Python packages with extras dependencies

Python packages can specify extras dependencies, which are sets of dependencies not required for core functionality, and which generally correspond to some feature. These can then be specified by downstream consumers of the package. For example, requests has an entry in extras called security[1], which currently adds requirements of python packages pyOpenSSL >= 0.14, cryptography >= 1.3.4, and idna >= 2.0.0. A downstream consumer that wants to use this would add a dependency on requests[security].

From what I can tell, the current practice in Fedora packaging is to ignore these. This simplifies packaging Python modules that have extras specified, but ultimately pushes the specification of those dependencies down into every consumer of the package, whether users or other packages.

As an example of this, I currently maintain the python-dns-lexicon package, which provides a common CLI and API for various different DNS providers. Some of the providers have additional dependencies that are necessary to function, and which are specified as extras. The Plesk provider, for example, also requires python-xmltodict[2]. In line with what appears to standard practice, extra dependencies are not currently installed with the broader python-dns-lexicon package. If, however, a user or dependent package wants to utilize the Plesk functionality of python-dns-lexicon, they now need to know that python-xmltodict needs to be installed, and will need to check whenever the package updates as to whether or not that has changed.

How should we be handling this? Right now, it seems that most packages follow this behavior of punting on the responsibility to package consumers. Should this continue? If not, how should we handle things? Should we just include all extras dependencies in the parent package? Alternatively, should we have dummy/meta subpackages for extras that require the parent package as well as any extras dependencies (e.g. python-dns-lexicon-plesk would require python-dns-lexicon and python-xmltodict)?

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Re: Python packages with extras dependencies

By Petr Viktorin at 02/05/2019 - 05:53

On 2/5/19 12:44 AM, Eli Young wrote:


AFAIK, there are currently no official guidelines for Python extras, and
there's some fame & glory waiting if you'd like to help draft them :)

That said, I believe subpackages are the answer here.
In addition you could use weak dependencies. I think the main package
should have "Suggests:" for all the extras, and even "Recommends:" for
ones that are almost always useful.
I believe the cost of extra dependencies is lower in Fedora than on
PyPI. Some common reasons for punting deps to extras don't apply (like
bootstrapping issues or requiring compiled modules from pure-Python
ones), so sometimes it's good to just go for hard Requires.

Purely as a packager, I'd like to see "boolean Provides", something like:
Provides: (python3dist(dns-lexicon[plesk]) if python3dist(xmltodict))
... but at a chat with RPM devs on the last Flock, I learned that is not

The Python SIG (<a href="mailto:python- ... at lists dot">python- ... at lists dot</a>) would be a good
place to discuss specific details.

Re: Python packages with extras dependencies

By =?UTF-8?B?TWlyb... at 02/04/2019 - 18:59

On 05. 02. 19 0:44, Eli Young wrote:
This (metapackages) is what I've done in python-trimesh:

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I'd still consider this on case by case basis instead of developing a general
solution, sometimes a simple Recommends works. Sometimes, it's more complicated.

I'm CCing packaging and python to get more attention to this, but please keep
the discussion on devel, so it's not shattered.

Re: Python packages with extras dependencies

By Eli Young at 02/15/2019 - 15:27

That's fair and makes a lot of sense. I do like the pattern mentioned by Igor as used for Rust packages of using + as a separator for the features/extras.

Re: Python packages with extras dependencies

By Igor Gnatenko at 02/17/2019 - 03:50

Just make sure before start using it to implement support into the RPM's
Python dependency generator.

Re: Python packages with extras dependencies

By Igor Gnatenko at 02/05/2019 - 03:13

In Rust we have similar problem (we have "features" than "extras") and we
always package them as a subpackages.

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And the dependencies for all those subpackages are automatically generated.
We could do the same with Python.