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In javascript, foo = x || "default" is often coded. If x = null, foo = "default".

In python, is foo = x or "default" correct?

Thomas Orozco
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hashikami
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  • If x is `0` or any "falsy" value `foo` will be set to "default". – Akavall Sep 13 '15 at 13:28
  • Just to add this: Often Python programmers use the `x and y or z` in Python to express "return `y` if `x` is true else return `z`. Since Python 2.5. there is the inline if to replace that so-called boolean shortcut, since in some situations it might not give the wanted results. – Klaus D. Sep 13 '15 at 13:36
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    Define "correct". Do you mean that it's not a `SyntaxError`? That its result is the same a different statement that you didn't provide in the question making it useless? Or what? In any case questions should be researched before being asked. – Bakuriu Sep 13 '15 at 14:08

5 Answers5

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Your code as written will produce the behavior you describe, but I would suggest the following, which is the Python canonical form of a ternary expression

>>> x = None
>>> foo = x if x else 'default'
>>> foo
'default'

If you are familiar with C++, you can read the above as if it were the below ternary expression

foo = x ? x : "default";
Cory Kramer
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2

Yes, it works. From documentation -

The expression x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned.

The expression x or y first evaluates x; if x is true, its value is returned; otherwise, y is evaluated and the resulting value is returned.

(Note that neither and nor or restrict the value and type they return to False and True, but rather return the last evaluated argument. This is sometimes useful, e.g., if s is a string that should be replaced by a default value if it is empty, the expression s or 'foo' yields the desired value.

(Emphasis mine)

Please note when you do -

foo = x or "default"

if x is None or empty string, in both cases "default" would be the return value of the expression - x or "default" .

Community
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Anand S Kumar
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2

It is 'correct'/valid code (as long as x is defined), but bear in mind that foo will be set to "default" for any falsy value such as 0, "", [] or {}.

If you only want to use the default value if x is not None, you can use: foo = x if x is not None else "default"

This is demonstrated below.

x = 0
print(x if x is not None else "default")  # prints 0
print(x or "default")  # prints default

If you are defining a function, you will probably just want to use default parameters.

Again, demonstrated below:

def myfunc(x="default"):
    print(x)

myfunc()  # prints default
myfunc("custom value")  # prints custom value
reupen
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1

If by "correct", you mean "does it work?", then yes, it is correct. It works fine and will work as you expect it to. Note, however, that if x is something like False, 0, None, or "", foo will also be set to default due to the way boolean logic works in Python.

If by "correct", you mean "is it correct conventions?", then yes, I would also say it is correct. This is a common pattern that I have seen many developers use throughout several projects. It is also given as an example in the Python documentation.

AppleDash
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0

The code works fine if x is already defined, else you will get an error.

Correct usage:

 >>> x= None
 >>> foo = x or "default"
 >>> foo
 'default'

Error case :

>>> foo = x or "default"

Traceback (most recent call last):
  File "<pyshell#12>", line 1, in <module>
    foo = x or "default"
NameError: name 'x' is not defined
Roman Marusyk
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Sonu
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