Assignment with "or" in python

No, it's a common practice. It's only considered bad style for expressions that are considerably longer than yours.

I also feel a bit unconfortable using that kind of expressions. In Learning Python 4ed it is called a "somewhat unusual behavior". Later Mark Lutz says:

...it turns out to be a fairly common coding paradigm in Python: to select a nonempty object from among a fixed-size set, simply string them together in an or expression. In simpler form, this is also commonly used to designate a default...

In fact, they produce concise one-line expressions that help to eliminate line noise from the code.
This behavior is the basis for a form of the if/else ternary operator:

A = Y if X else Z

The primary danger of doing something like this is the possibility that (in the second case) some_variable is False but not None (the integer 0, for instance) and you don't want to end up with y equal to None in that case.