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]?
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4Don't mean to be rude, but reading the doc is the first thing you should do before posting here, cf How to Ask.bruno desthuilliers– bruno desthuilliers2020-03-26 08:01:10 +00:00Commented Mar 26, 2020 at 8:01
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Related: What is Truthy and Falsy, in Python?smci– smci2020-03-26 08:04:33 +00:00Commented Mar 26, 2020 at 8:04
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Does this answer your question? What is Truthy and Falsy? How is it different from True and False?Tomerikoo– Tomerikoo2020-03-26 08:10:34 +00:00Commented Mar 26, 2020 at 8:10
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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:
NoneandFalse.- zero of any numeric type:
0,0.0,0j,Decimal(0),Fraction(0, 1)- ...
3 Comments
Nathan Kirui
From the docs.python.org/3/library/functions.html#filter it says " If a function is None, the identity function is assumed, that is, all elements of iterable that are false are removed". but the element zero(0) in our case is not false.
jonrsharpe
@NathanKirui as I've also linked above, 0 is also considered false; I've added another quote.
Tomerikoo
@NathanKirui note that it just says false as opposed to
False. This means objects that are considered to be false (not the constant False), and as the quote that jon provided you have the list of those objectsThe 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]
