In Python, is 'foo = x or "default"' correct?

Question

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

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

Answer 1

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";

Answer 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, eg, 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" .

Answer 3

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

Answer 4

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.

Answer 5

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