Using External Modules

Overview

Teaching: 15 min
Exercises: 5 min

Questions

• How can I extend Python’s functionality?

• How can I use code other people have written?

Objectives

• Import modules into your python code

• Call the functions provided by these modules just like other code

We’ve now covered a lot of the basic syntax and commands in Python. However, you may be wondering how you do anything useful with this without creating lots of code for all sorts of mundane jobs such as reading in data, performing statistical analyses or producing plots. Luckily, Python has many external bits of code (called ‘modules’) that you can easily use within your own code.

Importing an External Module

Importing a module is like getting a piece of lab equipment out of a storage locker and setting it up on the bench. Moduels provide additional functionality to the basic Python package, much like a new piece of equipment adds functionality to a lab space. Just like in the lab, importing too many libraries can sometimes complicate and slow down your programs - so we only import what we need for each program.

If you want access to an external module in you code, all you have to do is:

import math

From this point on in your code, you can use the math name to access this module. For example, to add up all the numbers in a list you can do:

import math
nums = [1., 6., 9.23]
total = math.fsum(nums)
print(total)

This code asks Python to run the function fsum which belongs to the math module in a similar way to how the append function belonged to the list object.

As an example, John Smith is the John that belongs to the Smith family, We could use the dot notation to write his name smith.john, just as fsum is a function that belongs to the math module.

It’s not just functions that a module can provide - there can also be variables as well:

import math
print(math.pi)

If you just want to import a specific function rather than a whole module, you can say this when importing. You then only need to call the function name without the module name as well:

from math import fsum
nums = [1., 6., 9.23]
total = fsum(nums)
print(total)

Finally, you can also rename what something is imported as if it conflicts with another name you are using or you just want to make it clearer:

import math as sys_math
nums = [1., 6., 9.23]
total = sys_math.fsum(nums)
print(total)

Using the sys module

Another module we will be using in the future is the sys module which contains functions and variables that are associated with Python system that you are running in. One variable in particular which we will use later is a list of all the command line arguments passed to Python.

Either using the built in help system or Google, find the variable in the sys module that corresponds to the command line arguments and write a loop to print out each one.

Solution

import sys
for opt in sys.argv:
   print(opt)

Key Points

• To import a module, simply call import <module>

• You can import particular elements of a module using from <module> import <func>

• Rename a module in your code using import <module> as <myname>