Python code looks like Python code. Its structure is rigid enough to let you take on new Python code and (unless it's been too heavily inspired by Ruby code or similarly makes injudicious use of certain powerful Python features) you'll just "get it". Python's strict structure and syntax enhance this.
Ruby? Ruby is a language, but it's sort of more like a meta-language, with the popularity of the concept of using DSLs. Its studied lack of syntax makes it look to me like you often need to know the codebase to know what to do with the code.
(I speak this as one extensively familiar with Python but who has only a passing acquaintance with Ruby at present.)
