I strongly suspect that these three are not intended to be mutually exclusive.
For me, Python has at some point or another been in each category, though at most two simultaneously.
And for those of us who do web development, "bread and butter" can easily include a half dozen languages or more. After leaving a project where I personally (no joke) used AS3, Awk, C, C++, C#, Java, JavaScript, Perl, PHP, Python, and SQL (sqlite, MySQL, and Postgres!) [1], my current position is just a trivially "polyglot" mix of C++ and Java desktop apps. Before that, I was constantly afraid that I'd forever be doomed to be a jack of all trades, master of none, but now the majority of my time is gobbled up by languages I don't like very much. Can't have it both ways...
[1] mostly by accident, as the powers that be could never make up their minds about what they actually wanted
Happiness: Clojure
GTD: Clojure (for hacking stuff together on top of Java)
Bread and butter: Java
I get paid to develop Java, but use Clojure to call our Java libraries for quick prototyping of ideas and building tools for little tasks that come up. Other people at my company use Jython for this, so I end up doing some of that as well, which also works very well.
I had to answer Python/Python/what the project requires, just to be realistic. Not every organization jumps of joy hearing "python", some (who have no idea) perceive it like a perverted bash scripting. So have to use C/java/php depending on the task.
IMHO it is more important that you can reason why you chose a language as your Happiness/Hacking/Bread-n-Butter languange, than which language you chose. I think it's perfectly valid to have a language in more than one category.
Happiness: Python because it has such an amazing number of really fun libraries like numpy,matplotlib,openopt,mayavi, pybrain,simpy etc. etc. Clojure is a close second, but I think it might catch up with python as a) I get better at it and b) more interesting numeric libraries start to show up
Hacking: Python, because I know python really well, it has a very complete set of libraries and when all is said and done, for most tasks, I'll simply finish it faster in python than in any other language.
BnB: Python, because my last two jobs have been working on python apps. But also more and more javascript as we are moving some of our stuff to webapps.
For me, Python has at some point or another been in each category, though at most two simultaneously.
And for those of us who do web development, "bread and butter" can easily include a half dozen languages or more. After leaving a project where I personally (no joke) used AS3, Awk, C, C++, C#, Java, JavaScript, Perl, PHP, Python, and SQL (sqlite, MySQL, and Postgres!) [1], my current position is just a trivially "polyglot" mix of C++ and Java desktop apps. Before that, I was constantly afraid that I'd forever be doomed to be a jack of all trades, master of none, but now the majority of my time is gobbled up by languages I don't like very much. Can't have it both ways...
[1] mostly by accident, as the powers that be could never make up their minds about what they actually wanted