null value in python

user_9042000 0
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As per definition, the None keyword is used to define a null value, or no value at all. But why does:

inputs = [3, 0, 1, 2, None]
print(list(filter(None, inputs)))

return this list [3,1,2] and not [3,0,1,2]?

python
2 Answers

Per the filter docs:

If function is None, the identity function is assumed, that is, all elements of iterable that are false are removed.

The identity function is basically:

def identity(something):
    return something

so filtering on this means that any value that evaluates false-y will be excluded from the output.

Then if you look at truth-value testing you can see that, as well as None, 0 evaluates false-y in Python:

Here are most of the built-in objects considered false:

  • constants defined to be false: None and False.
  • zero of any numeric type: 0, 0.0, 0j, Decimal(0), Fraction(0, 1)
  • ...
user_30017610

The answer by @jonrsharpe explains why you get [3,1,2] instead of [3,0,1,2]. This is because 0 as well as None evaluates false-y in Python.

But in case you want the code to perform as expected, try this:

inputs = [3, 0, 1, 2, None]
print(list(filter(lambda x: x is not None, inputs)))

This should return the output as [3,0,1,2]

user_131263810
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