Bast practices for writing new code on existing project?

I wonder what it is better to do when working on an existing project and facing different "style" of coding and organization of source code in general.
It is better to keep my style of coding about conventions on naming, documentation, spacing, etc... (personally I work with python and always try to stick to standard PEPs) or adapt it to the existing one? What if the existing one is unclear or inconsistent itself and refactoring is not an option?

I tried reading articles about collaborative coding.