I just spent the better part of the evening and early night to hack together a Mercurial extension to check for and fix tabs and trailing whitespace in files to be committed.
Diving into a (relatively) poorly documented Python API is not easy. My previous attempts with Django (extremely good docs) and BuildBot (perfectly acceptable docs) went much better in terms of tackling the early learning curve.
As an example, I have not yet managed to figure out how to get the added diff lines of the working directory, or the files included in an upcoming commit (this are slightly different). The MercurialAPI wiki page only gives a brief overview, after that you're on your own and have to use the source. This is pretty fine, except that dynamic languages need more written docs if the source is to be understood better.
Anyway, the extension works pretty nicely as I decided to check the _entire_ file instead of just the changed lines. Shouldn't make a big difference performance wise, and it will allow a code base to go from mixes-tabs to spaces-only much faster. I can always revisit that part later if the need arises.
The next part in the plan is to write a small script that auto-injects this into each developers hgrc file on checkout. (I have a script that one must run, more or less, to deploy/unzip third-party libraries and set up environment variable, so it can just as well edit the hgrc file too.) I'm not worried about the security implications currently, as we're four developers on a corporate network, so everyone should play nicely at the moment, and secondly, I can always safeguard the paths to the extension (or put in in a subrepo) to which no-one but a select few, including myself, have commit/push rights (in case of a rogue developer).
The next little script I write will probably be something like that, to allow deployment of local extensions. I'll also add some extra meta-data to a commit and a central-repo hook to check for that, so that one can't push without the meta-data (ensuring that everyone starts using the hook ASAP) and also run the same checks there (in case someone decides to override the hook).
Hacking should not happen out of pure malice, but I've had developers disabling the global warnings-in-log-at-exit popup just because they where annoyed by them. (The best way would've been to fix the warnings inside the app, or at least have a chat with me on which log messages are warnings and if it's worthwhile to add a feature where said developer may disable the popup for a few hours.) Since the popup was disabled, many others in the team missed warnings or errors in the log and subsequently the CI build failed with warnings after commit. Sadly quite contraproductive and against the spirit of early-warnings.
Summing it up, I really enjoy hacking together small things in Python, but for larger projects it seems to become pretty unbearable and you really want to know the types of the variables. I really wish C# and WPF was available on more than one platform, so that we could go with that instead of C++/Python on Qt. It does seem to be a nicer road to walk.
(Yes, I've thought about Java, and I've worked with it for a few years. And no, I don't really like it, but I suppose SWT is at leats a bearable GUI-API (i.e. better than Swing), but lately with the Oracle takeover I'm very wary of betting on Java going anywhere but down the drain for non-enterprise projects.)