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'.
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5That's not what a "side effect" is.Greg Hewgill– Greg Hewgill2012-01-05 18:42:01 +00:00Commented Jan 5, 2012 at 18:42
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4@Ramin I know you're not assigning a boolean. My point is that I don't see any side effect.jcollado– jcollado2012-01-05 19:28:54 +00:00Commented Jan 5, 2012 at 19:28
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5@Ramin: "A somewhat unusual behavior", "side effect" has a very different meaning in computer science. en.wikipedia.org/wiki/Side_effect_(computer_science)joaquin– joaquin2012-01-05 19:47:02 +00:00Commented Jan 5, 2012 at 19:47
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4Yes Python has them, terrible, dark, dangerous (a Haskeller)joaquin– joaquin2012-01-05 20:19:03 +00:00Commented Jan 5, 2012 at 20:19
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2Related: What does an 'x = y or z' assignment do in Python?Stevoisiak– Stevoisiak2018-05-04 13:55:47 +00:00Commented May 4, 2018 at 13:55
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4 Answers
No, it's a common practice. It's only considered bad style for expressions that are considerably longer than yours.
2 Comments
diginoise
It is considered an anti-pattern in JavaScript. It does not read naturally to uninitiated and you cannot assign falsy values.
cjs
@diginoise Nothing in Javascript or any other language reads naturally to the sufficiently uninitiated. But once a developer is familiar with short-cut evaluation (as used in, e.g.,
if foo or bar..., which is quite a common idiom in JS, Python and many other languages) it reads just as naturally, though you may have to think about it for a moment the very first time you see it. However, "falsy" values are a problem in Python, too, as with 'list = []; return list or False`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.
Comments
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! https://en.wikipedia.org/wiki/Null_coalescing_operator
This question may be useful as well:
Is there a Python equivalent of the C# null-coalescing operator?
Comments
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
2 Comments
Roee Gavirel
But it may cause to repeat a long parameters where
Y = X. For instance: A = my_main_dictionary.my_sub_dictionary.my_value if my_main_dictionary.my_sub_dictionary.my_value else None or A = my_main_dictionary.my_sub_dictionary.my_value or None
winklerrr
@RoeeGavirel just assign
my_value to a local variable