# define a function
def my_function(variable1, variable2):
    # contents of a function must be indented
    print(variable1, variable2)

# define variables (back "outside" of the function)
a = "Hello"
b = "world!"

# use (or "call") the function with the two variables a and b
my_function(a, b)
Hello world!

The function name must start with an upper or lowercase letter, but can then contain any letters, numbers or the underscore _ character. It cannot contain spaces or other characters. It is useful if the function name is descriptive of what the function does. Some fairly common ways of defining function names are to split multiple descriptive words using an underscore, or use camel case (having words together, but starting each with a capital letter), e.g.,

def add_two_numbers(a, b):
    c = a + b
    return c

# use the function
num = add_two_numbers(1, 2)
print(num)
3

def countToTen():
    # indent the function
    for i in range(1, 11):
        # indent the for loop within the function
        print(i)

# use the function
countToTen()
1
2
3
4
5
6
7
8
9
10

Function arguments

In the above examples there are some functions that take arguments, i.e., variables that are passed to them that they then use. In the function definition each variable within the brackets is separated by a comma.

Note

The name of the variable used in the function definition is what that variable will be known as within the function even if the variable passed to the function has a different name.

As with function names, it is useful for variable names to be descriptive. You can have as many arguments to a function as you want, including no arguments at all!

# a function with no arguments
def PiratePete():
    print("Aarrrrrrrr! Where's me treasure.")

# use the function (remember the brackets)
PiratePete()
Aarrrrrrrr! Where's me treasure.

Note

Notice that when using the function, even if it takes no arguments, you need to include the brackets.

# a function with two arguments
def full_name(firstname, surname):
    name = firstname + " " + surname
    return name

myname = full_name("Joe", "Bloggs")
print(myname)
Joe Bloggs

# a function with lots of arguments
def sum_many_numbers(a, b, c, d, e, f, g, h, i, j, k):
    total = sum([a, b, c, d, e, f, g, h, i, j, k])
    return total

total = sum_many_numbers(2, 3, 1, 5, 6, 1, 3, 4, 9, 7)
TypeError: sum_many_numbers() missing 1 required positional argument: 'k'

total = sum_many_numbers(2, 3, 1, 5, 6, 1, 3, 4, 9, 7, 1)
print(total)
42

This style of argument is known as a "positional" argument. This means that when you pass values to the function their order (or position) matters. Positional arguments are always all required when you use the function, i.e., you cannot leave any out when calling the function.

It is best not to have too many arguments or you can lose track of them. If you do require lots of inputs to a function it can be useful to group them and instead define the function to take a single variable that has a type such as a list or dictionary.

Note

You can use the names of positional arguments (see keyword arguments below) when parsing values to a function if you want, and they can then be passed in any order:

def hey_there(firstname, lastname):
    print(f"Hello {firstname} {lastname}")

hey_there(lastname="Pitkin", firstname="Matt")
Hello Matt Pitkin

Keyword arguments

As well as positional arguments, you can define a function to be passed keyword arguments. With these you pass a variable to a function using a specific "key" word that is then used as that variable's name within the function. Keyword arguments and defined in the function definition using a keyword=value pair.

Unlike positional arguments, you do not have to call the function using all of (or even any of) the keywords. However, this means that when defining a function with keyword arguments you must give them default values that will be used if they are not explicitly set.

# a function with one positional argument and two keyword arguments
def in_range(value, minimum=-100, maximum=100):
    # return True if value is within minimum and maximum
    return minimum < value < maximum

# use function without keywords (i.e., using defaults)
print(in_range(20))
True

# set minimum value using the "minimum" keyword
print(in_range(12, minimum=15))
False

# use both minimum and maximum keywords
lower = 34
upper = 90
print(in_range(56, minimum=lower, maximum=upper))
True

You do not have to pass keyword arguments in the same order that they come in the function definition:

print(in_range(99, maximum=lower, minimum=upper))
False

Note

Sometimes you might see code where the user has given the variable being passed to a keyword argument the same name as the keyword argument itself, e.g.:

minimum = 12
maximum = 100
is_in_range = in_range(34, minimum=minimum, maximum=maximum)

While this may look a bit confusing it is perfectly valid.

Returning values

Many of the above example use the print built-in function to print a message to the screen. However, you cannot then use the output of the function if it just prints it, e.g., if you had a function that calculated the sine of an angle and it just prints it to the screen it would be fine if that is all you ever used it for, but if you wanted to use that output as part of a larger equation it would not be much use.

Most functions should return something to the user. This returned value can be assigned to a variable that can then be used later in the code. To do this the return keyword is used within the function definition. As well as returning a variable from the function it also tells the function to exit, i.e., there should not be code after the return (unless you're using it in a conditional statement) as it will not get run.

def add_two_numbers(a, b):
    # add the two input arguments
    c = a + b

    # return the result
    return c

# use the function and assign the output to a variable called result
result = add_two_numbers(3, 5)

# use the "result" variable
print(result + 10)
18

Multiple outputs

The returned item can be any Python object, but you can explicitly return multiple variables by separating them with commas in the return statement:

def sum_and_product(a, b):
    totalsum = a + b
    totalproduct = a * b

    # return both values
    return totalsum, totalproduct

# call the function and assign the answers to two variables
sumres, prodres = sum_and_product(5, 10)
print(sumres, prodres)
15 50

# return them as a tuple
result = sum_and_product(12, 8)
print(result)
(20, 96)

The results are returned with a Python tuple object.

Functions as variables

Like everything within Python function are themselves objects. So, you can create a variable out of a function by omitting the bracket during the function call:

def sing_a_rainbow(forwards=True):
    colours = ["red", "yellow", "pink", "green", "purple", "orange", "blue"]
    if forwards:
        print(" and ".join(colours[:4]) + ", " + " and ".join(colours[4:]))
    else:
        print(" and ".join(colours[::-1][:4]) + ", " + " and ".join(colours[::-1][4:]))

# create a variable from the function (note the lack of brackets)
song = sing_a_rainbow
print(song)
<function sing_a_rainbow at 0x7f9254c7f840>

# call the function using the new variable
song()
red and yellow and pink and green, purple and orange and blue

# or
song(forwards=False)
blue and orange and purple and green, pink and yellow and red

This means you can easily pass functions as arguments to other functions:

def CanISing(aSong, forwards=True):
    # call the function aSong
    aSong(forwards=forwards)
    print("I can sing a rainbow too!")

song = sing_a_rainbow
CanISing(song)
red and yellow and pink and green, purple and orange and blue
I can sing a rainbow too!

Variable scope

A variable's scope, or nested scope, relates to where in a code a particular variable is recognised. It is easiest to demonstrate this with an example:

# a variable that can be recognised/used anywhere within the script
variable1 = 1

def testfunc1(arg1):
    # a variable that can be used anywhere within testfunc1, but not at lower levels
    variable2 = arg1 * variable1  # using variable1

    def testfunc2(arg2):
        # a variable that can be used anywhere within testfunc2, but not at lower levels
        variable3 = variable2 + arg2  # using variable2

Here, we see that a variable (variable1) defined at the lowest level, i.e., not indented, can be recognised and used anywhere within a code.

A variable defined within a function (or class), e.g., variable2, can be used anywhere within that function, but not at lower levels. I.e., its "scope" is limited to its indented level and higher.

It is worth noting that you can use the same variable name, without clashing, within different scopes, e.g.:

x = 2

def myfunc(y):
    # re-use variable name x, but within this scope
    x = 2 * y
    return x

z = myfunc(3)
print(z)
6

# x in the outer scope shouldn't change
print(x)
2

If you do this, then you cannot use the variable x from the outer most scope within the inner scope.

Tip

If you want to use a value within a function it is best practice to pass it as an argument rather than rely on it having an appropriate scope.

Unknown numbers of keyword arguments

Question

What if your function calls another function that takes in multiple keyword arguments. Does your function then have to be defined with all those keyword arguments too?

def somefunc(firstname="Default", lastname="Name"):
    # do something with args...
    return f"{firstname} {lastname}"

# another function that uses somefunc
def anotherfunc(message="Hello", firstname="Default", lastname="Name"):
    # we've had to define this function to take firstname and last name as well,
    # and redo their default values too

    # use somefunc
    output = somefunc(firstname=firstname, lastname=lastname)

    return f"{message} {output}"

There are a couple of ways to make this slightly simpler, both of which use the ability to "unpack" a dictionary.

A function can be defined to take an argument named **kwargs. This is special syntax that means that when you use the function you can pass it any number of keyword arguments. **kwargs must be the final argument in your function definition, but you can have other positional and explicit keyword arguments before it.

Within the function kwargs (without the preceding two asterisks) is just a Python dictionary containing the keywords and their values as key-value pairs.

If you know that you are only ever going to pass the additional keywords "firstname" and "lastname" to anotherfunc then you could do:

def anotherfunc(message="Hello", **kwargs):
    print(kwargs)  # print statement is just for show here

    # use somefunc
    output = somefunc(**kwargs)

    return f"{message} {output}"

print(anotherfunc(firstname="Matt", lastname="Pitkin"))
{'lastname': 'Pitkin', 'firstname': 'Matt'}
Hello Matt Pitkin

By passing **kwargs to somefunc it "unpacks" the dictionary, so that essentially {'lastname': 'Pitkin', 'firstname': 'Matt'} becomes lastname="Pitkin", firstname="Matt" when input into function. If you did not supply either firstname or lastname to anotherfunc then the default values defined in somefunc would be used.

If there are other keyword arguments that you could pass to anotherfunc then trying to pass them to somefunc will not work:

print(anotherfunc(firstname="Matt", lastname="Pitkin", random="blah"))
{'random': 'blah', 'lastname': 'Pitkin', 'firstname': 'Matt'}
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-8-cf7c1c75bd94> in <module>()
----> 1 print(anotherfunc(firstname="Matt", lastname="Pitkin", random="blah"))

<ipython-input-6-b382a5e46b0e> in anotherfunc(message, **kwargs)
      3 
      4     # use somefunc
----> 5     output = somefunc(**kwargs)
      6 
      7     return f"{message} {output}"

TypeError: somefunc() got an unexpected keyword argument 'random'

So, alternatively you could have:

def anotherfunc(message="Hello", **kwargs):
    print(kwargs)  # print statement is just for show here

    # only get keyword args required for somefunc
    somefunckwargs = {}
    if "firstname" in kwargs:
        somefunckwargs["firstname"] = kwargs.pop("firstname")
    if "lastname" in kwargs:
        somefunckwargs["lastname"] = kwargs.pop("lastname")

    # use somefunc, but unpacking somefunckwargs
    output = somefunc(**somefunckwargs)

    return f"{message} {output}"

There is similar unpacking for positional arguments using the *args syntax in the function definition, but it is generally safer to use keyword arguments.

Documenting functions

The above examples are lacking one key feature: Documentation! It is very good practice to document your code. This helps you and others know what your code is supposed to do. The documentation of a function should briefly describe what it does, list its arguments, and state what it returns. We will demonstrate this using a couple of the above examples.

Example 1

def sum_and_product(a, b):
    """
    Calculate the sum and product of two numbers.

    Parameters
    ----------
    a: float
        A floating point number
    b: float
        A floating point number

    Returns
    -------
    sum, product: tuple
        The resulting sum and product
    """

    totalsum = a + b
    totalproduct = a * b

    # return both values
    return totalsum, totalproduct

The text between the """'s is known as a "docstring". In IPython you can access a function's docstring using ?, e.g.,

sum_and_product?

Signature: sum_and_product(a, b)
Docstring:
Calculate the sum and product of two numbers.

Parameters
----------
a: float
    A floating point number
b: float
    A floating point number

Returns
-------
sum, product: tuple
    The resulting sum and product
File:      ~/repositories/<ipython-input-27-b49e20c31d46>
Type:      function

Note

There are various styles of docstrings. The above example, and others in these notes, use the numpydoc style. But others are available. The best advice is to pick one and be consistent when using it.

As well as the docstring you should liberally comment your code using comment lines starting with #. Using descriptive variable names and leaving blank space lines between less closely related code blocks will also help with code readability.

Example 2

def in_range(value, minimum=-100, maximum=100):
    """
    Check whether a given value is within a given range, excluding the limits
    of the range.

    Parameters
    ----------
    value: (int, float, required)
        The value for checking
    minimum: (int, float)
        The lower limit of the range (defaults to -100).
    maximum: (int, float)
        The upper limit of the range (defaults to 100).
    """

    # return True if value is within minimum and maximum
    return minimum < value < maximum

Type hinting

When defining a function you do not need to specify the type of each argument. When writing a docstring you should however let the user know what type of object each argument should be. Another way of doing this is to use "type-hints". This is way of "hinting" at what type each argument should be in the function definition. If we take the examples above, we could have:

# use Tuple to show that the returned value is a tuple
from typing import Tuple

def sum_and_product(a: float, b: float) -> Tuple(float, float):
    """
    Calculate the sum and product of two numbers.

    Parameters
    ----------
    a: float
        A floating point number
    b: float
        A floating point number

    Returns
    -------
    sum, product: tuple
        The resulting sum and product
    """

    totalsum = a + b
    totalproduct = a * b

    # return both values
    return totalsum, totalproduct

or

# use Union to define multiple allowed types
from typing import Union

def in_range(value: Union[float, int], minimum: Union[float, int] = -100, maximum: Union[float, int] = 100) -> bool:
    """
    Check whether a given value is within a given range, excluding the limits
    of the range.

    Parameters
    ----------
    value: (int, float, required)
        The value for checking
    minimum: (int, float)
        The lower limit of the range (defaults to -100).
    maximum: (int, float)
        The upper limit of the range (defaults to 100).
    """

    # return True if value is within minimum and maximum
    return minimum < value < maximum

Note

Type hints are purely for documentation purposes. Python will not check that input arguments are actually of the "correct" type unless you explicitly do it within the function (see here for an example).