When asked to create system XYZ and you ask to do it in Python over PHP or Ruby, what are the main features you can mention when they require you to explain it?
This is one of those cases that really boil down to personal preference or situational details. If you're more comfortable and experienced with Python, then say so. Are they asking you to justify it because they're more comfortable with one of the other environments? After you're done, will the system be passed off to someone else for long-term maintenance?
If they ask you to use a technology or language that you're not as familiar with, then make sure they know up-front that it's going to take you longer.
How to sell Python to a client/boss/person