1. A personal project to help you learn Python, some Python library, etc. (Note, learning how a framework in another language does something is an excellent way to open your mind to new ways to do it in Perl.)
2. An existing Python project has a lot of the functionality you need, and extending it looks easier than rewriting it.
3. The team that will have to support and maintain it are Python developers rather than Perl developers.
4. A client is willing to pay you to write in Python, and is willing to accept your reduced productivity.

The only time I would want to leave an interviewer with those types of negative impressions of me is if working with a particular language was so distasteful that I really don't want a job where I have to do that. The only language that I feel that way about is VB. Certainly I don't feel that way about Python. Furthermore Python has some good features that I wish were in Perl. For instance Perl has nothing equivalent to Python's list comprehensions and generators.

As it happens, most of what I know about Python comes from three out of four of the motivations you list. I wanted to add a feature to an existing (open source) Python project (maintained by Python programmers, natch), in part because I wanted to learn about the language. I also discussed that in the same interview, just not in answer to that question.

That's good, and I want to be clear that I'm not accusing you of having the negative traits. I'm just saying that when you give that answer, you don't know how it will be heard. And some interviewers will hear that answer in a very bad way.

The potential for harm is mitigated a lot if you have already discussed your willingness to work with Python in other circumstances. But if the job they envision is 70% Python and 30% Perl, you've still potentially argued yourself out of a job for a skill that you realistically could come up to speed on well within the usual window of 1-3 months that most people need to become productive.

"what kind of project would you rather write in this other language you know rather than in Perl?"

• The environment it has to work on doesn't have the resources to have Perl. I've worked on Linux systems that only had 25 Mb of disk space, 8 Mb of RAM, and where installed from a single floppy disk. No way to cram Perl in there.
• Speed or memory usage is an issue. In such cases, I use C.
• There's a domain specific language much more suitable to solve the problem. SQL being an obvious example.
• Lack of enough Perl knowledge (or willingness to use it) in the rest of the team.
• Program needs to be distributed in an environment that may not have Perl. Hasn't been a reason for me in the last 10 years, but was before.
• Low level enough that it's easier to do in C than it would be in Perl.

Didn't one of the Fredericks of Germany once say something to the effect of "I speak English to my wife, French to my ministers, Italian to my mistress, and German to my horse?"

Maybe you're thinking of Charles V: I speak Spanish to God, Italian to women, French to men, and German to my horse.

I'm sure you're correct. Alas, my memory seems to suffer from bit rot.

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Information about American English usage here and here. Floating point issues? Please read this before posting. — emc

I'm quite sure it's this Charles V.