CODEWITHSUNDEEP
pythonlist
Roman
Roman

Reputation: 3241

Test if all elements of a python list are False

How to return False if all elements are in a list are False?

The given list is:

data = [False, False, False]

Upvotes: 139

Views: 206231

Answers (5)

Abhijit Manepatil
Abhijit Manepatil

Reputation: 907

True and False are the Boolean representations of 0 and 1.
True = 1 and False = 0

data = [False, False, False]    
if sum(data) == 0:
   return False
if sum(data) == len(data):
   return True

sum(data) represents the addition of 1 and 0 with respective values of True(1) and False(0) in a list.

In the case of all False sum is 0,
and in the case of all True` sum is equal to the length of the list.

Any other sum values would mean not all is False or True.

Upvotes: 6

XMehdi01
XMehdi01

Reputation: 1

We all know that False is also considered 0, So if sum of all elements is 0, which means all elements within list are False.
But since you want:

to return 'false' because all elements are 'false'

To do that use negation operator not or !.

data = [False, False, False]
print(sum(data)!=0) #False

Upvotes: 2

Krishna Chaurasia
Krishna Chaurasia

Reputation: 9572

Here is another approach using the generator expression:

data = [False, False, False]
try:
    out = next(elt for elt in data if elt) # holds the value of first non-empty element
except StopIteration:
    print("all elements are empty")

data = [[], [], [1], [2]]
try:
    out = next(elt for elt in data if elt) # [1]
except StopIteration:
    print("all elements are empty")

Upvotes: 1

Alexey Orlenko
Alexey Orlenko

Reputation: 722

Basically there are two functions that deal with an iterable and return True or False depending on which boolean values elements of the sequence evaluate to.

  1. all(iterable) returns True if all elements of the iterable are considered as true values (like reduce(operator.and_, iterable)).

  2. any(iterable) returns True if at least one element of the iterable is a true value (again, using functional stuff, reduce(operator.or_, iterable)).

Using the all function, you can map operator.not_ over your list or just build a new sequence with negated values and check that all the elements of the new sequence are true:

>>> all(not element for element in data)

With the any function, you can check that at least one element is true and then negate the result since you need to return False if there's a true element:

>>> not any(data)

According to De Morgan's law, these two variants will return the same result, but I would prefer the last one (which uses any) because it is shorter, more readable (and can be intuitively understood as "there isn't a true value in data") and more efficient (since you don't build any extra sequences).

Upvotes: 53

falsetru
falsetru

Reputation: 369044

Using any:

>>> data = [False, False, False]
>>> not any(data)
True

any will return True if there's any truth value in the iterable.

Upvotes: 244

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