Python interface validation

When I started working on update-manager I thought using zope.interface for my interfaces was a good idea, but soon realized that it lacked a way of actually validating a given interface against an implementation. The only thing it did was checking whether the implementation defined that it implements the interface.

Now, whilst writing some unit tests for update-manager I came up with a simple way of doing "real" validation, and I would like to share that Python code with you.

Firstly, I'd like to give you an overview of which checks my code carries out:

  • Mandatory method (raises NotImplementedError in interface definition) is not implemented (also raises NotImplementedError in implementation)

  • Optional or mandatory method is of correct type (static method versus instance method)

  • Optional or mandatory method has a different signature (argument count is different)

I consider at least the first and last check viable for validation of an interface against its implementation. The second check I listed is not that useful, and may produce false positives when someone uses certain decorators, I did not carry out any tests on that myself though.

The code can be found in update-manager's repository (link) and (for now) is licensed under the GPLv2 or later. I am willing to distribute this code as a separate Python module (maybe under a more permissive license like the LGPL) if enough (let's say at least two) people are interested in it, so please let me know if you like it.

Apart from the code itself the unit tests in the file linked above should explain how this beast exactly works.

Happy hacking!


update-manager weekly update #2

First of all: yes, I skipped update #1. I was rather busy with some assignments and exams at university and didn't work that much on update-manager the past two weeks.

Anyways, this update contains everything that has happened since update #0.

Changelog fetching

The changelog fetching code has been added to update-manager. This means that the changelog will be shown in the details section now and should look the same it looked before. However, I have only written that code for Debian so far, but the Ubuntu part is on my TODO list.


The documentation has been updated and uploaded to alioth and can be viewed here. I have set up a python environment on alioth which allows building the documentation directly, rather than building it locally and uploading it then. Basically this works by having a separate python packages directory, containing some mock modules that are needed (think gtk and friends here), allowing us to build the docs without having to install all dependencies.
I am planning on elaborating on this method and how to create such an environment in one of my upcoming posts, so stay tuned if you could use something like this too.

Additionally to this environment the documentation has been updated a great deal, including more modules and containing documentation for previously undocumented methods and classes.

Application module

I have reworked some aspects of the UpdateManager.Application module, allowing me to do unit testing on pretty much every aspect of the class. The problem I fixed here is that Application directly called sys.exit when something went wrong and now raises exceptions, which contain the status code and are handled in the respective scripts (ie. "update-manager").

Gtk Frontend and updates from another thread

One thing I fixed was the problem caused by the changelog fetching code running in a separate thread and invoking a callback function that updates the UI. It seems as Gtk isn't that happy when you do this and the UI wouldn't be updated immediatly (it seemed that this only happened after some events, like scrolling the update list). This has been reworked and the callback function now checks if it was called from the main thread or not and calls gtk.gdk.threads_enter/_leave accordingly.

Changelog Viewer

After finishing the changelog fetching code I added the ChangelogViewer widget from previous update-manager versions again, supporting creation of links to launchpad and debian bugs (ie. LP:NNNNNN and Closes: #NNNNNN are now links) and displaying the version number in bold, among other things.

Weeding out UpdateManager.Frontend.Gtk.utils

Initially I just copied over the utils module from old update-manager to the new implementation, leaving every single function in there, but now I decided to weed out the module. The result is that only the functions actually used by this implementation remained in there. Related to this documentation of that module is pending and on my TODO list.

Version number

After a chat with my mentor we decided to bump update-manager's version to 0.200-pre. This should make it easier to distinguish from the old version and indicates that a lot has changed. The first release following the -pre series will be 0.200.0, which should then include all functionality old update-manager included.

My TODO list for next week

Ordered by priority

  • Documentation of UpdateManager.Frontend.Gtk.utils and .ChangelogViewer modules

  • Ubuntu Distribution Specific code

  • More unit tests

  • Update list downloading in Gtk frontend


Should CLI debug output and error messages be localized in a GUI application?

Whilst working on update-manager I have been wondering whether I should use gettext for localizing debug output and error messages sent to stderr.
As for debug output itself I basically do not see the need for providing a localized version for each and every message sent to stderr, but as far as error messages are concerned I am uncertain.

The point is that update-manager (apart from its experimental text interface) is usually not launched from a terminal at all and so most users won't even see these messages ever. Also, I believe that every developer's English skills are good enough so that he or she is able to understand simple messages.
Error messages however might be useful to all users when they experience a problem with the software, but localizing those could make handling bug reports a bit harder, possibly having to translate the error message back to English before being able to see what has gone wrong.

So basically I am asking you: What do you think? Is it worth localizing these messages? What is your experience with localized or non-localized error and debug messages?

I would be glad if I could get some input from you, either as a comment to this article, via email to debian(dot)sp(dot)or(dot)at or through the update-manager-devel mailing list.


sphinx-aware Enums in Python

As I promised to keep you updated on recent developments on update-manager I am writing this article. Just as a disclaimer: I am not going to write about any recent developments here, but would rather like to point at a piece of code I added to update-manager that could be useful in other applications too.

Now, as the title suggests there are sphinx-aware Enums in update-manager. Enums are common constructs in other programming languages like C and allow simple creation of constants with, for example, ascending values (first constant has value 0, second has value 1 and so on). Python unfortunately does not include support for Enums itself, but I found it rather easy to write classes that emulate such a construct.

Nothing is new about Enums in Python and there are probably quite a few different implementations out there, but I believe mine is different. The sphinx-aware part means that my implementation automagically updates the docstrings of the created instances and thus allows sphinx' "autodata" method to include sensible information in generated API documentation.

I could go on writing about and praising my method, but I believe a short example gives you a better idea how my implementation works and what I wanted to achieve with this. Have a look at this page, which is part of update-manager's new API documentation. You should see rather well-looking documentation of the UpdateManager.Backend.RELOAD_CACHE_STATUS NegativeEnum, the defined constants, their values and some additional information about each value now.

Still, nothing too fancy, HTML documentation generated from docstrings. What makes this special is the code from which it was generated:

BEGIN = "Started reloading package cache",
DONE = "Finished reloading package cache")

This not only gives us a RELOAD_CACHE_STATUS enum, along with the RELOAD_CACHE_STATUS.BEGIN and RELOAD_CACHE_STATUS.DONE, but also some documentation, included in RELOAD_CACHE_STATUS' docstring, that can be used by sphinx.

You can find the Enum code, which is rather short and should be quite easy to understand, here. I hope you find this code as useful as I do.