Assignment with "or" in python


Is it considered bad style to assign values to variables like this?

x = "foobar" or None
y = some_variable or None

In the above example, x gets the value ‘foobar’.

Asked By: TheOne



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

Answered By: tback

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
Answered By: joaquin

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.

Answered By: Free Monica Cellio

OP’s syntax is perfectly fine.
The official name for "assignment with or" is null coalescing and there’s actually a Wikipedia page about it now!
This question may be useful as well:
Is there a Python equivalent of the C# null-coalescing operator?

Answered By: DonCarleone