Python object is nullified after calling-CodePudding

If I make any operation on an object, it is nullified after. Why is it so? Looks like list() method is doing it.

from itertools import permutations

my_string = 'ada'

result = permutations(my_string)
result2 = permutations(my_string)

print(result)
print('Number of variations is: ', len(list(result)))
print(list(result))


print(' ')

print(result)
print('Number of variations is: ', len(list(result)))
print(list(result))

The first three attempts to print() the object are a success.

<itertools.permutations object at 0x0000015B4B4555E0>
Number of variations is:  6
[('a', 'd', 'a'), ('a', 'a', 'd'), ('d', 'a', 'a'), ('d', 'a', 'a'), ('a', 'a', 'd'), ('a', 'd', 'a')]

The second attempt to print() them gives Null.

<itertools.permutations object at 0x0000015B4B4555E0>
Number of variations is:  0
[]

CodePudding user response：

You cannot iterate multiple times through the result of itertools.permutations(). From the results you are showing, it looks like you have done:

result = permutations(my_string)
len(list(result))
len(list(result))

An iterator can be used once and only once. If you need to iterate through the permutation a second time, either call permuations() again, or hold onto list(result).

I noticed that you printed the value of the iterator, and both times is showed the exact same iterator.

CodePudding user response：

itertools.permutations returns an iterator. Iterators can only be looped over once.

After you do list(result), the result iterator is exhausted and thus len(result) will be 0.

I also assume you made a mistake while copying the output since when you actually run the code, only the first call(len(list(result))) gives the expected output.