[Distutils] How to specify dependencies in Python

Paul Moore p.f.moore at gmail.com
Fri Jan 13 08:07:14 EST 2017

On 13 January 2017 at 12:23, Thomas Güttler
<guettliml at thomas-guettler.de> wrote:
> Am 12.01.2017 um 13:43 schrieb Nick Coghlan:
>> On 12 January 2017 at 22:04, Thomas Güttler
>> <guettliml at thomas-guettler.de> wrote:
>>> I came across a python library which has docs, which start like this:
>>> {{{
>>>  Quickstart
>>>  Include foolib in your requirements.txt file.
>>> }}}
>>> AFAIK dependencies should be specified via `install_requires` in
>>> `setup.py`.
>> Applications and services don't necessarily have a setup.py file -
>> setup.py is more for pip-installable libraries and frameworks (see
>> https://caremad.io/posts/2013/07/setup-vs-requirement/ for more on
>> that topic).
> What is an application?
> The django project uses this definition:
> https://docs.djangoproject.com/en/1.10/ref/applications/
> I guess you mean something else, if you talk about "application".
> What is an application for you?

IMO, an "application" in this context is a standalone program
(implemented in Python). This is as opposed to a "library" which is
written to be imported in other Python code. Obviously, there's some
overlap - some "applications" provide a "programming API" that can be
imported, and some libraries provide command line entry points.

A library (something that you import) should declare in its metadata
that you need to have its dependencies installed before it will work.
That lets you (the user of the library) be ignorant of the
dependencies of the library (which are implementation details as far
as you are concerned) - pip just sorts it out for you from the
metadata. To get the dependency metadata, the *library* should include
its dependencies in install_requires in its setup.py.

For applications on the other hand, you need to install exactly the
environment you tested, so you *don't* use dependency metadata.
Rather, you create a requirements.txt file that contains the exact
versions of everything (direct dependencies) your application needs,
then deploy your application script to a Python environment where
you've run "pip install -r requirements.txt" to set it up correctly.

As I say, there are lots of grey areas here. But from your
description, the library you found sounds like it should have
dependency metadata, and not need you to use a requirements file. On
the other hand, the "Quickstart" may be trying to tell people how to
use foolib in their *application*, in which case it's correct. (It's
oversimplified, as if you're writing a library that depends on foolib
you'd use install_requires, but simply changing the text to just say
"put foolib in your install_requires" is no better, as then you'd have
ignored the application use case...)

If you can point to the actual library you're referring to, it would
be easier to be specific :-)


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