Using Python modules

Standard Python provides a very wide range of functions and objects that allow you to easily perform various advanced operations. This makes writing Python programs much easier than using other popular programming languages ​​such as C++. The most common and frequently used functions (e.g. print, input, len, and range) are always available. They are so-called built-in functions. However, the vast majority are grouped in packages and modules that must be imported when needed.

To import a module, you should issue the command:

import module_name

An example would be a module random that provides functions for generating pseudo-random numbers. We import it with the command

import random

This command does not make all functions and objects defined in this module directly available. To get to them you have to use notation module_name.function(...). For example, to generate a random number in the range [a, b] we can use the function uniform from the module random:

random_number = random.uniform(-1, 1)

A complete list of built-in Python modules and functions can be found in the documentation. In particular, it is worth reviewing the module descriptions sys and os which contain a set of basic functions and objects for using the operating system tools. For example, you can check the current version of Python with:

import sys

print(sys.version)

In this case, sys.version is not a function, but just a string. However, it is an object defined inside a module.

Modules may contain other modules nested in a hierarchical manner: any module may contain sub-modules (master modules are then called packages). To import the selected sub-module, use the command import package.submodule. There can be any number of nesting levels: in such case, enter all of them, separating subsequent levels with a dot. Useful functions and objects can be found both in the top module and in submodules. Submodules are used to group a set of functions. For example, a module os contains functions that allow you to perform various operations on files on the disk, and a module os.path provides functions that allow you to manipulate filesystem paths:

import os.path

print(os.path.join('directory', 'file.txt'))

Simplified module loading

The fact that the command import only defines the name of the module keeps the program code in order. In particular, there is no problem with functions with the same names in several modules. As their use requires the name of the module before the dot, there will be no collisions between them. However, sometimes—especially with nested modules—this can be cumbersome (e.g. when you have to frequently type moduleA.moduleB.moduleC.function1(), moduleA.moduleB.moduleC.function2() etc.) There are two methods to deal with this. The first one is to import any module (also nested) with its own name, using the keyword as:

import moduleA.moduleB.moduleC as mc

Then instead of moduleA.moduleB.moduleC.function1() we can just write mc.function1(), which is much shorter. For example

import os.path as p

print(p.join('directory', 'file.txt'))

Remember that the names we give the imported modules should be meaningful: even if it is a one- or two-letter abbreviation (in the example above, the first letter of the submodule name path was used).

The second method allows you to directly load selected functions and other objects from the module and access them without prefixing them with the module name. For this, we use the following syntax:

from module_name import function

module_name can have the form package.submodule, as in the previously described cases. You can import more than one function from a single module in such a way. If there is more than one, they should be separated by a comma. Functions imported in this way can be used directly. E.g:

from numpy import pi, sin, cos

print(sin(pi / 2))

which is equivalent to:

import numpy

print(numpy.sin(numpy.pi / 2))

Note, however, that in using this approach, we run the risk that the imported function will replace the built-in function. Moreover, in the program code we do not clearly see which module it comes from. Therefore, please use this method with caution.

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