One of the strongest capabilities of Python is the ability to use an impressive catalog of third-party packages that cover an amazing amount of ground in different areas, from modules specialized in performing numerical operations, machine learning, and network communications, to command-line convenience tools, database access, image processing, and much more!

Most of them are available on the official Python Package Index (https://pypi.org/), which has more than 130,000 packages ready to use. In this book, we'll install some of them, and in general spending a little time researching external tools when trying to solve a problem is time well spent. It's very likely that someone else has created a tool that solves all, or at least part, of the problem.

As important as finding and installing a package is keeping track of which packages are being used. This greatly helps with replicability, meaning the ability to start the whole environment from scratch in any situation.

The starting point is to find a package that will be of use in our project.

A great one is requests, a module that deals with HTTP requests and is known for its easy and intuitive interface, as well as its great documentation. Take a look at the documentation, which can be found here: http://docs.python-requests.org/en/master/.

We'll use requests throughout this book when dealing with HTTP connections.

The next step will be to choose the version to use. In this case, the latest (2.18.4, at the time of writing) will be perfect. If the version of the module is not specified, by default, it will install the latest version, which can lead to inconsistencies in different environments.

We'll also use the great delorean module for time handling (version 1.0.0 http://delorean.readthedocs.io/en/latest/).

1. Create a requirements.txt file in our main directory, which will specify all the requirements for our project. Let's start with delorean and requests:

delorean==1.0.0
requests==2.18.4

2. Install all the requirements with the pip command:

$ pip install -r requirements.txt
...
Successfully installed babel-2.5.3 certifi-2018.4.16 chardet-3.0.4 delorean-1.0.0 humanize-0.5.1 idna-2.6 python-dateutil-2.7.2 pytz-2018.4 requests-2.18.4 six-1.11.0 tzlocal-1.5.1 urllib3-1.22

3. You can now use both modules when using the virtual environment:

$ python
Python 3.6.5 (default, Mar 30 2018, 06:41:53)
[GCC 4.2.1 Compatible Apple LLVM 9.0.0 (clang-900.0.39.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import delorean
>>> import requests

The requirements.txt file specifies the module and version, and pip performs a search on pypi.org.

Note that creating a new virtual environment from scratch and running the following will completely recreate your environment, which makes replicability very straightforward:

$ pip install -r requirements.txt

Note that step 2 of the How to do it... section automatically installs other modules that are dependencies, such as urllib3.

If any of the modules need to be changed to a different version because a new version is available, change it using requirements and run the install command again:

$ pip install -r requirements.txt

This is also applicable when a new module needs to be included.

At any point, the freeze command can be used to display all installed modules. freeze returns the modules in a format compatible with requirements.txt, making it possible to do this to generate a file with our current environment:

$ pip freeze > requirements.txt

This will include dependencies, so expect a lot more modules in the file.

In some cases, installing packages may require additional tools, such as compilers or a specific library that supports some functionality (for example, a particular database driver). If that's the case, the documentation will normally explain the dependencies.

• The Creating a virtual environment recipe
• The Using a third-party tool—parse recipe