Building programs with Python

Modularising your code using functions

Learning Objectives

Define a function that takes parameters.
Return a value from a function.
Understand the scope of function variables and parameters.
Documenting a function.
Understand why we should divide programs into small, single-purpose functions.
Define and use a module that contains functions.

At this point, we’ve written some scripts to do various things, including one to loop through a data file and output its contents. But it’s not hard to imagine our code getting more complicated as we add more features.

We’ll see how we can amend our code to be better structured to further increase its readability, as well as its maintainability and reuse in other applications.

Converting from Fahrenheit to Celsius

Let’s look at adding a feature to our code to perform a conversion from Fahrenheit to Celsius on the temperature data we are looking at:

celsius = ((data[3] - 32) * (5/9))

Now this wouldn’t work as it is - we can’t just apply this formula directly to data[3] since it’s a string. We need to convert it to a number first. To be specific, a floating point number.

Fortunately, Python has some built-in functions to do these type conversions:

climate_data = open('../data/sc_climate_data_10.csv', 'r')

for line in climate_data:
    data = line.split(',')

    if data[0][0] == '#':
        # don't want to process comment lines, which start with '#'
        pass
    else:
        # extract our max temperature in Fahrenheit - 4th column
        fahr = float(data[3])

        # apply standard Fahrenheit to Celsius formula
        celsius = ((fahr - 32) * (5/9)) 

        print('Max temperature in Celsius', celsius)

So we first convert our data[3] value to a floating point number using float(), then we are free to use it in our conversion formula. Depending on the structure of your own data, you may find you end up doing this a lot!

So now we get:

Max temperature in Celsius 14.73888888888889
Max temperature in Celsius 14.777777777777779
Max temperature in Celsius 14.61111111111111
Max temperature in Celsius 13.838888888888887
Max temperature in Celsius 15.477777777777778
Max temperature in Celsius 14.972222222222225
Max temperature in Celsius 14.85
Max temperature in Celsius 16.33888888888889
Max temperature in Celsius 16.261111111111113
Max temperature in Celsius 16.33888888888889

Modularising conversion code into a function

Whilst this is a simple calculation, there are many things we may want to do that are more complex. What is essentially a single task may require a number of lines of code to accomplish it, and with many of these our code could become quite messy.

We’d also like a way to package our code so that it is easier to reuse, and Python provides for this by letting us define things called ‘functions’ - a shorthand way of re-executing longer pieces of code.

climate_data = open('../data/sc_climate_data_10.csv', 'r')

def fahr_to_celsius(fahr):
    # apply standard Fahrenheit to Celsius formula
    celsius = ((fahr - 32) * (5/9)) 
    return celsius

for line in climate_data:
    data = line.split(',')

    if data[0][0] == '#':
        # don't want to process comment lines, which start with '#'
        pass
    else:
        # extract our max temperature in Fahrenheit - 4th column
        fahr = float(data[3])

        celsius = fahr_to_celsius(fahr)

        print('Max temperature in Celsius', celsius)

The definition opens with the word def, which is followed by the name of the function and a parenthesized list of parameter names. The body of the function — the statements that are executed when it runs — is indented below the definition line, typically by four spaces.

When we call the function, the values we pass to it are assigned to those variables so that we can use them inside the function. Inside the function, we use a return statement to send a result back to whoever asked for it.

Note that the function is at the top of the script. This is because Python reads the script from top to bottom, and if we called the function before we defined it, Python wouldn’t know about it and throw an error like this:

Traceback (most recent call last):
  File "climate_analysis-6.py", line 13, in <module>
    celsius = fahr_to_celsius(fahr)
NameError: name 'fahr_to_celsius' is not defined

And when we run it again — which we most definitely should, to make sure it’s still working as expected — we see the same output, which is correct.

How do function parameters work?

We actually used the same variable name fahr in our main code and and the function. But it’s important to note that even though they share the same name, they don’t refer to the same thing. This is because of variable scoping.

Within a function, any variables that are created (such as parameters or other variables), only exist within the scope of the function.

For example, what would be the output from the following:

f = 0
k = 0

def multiply_by_10(f):
  k = f * 10
  return k

multiply_by_10(2)
multiply_by_10(8)

print(k)

This is really useful, since it means we don’t have to worry about conflicts with variable names that are defined outside of our function that may cause it to behave incorrectly.

Combining strings

“Adding” two strings produces their concatenation: 'a' + 'b' is 'ab'. Write a function called fence that takes two parameters called original and wrapper and returns a new string that has the wrapper character at the beginning and end of the original. A call to your function should look like this:

print(fence('name', '*'))
*name*

Palindrome String check

A “Palindrome” is a word, phrase, number, or other sequence of characters which reads the same backward or forward. Write a function (any name of your choice) that takes input_string as a parameter and returns either True or False based on whether the input string is a palindrome or not respectively. Try to make it case insensitive.
Following the function definition, a call to your function should look like this:

is_palindrome("hello")
False
is_palindrome("Deed")
True

Performing more temperature conversions

Of course, we can also add more functions. Let’s add another, which performs a conversion from Fahrenheight to Kelvin. The formula looks like this:

kelvin = ((fahr - 32) * (5/9)) + 273.15

Now, we could just add a new function that does this exact conversion. But Kelvin uses the same units as Celsius, the part of the formula that converts to Celsius units is the same. We could just used our fahr_to_celsius function for the unit conversion, and add 273.15 to that to get Kelvin. So our new function becomes:

def fahr_to_kelvin(fahr):
    # apply standard Fahrenheit to Kelvin formula
    kelvin = fahr_to_celsius(fahr) + 273.15
    return kelvin

Which we insert after the fahr_to_celsius function (since our new function needs to call that one). We can then amend our code to also call that new function and output the result. Our code then becomes:

climate_data = open('../data/sc_climate_data_10.csv', 'r')

def fahr_to_celsius(fahr):
    # apply standard Fahrenheit to Celsius formula
    celsius = ((fahr - 32) * (5/9)) 
    return celsius

def fahr_to_kelvin(fahr):
    # apply standard Fahrenheit to Kelvin formula
    kelvin = fahr_to_celsius(fahr) + 273.15
    return kelvin

for line in climate_data:
    data = line.split(',')

    if data[0][0] == '#':
        # don't want to process comment lines, which start with '#'
        pass
    else:
        # extract our max temperature in Fahrenheit - 4th column
        fahr = float(data[3])

        celsius = fahr_to_celsius(fahr)
        kelvin = fahr_to_kelvin(fahr)

        print('Max temperature in Celsius', celsius, 'Kelvin', kelvin)

Hmm… our code is starting to get a little large with these functions. What could we do to make it clearer and less cluttered?

Modularising conversion code into a library

Words are useful, but what’s more useful are the sentences and stories we build with them. Similarly, while a lot of powerful tools are built into languages like Python, even more live in the libraries they are used to build.

A library is a collection of code (precompiled routines, functions) that a program can use. They are particularly useful for storing frequently used routines because you don’t need to explicitly link them to every program that uses them. Libraries will be automatically looked for routines that are not found elsewhere.

So we can go one step further to improve the structure of our code. We can separate out the two functions and have them in a separate Python module (or library) which we can use.

Create a new file called temp_conversion.py and copy and paste those two functions into it, then save it, and remove those functions from the original climate_analysis.py script and save that. We’ll see how to use those library functions shortly. But first, let’s take this opportunity to improve our documentation of those functions!

The usual way to put documentation in software is to add comments, as we’ve already seen. But when describing functions, there’s a better way. If the first thing in a function is a string that isn’t assigned to a variable, that string is attached to the function as its documentation:

"""A library to perform temperature conversions"""

def fahr_to_celsius(fahr):
    """Convert Fahrenheit to Celsius.

    Uses standard Fahrenheit to Celsius formula

    Arguments:
    fahr -- the temperature in Fahrenheit
    """
    celsius = ((fahr - 32) * (5/9)) 
    return celsius

def fahr_to_kelvin(fahr):
    """Convert Fahrenheight to Kelvin.

    Uses standard Fahrenheit to Kelvin formula

    Arguments:
    fahr -- the temperature in Fahrenheit
    """
    kelvin = fahr_to_celsius(fahr) + 273.15
    return kelvin

A string like this is called a docstring. We don’t need to use triple quotes when we write one, but if we do, we can break the string across multiple lines. This also applies to modules

So how would we use this module and its functions in code? We do this by importing the module into Python.

Python 3.4.3 |Anaconda 2.3.0 (x86_64)| (default, Mar  6 2015, 12:07:41) 
[GCC 4.2.1 (Apple Inc. build 5577)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import temp_conversion

When modules and functions are described in docstrings, we can ask for these explanations directly from the interpreter which can be useful. Following on from the above:

>>> help(temp_conversion)

So here’s the help we get for the module:

Help on module temp_conversion:

NAME
    temp_conversion - A library to perform temperature conversions

FUNCTIONS
    fahr_to_celsius(fahr)
        Convert Fahrenheit to Celsius.
        
        Uses standard Fahrenheit to Celsius formula
        
        Arguments:
        fahr -- the temperature in Fahrenheit
    
    fahr_to_kelvin(fahr)
        Convert Fahrenheight to Kelvin.
        
        Uses standard Fahrenheit to Kelvin formula
        
        Arguments:
        fahr -- the temperature in Fahrenheit

FILE
    /Users/user/Projects/RSG/Training/2016-08-31-Southampton/novice/python/code/temp_conversion.py

Here, note we’ve used the term library in the code documentation. This is a more conventional, general term for a set of routines in any language.

Similarly, for Docstrings in functions, e.g.:

>>> help(temp_conversion.fahr_to_celsius)

Note that we need to put in temp_conversion. prior the function name. We need to do this to specify that the function we want help on is within the temp_conversion module.

So we get:

Help on function fahr_to_celsius in module temp_conversion:

fahr_to_celsius(fahr)
    Convert Fahrenheit to Celsius.
    
    Uses standard fahrenheit to Celsius formula
    
    Arguments:
    fahr -- the temperature in Fahrenheit

And then we need to import that function from our module into our script, so we can use it.

import temp_conversion

climate_data = open('../data/sc_climate_data_10.csv', 'r')

for line in climate_data:
    data = line.split(',')

    if data[0][0] == '#':
        # don't want to process comment lines, which start with '#'
        pass
    else:
        # extract our max temperature in Fahrenheit - 4th column
        fahr = float(data[3])

        celsius = temp_conversion.fahr_to_celsius(fahr)
        kelvin = temp_conversion.fahr_to_kelvin(fahr)

        print('Max temperature in Celsius', celsius, 'Kelvin', kelvin)

Like when we used the interpreter to ask for help on the fahr_to_celsius() function, we need to prefix the function with its temp_conversion module name.

Again, the results should be the same as before.