All Python implementations that I've used on various platforms use IEEE-754 and would behave as you describe.
However, this does not appear to be formally mandated by the language. So much so that the tutorial has the following to say (emphasis mine):
Almost all machines today (July 2010) use IEEE-754 floating point arithmetic, and almost all platforms map Python floats to IEEE-754 “double precision”.
Here is a relevant thread from 2008: Python on non IEEE-754 platforms: plea for information.
It is very easy to check whether your interpreter/platform behave the way you expect. If you're critically dependent on this, you could easily detect non-compliance when your program starts up.