Editing Python Project Dependencies

[[Category:Python]] [[Category:Web Development]]

== Workflow overview ==

Add a new pip dependencies to `requirements-to-freeze.txt`.

<syntaxhighlight lang="bash">
# Install the dependencies with:
$ pip install -r requirements-to-freeze.txt --upgrade

# Save the specific dependency versions with:
$ pip freeze > requirements.txt
</syntaxhighlight>

See [https://www.kennethreitz.org/essays/a-better-pip-workflow A Better Pip Workflow] for more details.

== requirements.txt ==

First, for every project create and load a [[Virtual Environments with Python|virtual environment]] specific to the project before installing dependencies.

Project dependencies are stored in a file named `requirements.txt` in the top level of the project, e.g.: <ref>[https://www.fullstackpython.com/application-dependencies.html Application Dependencies], Full Python Stack</ref>

`requirements.txt` defines ''concrete'' dependencies for non-redistributable applications. It defines the dependencies between packages and the source for those packages, especially if they don't come from PyPi.

An example `requirements.txt`:

<syntaxhighlight lang="txt">
django==1.6
bpython==0.12
django-braces==0.2.1
django-model-utils==1.1.0
logutils==0.3.3
South==0.7.6
requests==1.2.0
stripe==1.9.1
dj-database-url==0.2.1
django-oauth2-provider==0.2.4
djangorestframework==2.3.1
</syntaxhighlight>

=== Saving dependencies ===

Anytime a package is installed with `pip`, it must be added to `requirements.txt`. Or better yet, add the package to `requirements.txt` and use the file to install the new package and verify the project's environment.

A quick and reliable way to track all the packages currently installed with pip is with 

<syntaxhighlight lang="bash">
$ pip freeze > requirements.txt
</syntaxhighlight>

This saves the packages along with their current version. This means that installing from `requirements.txt` in the future might mean installing older versions of packages. [https://www.kennethreitz.org/essays/a-better-pip-workflow This article] has a recommendation for tracking both the top-level packages and all their dependencies. <ref>[https://www.kennethreitz.org/essays/a-better-pip-workflow A Better Pip Workflow] - Kenneth Reitz</ref>

=== Installing dependencies ===

Dependencies in `requirements.txt` can be installed with:

<syntaxhighlight leng="bash">
$ pip install -r requirements.txt
</syntaxhighlight>

== setup.py ==

In addition to `requirements.txt`, `setup.py` is another file used to check for dependencies when installing a package. <ref>[https://caremad.io/posts/2013/07/setup-vs-requirement/ setup.py vs requirements.txt], Donald Stufft</ref>

`setup.py` defines ''abstract'' dependencies for redistributable packages. The specific ''source'' for those dependencies should be defined elsewhere, e.g. `requirements.txt`. 

Given a directiory containing a `setup.py` file, a `requirements.txt` file can be written that will install all dependencies found in `setup.py` from PyPi: 

<pre>
--index-url https://pypi.python.org/simple

-e .
</pre>

If any dependencies should come from another source, `requirements.txt` can be modified similar to this:

<pre>
--index-url https://pypi.python.org/simple

-e https://github.com/foo/bar.git#egg=bar
-e .
</pre>

`pip` will first install the `bar` library from GitHub, then move on to install all the dependencies found in `setup.py`. Since the `bar` library has already been installed at this point, it will be skipped over when installing libraries from PyPi.

=== Installing packages defined in setup.py ===

Command to install the project dependencies defined in `setup.py`: <ref>[https://stackoverflow.com/questions/1471994/what-is-setup-py What is setup.py] - Stackoverflow</ref> <ref>[https://docs.python.org/3/installing/index.html#installing-index Installing Python Modules] - Python Software Foundation</ref>

<syntaxhighlight lang="bash">
$ python setup.py install
</syntaxhighlight>

== Alternate workflow ==

Save the top-level dependencies in one file, then save the specific dependencies in `requirements.txt`.

See [https://kenreitz.org/essays/a-better-pip-workflow A Better Pip Workflow] for details. 

But basically, on the development server create a file named `requirements-to-freeze.txt`. In that file list the top-level packages used by the project. 

<pre>
$ pip install -r requirements-to-freeze.txt --upgrade
$ pip freeze > requirements.txt
</pre>

== Notes ==

=== See also ===

* [[Defining Django App Dependencies]]

=== References ===

<references/>