Why does updating a set with map object destroy the map object?












0















The following code shown below:



y=set()
y.update([1])
x=map(chr, [65, 66, 67, 68])
for ch in x:
print(ch)
print(y)


Outputs:




A



B



C



D



{1}




Which is expected. However, when I update the y set with the x map object, it destroys the map object. This is shown here with this code:



y=set()
y.update([1])
x=map(chr, [65, 66, 67, 68])
y.update(x)
for ch in x:
print(ch)
print(y)


which outputs:




{'C', 1, 'D', 'A', 'B'}




When my expected output is this:




A



B



C



D



{'C', 1, 'D', 'A', 'B'}




The variable x is no longer there and destroyed?



Can someone explain this please?



and also how do I keep the x map object after updating the y set?










share|improve this question


















  • 3





    A map object can only be iterated once. If you want to iterate it multiple times, transform it to a list. x = list(map(...))

    – khelwood
    Nov 23 '18 at 21:54













  • @khelwood ah didn't know that. Thanks!

    – Ricky Kim
    Nov 23 '18 at 21:58
















0















The following code shown below:



y=set()
y.update([1])
x=map(chr, [65, 66, 67, 68])
for ch in x:
print(ch)
print(y)


Outputs:




A



B



C



D



{1}




Which is expected. However, when I update the y set with the x map object, it destroys the map object. This is shown here with this code:



y=set()
y.update([1])
x=map(chr, [65, 66, 67, 68])
y.update(x)
for ch in x:
print(ch)
print(y)


which outputs:




{'C', 1, 'D', 'A', 'B'}




When my expected output is this:




A



B



C



D



{'C', 1, 'D', 'A', 'B'}




The variable x is no longer there and destroyed?



Can someone explain this please?



and also how do I keep the x map object after updating the y set?










share|improve this question


















  • 3





    A map object can only be iterated once. If you want to iterate it multiple times, transform it to a list. x = list(map(...))

    – khelwood
    Nov 23 '18 at 21:54













  • @khelwood ah didn't know that. Thanks!

    – Ricky Kim
    Nov 23 '18 at 21:58














0












0








0








The following code shown below:



y=set()
y.update([1])
x=map(chr, [65, 66, 67, 68])
for ch in x:
print(ch)
print(y)


Outputs:




A



B



C



D



{1}




Which is expected. However, when I update the y set with the x map object, it destroys the map object. This is shown here with this code:



y=set()
y.update([1])
x=map(chr, [65, 66, 67, 68])
y.update(x)
for ch in x:
print(ch)
print(y)


which outputs:




{'C', 1, 'D', 'A', 'B'}




When my expected output is this:




A



B



C



D



{'C', 1, 'D', 'A', 'B'}




The variable x is no longer there and destroyed?



Can someone explain this please?



and also how do I keep the x map object after updating the y set?










share|improve this question














The following code shown below:



y=set()
y.update([1])
x=map(chr, [65, 66, 67, 68])
for ch in x:
print(ch)
print(y)


Outputs:




A



B



C



D



{1}




Which is expected. However, when I update the y set with the x map object, it destroys the map object. This is shown here with this code:



y=set()
y.update([1])
x=map(chr, [65, 66, 67, 68])
y.update(x)
for ch in x:
print(ch)
print(y)


which outputs:




{'C', 1, 'D', 'A', 'B'}




When my expected output is this:




A



B



C



D



{'C', 1, 'D', 'A', 'B'}




The variable x is no longer there and destroyed?



Can someone explain this please?



and also how do I keep the x map object after updating the y set?







python python-3.x set






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked Nov 23 '18 at 21:52









Ricky KimRicky Kim

1,033212




1,033212








  • 3





    A map object can only be iterated once. If you want to iterate it multiple times, transform it to a list. x = list(map(...))

    – khelwood
    Nov 23 '18 at 21:54













  • @khelwood ah didn't know that. Thanks!

    – Ricky Kim
    Nov 23 '18 at 21:58














  • 3





    A map object can only be iterated once. If you want to iterate it multiple times, transform it to a list. x = list(map(...))

    – khelwood
    Nov 23 '18 at 21:54













  • @khelwood ah didn't know that. Thanks!

    – Ricky Kim
    Nov 23 '18 at 21:58








3




3





A map object can only be iterated once. If you want to iterate it multiple times, transform it to a list. x = list(map(...))

– khelwood
Nov 23 '18 at 21:54







A map object can only be iterated once. If you want to iterate it multiple times, transform it to a list. x = list(map(...))

– khelwood
Nov 23 '18 at 21:54















@khelwood ah didn't know that. Thanks!

– Ricky Kim
Nov 23 '18 at 21:58





@khelwood ah didn't know that. Thanks!

– Ricky Kim
Nov 23 '18 at 21:58












1 Answer
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oldest

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3














In Python 2, map would return a list. In Python 3, it returns a special kind of object, which can only be iterated once.



If you want to iterate it multiple times, you can transform it to a list.



x = list(map(chr, [65, 66, 67, 68]))





share|improve this answer























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    1 Answer
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    active

    oldest

    votes








    1 Answer
    1






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    3














    In Python 2, map would return a list. In Python 3, it returns a special kind of object, which can only be iterated once.



    If you want to iterate it multiple times, you can transform it to a list.



    x = list(map(chr, [65, 66, 67, 68]))





    share|improve this answer




























      3














      In Python 2, map would return a list. In Python 3, it returns a special kind of object, which can only be iterated once.



      If you want to iterate it multiple times, you can transform it to a list.



      x = list(map(chr, [65, 66, 67, 68]))





      share|improve this answer


























        3












        3








        3







        In Python 2, map would return a list. In Python 3, it returns a special kind of object, which can only be iterated once.



        If you want to iterate it multiple times, you can transform it to a list.



        x = list(map(chr, [65, 66, 67, 68]))





        share|improve this answer













        In Python 2, map would return a list. In Python 3, it returns a special kind of object, which can only be iterated once.



        If you want to iterate it multiple times, you can transform it to a list.



        x = list(map(chr, [65, 66, 67, 68]))






        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered Nov 23 '18 at 21:58









        khelwoodkhelwood

        30.8k74263




        30.8k74263






























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