# 2.1 Functions#

Estimated time for this notebook: 15 minutes

Defining functions which put together code to make a more complex task seem simple from the outside is the most important thing in programming. We can wrap code up in a function, so that we can repeatedly get just the information we want.

## 2.1.1 Definition#

We use def to define a function, and return to pass back a value: The input comes in in brackets after the function name:

def double(x):
return x * 2

print(double(5), double([5]), double("five"))

10 [5, 5] fivefive


## 2.1.2 Default Parameters#

We can specify default values for parameters:

def jeeves(name="Sir"):
return f"Very good, {name}"

jeeves()

'Very good, Sir'

jeeves("James")

'Very good, James'


If you have some parameters with defaults, and some without, those with defaults must go later.

If you have multiple default arguments, you can specify neither, one or both:

def jeeves(greeting="Very good", name="Sir"):
return f"{greeting}, {name}"

jeeves()

'Very good, Sir'

jeeves("Hello")

'Hello, Sir'

jeeves(name="James")

'Very good, James'

jeeves(greeting="Suits you")

'Suits you, Sir'

jeeves("Hello", "Sailor")

'Hello, Sailor'


## 2.1.3 Early Return#

Return without arguments can be used to exit early from a function

Here’s a slightly convoluted example of a function which will return early under specific conditions. In this case if a list contains the string ‘cat’.

def are_there_cats(my_input_list):

if "cat" in my_input_list:  # If the string "cat" is in the list
print("There is a cat in here")  # print a statement to screen
return

print("Nothing to see here")

first_list = ["cat", "dog", "hamster", 42]

second_list = ["duck", 17, "elk"]

are_there_cats(first_list)

There is a cat in here

are_there_cats(second_list)

Nothing to see here


## 2.1.4 Scoping#

There are differences in how variables and names are accessed by your code based on where they are defined.

Within this notebook any variables that have been defined outside of a function will be available to the rest of the notebook. At this point in the notebook, x has not been defined.

x

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
Cell In[15], line 1
----> 1 x

NameError: name 'x' is not defined


If we now define x and write and call a function in which uses it; the function can still access x, even if x isn’t given as an argument.

x = 5  # Define x now

def can_we_see_x():
print(f"x = {x}")

can_we_see_x()

x = 5


However if we define y locally - in a function - we can access it from within that function:

def can_we_see_y():
y = 7  # Define y in the function
print(f"x = {x}")
print(f"y = {y}")

can_we_see_y()

x = 5
y = 7


However y isn’t accessible globally - that is it isn’t available outside of the function in which it was defined

y

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-18-9063a9f0e032> in <module>
----> 1 y

NameError: name 'y' is not defined


Note for the two functions above we used syntax for building strings that contain the values of variables. You can read more about it here or in the official documentation for formatted string literals; f-strings.

## 2.1.5 Side effects#

Functions can do things to change their mutable arguments, so return is optional.

This is pretty awful style, in general, functions should normally be side-effect free.

Here is a contrived example of a function that makes plausible use of a side-effect

def double_inplace(vec):
vec[:] = [element * 2 for element in vec]

z = list(range(4))
double_inplace(z)
print(z)

[0, 2, 4, 6]

letters = ["a", "b", "c", "d", "e", "f", "g"]
letters[:] = []


In this example, we’re using [:] to access into the same list, and write its data.

vec = [element*2 for element in vec]


would just move a local label, not change the input.

See Module 1.5 - Memory and Containers for a refresher

But I’d usually just write this as a function which returned the output:

def double(vec):
return [element * 2 for element in vec]


Let’s remind ourselves of the behaviour for modifying lists in-place using [:] with a simple array:

x = 5
x = 7
x = ["a", "b", "c"]
y = x

x

['a', 'b', 'c']

x[:] = ["Hooray!", "Yippee"]

y

['Hooray!', 'Yippee']


## 2.1.6 Unpacking arguments#

def arrow(before, after):
return str(before) + " -> " + str(after)

arrow(1, 3)

'1 -> 3'


If a function that takes multiple arguments is given an iterable object prepended with ‘*’, each element of that object is taken in turn and used to fill the function’s arguments one-by-one.

x = [1, -1]
arrow(*x)

'1 -> -1'


This can be quite powerful:

charges = {"neutron": 0, "proton": 1, "electron": -1}
for particle in charges.items():
print(arrow(*particle))

neutron -> 0
proton -> 1
electron -> -1


## 2.1.7 Sequence Arguments#

Similiarly, if a * is used in the definition of a function, multiple arguments are absorbed into a list inside the function:

def doubler(*sequence):
return [x * 2 for x in sequence]

doubler(1, 2, 3)

[2, 4, 6]

doubler(5, 2, "Wow!")

[10, 4, 'Wow!Wow!']


## 2.1.8 Keyword Arguments#

If two asterisks are used, named arguments are supplied inside the function as a dictionary:

def arrowify(**args):
for key, value in args.items():
print(key + " -> " + value)

arrowify(neutron="n", proton="p", electron="e")

neutron -> n
proton -> p
electron -> e


These different approaches can be mixed:

def somefunc(a, b, *args, **kwargs):
print("A:", a)
print("B:", b)
print("args:", args)
print("keyword args", kwargs)

somefunc(1, 2, 3, 4, 5, fish="Haddock")

A: 1
B: 2
args: (3, 4, 5)