Depends on what kind of programming the given Python programmer does! If it involves computation on numerical arrays, numpy; for more general scientific programming, other parts of scipy; for scraping often-badly-written HTML, BeautifulSoup (perhaps but not necessarily as included in lxml; for SSH connections, tunneling, etc, paramiko; and so on, and so forth.
Few programmers find a need to develop all these kinds of software (and many, many more besides). Besides, in many important and popular areas, there isn't a single package that dominates the scene so utterly that "every" Python programmer "has" to know it -- such areas as GUIs and web server/application frameworks, for example, offer several worthwhile alternatives. Nowhere it is written that (for any given area of software development) "there must be only one";-).