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Is it possible without loops initialize all list values to some bool? For example I want to have a list of N elements all False.

Rohit Jain
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Alecs
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3 Answers3

117

You can do it like this: -

>>> [False] * 10
[False, False, False, False, False, False, False, False, False, False]

NOTE: - Note that, you should never do this with a list of mutable types with same value, else you will see surprising behaviour like the one in below example: -

>>> my_list = [[10]] * 3
>>> my_list
[[10], [10], [10]]
>>> my_list[0][0] = 5
>>> my_list
[[5], [5], [5]]

As you can see, changes you made in one inner list, is reflected in all of them.

Rohit Jain
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    This is absolutely the correct idiom for this problem. However, it is absolutely the incorrect idiom to use when you're dealing with a mutable type: `[[]]*10` has some surprising consequences for new comers :). – mgilson Nov 14 '12 at 16:36
  • See [this link](https://bugs.python.org/issue27135) why you shouldn't create a list of mutable types using multiplication. – jdhao Nov 19 '20 at 10:57
18
    my_list = [False for i in range(n)]

This will allow you to change individual elements since it builds each element independently.

Although, this technically is a loop.

Joel Christophel
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Noah
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5

When space matters, bytearray is a better choice. It's roughly five times more space efficient than the list of boolean solution.

This creates an array of N values, initialized to zero:

B = bytearray(N)
Waylon Flinn
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