Unexpected behavior in Python 3: The list contents disappear after printed

Question

I’m learning Python and while doing some coding to practice I duplicated one line and the result was unexpected.

def myfunc(x):
    return x*2 
myList = [1,2,3,4,5]
newList = map(myfunc, myList)
print('Using myfunc on the original list: ',myList,' results in: ',list(newList))
print('Using myfunc on the original list: ',myList,' results in: ',list(newList))

I would expect to see the same result twice, but I’ve got this:

Using myfunc on the original list:  [1, 2, 3, 4, 5]  results in:  [2, 4, 6, 8, 10]
Using myfunc on the original list:  [1, 2, 3, 4, 5]  results in:  []

Why does this happen and how to avoid it?

Asked By: Prates

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Source

Answer 1

newList is not a list. It’s a generator that yields data on demand and will be exhausted after you retrieve all the data it holds with list(newList). So, when you call list(newList) again, there’s no more data to put in the list, so it remains empty.

Answered By: ForceBru

Unexpected behavior in Python 3: The list contents disappear after printed

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