Strange behaviour with Python Generator












7















I was running a piece of code that unexpectedly gave a logic error at one part of the program. When investigating the section, I created a test file to test the set of statements being run and found out an unusual bug that seems very odd.



I tested this simple code:



array = [1, 2, 2, 4, 5] # Original array
f = (x for x in array if array.count(x) == 2) # Filters original
array = [5, 6, 1, 2, 9] # Updates original to something else

print(list(f)) # Outputs filtered


And the output was:



>>> 


Yes, nothing. I was expecting the filter comprehension to get items in the array with a count of 2 and output this, but I didn't get that:



# Expected output
>>> [2, 2]


When I commented out the third line to test it once again:



array = [1, 2, 2, 4, 5] # Original array
f = (x for x in array if array.count(x) == 2) # Filters original
### array = [5, 6, 1, 2, 9] # Ignore line

print(list(f)) # Outputs filtered


The output was correct (you can test it for yourself):



>>> [2, 2]


At one point I outputted the type of the variable 'f':



array = [1, 2, 2, 4, 5] # Original array
f = (x for x in array if array.count(x) == 2) # Filters original
array = [5, 6, 1, 2, 9] # Updates original

print(type(f))
print(list(f)) # Outputs filtered


And I got:



>>> <class 'generator'>
>>>


TL;DR: Why is updating a list in Python changing the output of another generator variable? This seems very odd to me.










share|improve this question




















  • 2





    You keep printing list(g), but g is an undefined symbol. Perhaps you have a typo, using g for f?

    – Prune
    3 hours ago











  • Yh that is what I meant sorry

    – Suraj Kothari
    3 hours ago






  • 2





    You redefine array and your new array is what gets referenced by the lazy generator comprehension.

    – jpp
    3 hours ago


















7















I was running a piece of code that unexpectedly gave a logic error at one part of the program. When investigating the section, I created a test file to test the set of statements being run and found out an unusual bug that seems very odd.



I tested this simple code:



array = [1, 2, 2, 4, 5] # Original array
f = (x for x in array if array.count(x) == 2) # Filters original
array = [5, 6, 1, 2, 9] # Updates original to something else

print(list(f)) # Outputs filtered


And the output was:



>>> 


Yes, nothing. I was expecting the filter comprehension to get items in the array with a count of 2 and output this, but I didn't get that:



# Expected output
>>> [2, 2]


When I commented out the third line to test it once again:



array = [1, 2, 2, 4, 5] # Original array
f = (x for x in array if array.count(x) == 2) # Filters original
### array = [5, 6, 1, 2, 9] # Ignore line

print(list(f)) # Outputs filtered


The output was correct (you can test it for yourself):



>>> [2, 2]


At one point I outputted the type of the variable 'f':



array = [1, 2, 2, 4, 5] # Original array
f = (x for x in array if array.count(x) == 2) # Filters original
array = [5, 6, 1, 2, 9] # Updates original

print(type(f))
print(list(f)) # Outputs filtered


And I got:



>>> <class 'generator'>
>>>


TL;DR: Why is updating a list in Python changing the output of another generator variable? This seems very odd to me.










share|improve this question




















  • 2





    You keep printing list(g), but g is an undefined symbol. Perhaps you have a typo, using g for f?

    – Prune
    3 hours ago











  • Yh that is what I meant sorry

    – Suraj Kothari
    3 hours ago






  • 2





    You redefine array and your new array is what gets referenced by the lazy generator comprehension.

    – jpp
    3 hours ago
















7












7








7








I was running a piece of code that unexpectedly gave a logic error at one part of the program. When investigating the section, I created a test file to test the set of statements being run and found out an unusual bug that seems very odd.



I tested this simple code:



array = [1, 2, 2, 4, 5] # Original array
f = (x for x in array if array.count(x) == 2) # Filters original
array = [5, 6, 1, 2, 9] # Updates original to something else

print(list(f)) # Outputs filtered


And the output was:



>>> 


Yes, nothing. I was expecting the filter comprehension to get items in the array with a count of 2 and output this, but I didn't get that:



# Expected output
>>> [2, 2]


When I commented out the third line to test it once again:



array = [1, 2, 2, 4, 5] # Original array
f = (x for x in array if array.count(x) == 2) # Filters original
### array = [5, 6, 1, 2, 9] # Ignore line

print(list(f)) # Outputs filtered


The output was correct (you can test it for yourself):



>>> [2, 2]


At one point I outputted the type of the variable 'f':



array = [1, 2, 2, 4, 5] # Original array
f = (x for x in array if array.count(x) == 2) # Filters original
array = [5, 6, 1, 2, 9] # Updates original

print(type(f))
print(list(f)) # Outputs filtered


And I got:



>>> <class 'generator'>
>>>


TL;DR: Why is updating a list in Python changing the output of another generator variable? This seems very odd to me.










share|improve this question
















I was running a piece of code that unexpectedly gave a logic error at one part of the program. When investigating the section, I created a test file to test the set of statements being run and found out an unusual bug that seems very odd.



I tested this simple code:



array = [1, 2, 2, 4, 5] # Original array
f = (x for x in array if array.count(x) == 2) # Filters original
array = [5, 6, 1, 2, 9] # Updates original to something else

print(list(f)) # Outputs filtered


And the output was:



>>> 


Yes, nothing. I was expecting the filter comprehension to get items in the array with a count of 2 and output this, but I didn't get that:



# Expected output
>>> [2, 2]


When I commented out the third line to test it once again:



array = [1, 2, 2, 4, 5] # Original array
f = (x for x in array if array.count(x) == 2) # Filters original
### array = [5, 6, 1, 2, 9] # Ignore line

print(list(f)) # Outputs filtered


The output was correct (you can test it for yourself):



>>> [2, 2]


At one point I outputted the type of the variable 'f':



array = [1, 2, 2, 4, 5] # Original array
f = (x for x in array if array.count(x) == 2) # Filters original
array = [5, 6, 1, 2, 9] # Updates original

print(type(f))
print(list(f)) # Outputs filtered


And I got:



>>> <class 'generator'>
>>>


TL;DR: Why is updating a list in Python changing the output of another generator variable? This seems very odd to me.







python






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited 3 hours ago







Suraj Kothari

















asked 3 hours ago









Suraj KothariSuraj Kothari

438212




438212








  • 2





    You keep printing list(g), but g is an undefined symbol. Perhaps you have a typo, using g for f?

    – Prune
    3 hours ago











  • Yh that is what I meant sorry

    – Suraj Kothari
    3 hours ago






  • 2





    You redefine array and your new array is what gets referenced by the lazy generator comprehension.

    – jpp
    3 hours ago
















  • 2





    You keep printing list(g), but g is an undefined symbol. Perhaps you have a typo, using g for f?

    – Prune
    3 hours ago











  • Yh that is what I meant sorry

    – Suraj Kothari
    3 hours ago






  • 2





    You redefine array and your new array is what gets referenced by the lazy generator comprehension.

    – jpp
    3 hours ago










2




2





You keep printing list(g), but g is an undefined symbol. Perhaps you have a typo, using g for f?

– Prune
3 hours ago





You keep printing list(g), but g is an undefined symbol. Perhaps you have a typo, using g for f?

– Prune
3 hours ago













Yh that is what I meant sorry

– Suraj Kothari
3 hours ago





Yh that is what I meant sorry

– Suraj Kothari
3 hours ago




2




2





You redefine array and your new array is what gets referenced by the lazy generator comprehension.

– jpp
3 hours ago







You redefine array and your new array is what gets referenced by the lazy generator comprehension.

– jpp
3 hours ago














5 Answers
5






active

oldest

votes


















6














As others have mentioned Python generators are lazy. When this line is run:



f = (x for x in array if array.count(x) == 2) # Filters original


nothing actually happens yet. You've just declared how the generator function f will work. Array is not looked at yet. Then, you create a new array that replaces the first one, and finally when you call



print(list(f)) # Outputs filtered


the generator now needs the actual values and starts pulling them from the generator f. But at this point, array already refers to the second one, so you get an empty list.



If you need to reassign the list, and can't use a different variable to hold it, consider creating the list instead of a generator in the second line:



f = [x for x in array if array.count(x) == 2] # Filters original
...
print(f)





share|improve this answer

































    2














    Generators are lazy, they won't be evaluated until you iterate through them. In this case that's at the point you create the list with the generator as input, at the print.






    share|improve this answer


























    • When am I iterating through them. Am I meant to?

      – Suraj Kothari
      3 hours ago











    • @SurajKothari when you create the list it will iterate for you without you needing to do it explicitly.

      – Mark Ransom
      3 hours ago











    • Also which list? When I declare the first one, or re-assign the second?

      – Suraj Kothari
      2 hours ago











    • What first & second? You define only one list, at the final line of your code.

      – Prune
      2 hours ago











    • @SurajKothari I updated the answer to be more clear.

      – Mark Ransom
      2 hours ago



















    1














    You are not using a generator correctly if this is the primary use of this code. Use a list comprehension instead of a generator comprehension. Just replace the parentheses with brackets. It evaluates to a list if you don't know.



    array = [1, 2, 2, 4, 5]
    f = [x for x in array if array.count(x) == 2]
    array = [5, 6, 1, 2, 9]

    print(f)
    #[2, 2]


    You are getting this response because of the nature of a generator. You're calling the generator when it't contents will evaluate to






    share|improve this answer


























    • Thank you. I seem to have used the wrong brackets. But in general using a generator comprehension seems odd.

      – Suraj Kothari
      2 hours ago











    • With your change, list(f) becomes redundant.

      – Mark Ransom
      2 hours ago











    • Lol @Mark Ransom, copy paste got me, I edited.

      – Jaba
      2 hours ago













    • @SurajKothari It is not odd, it's a great tool! It just takes some time to wrap the ole brain around. Do some research you'll find that generators are amazing!

      – Jaba
      2 hours ago



















    0














    Generator evaluation is "lazy" -- it doesn't get executed until you actualize it with a proper reference. With your line:



    Look again at your output with the type of f: that object is a generator, not a sequence. It's waiting to be used, an iterator of sorts.



    Your generator isn't evaluated until you start requiring values from it. At that point, it uses the available values at that point, not the point at which it was defined.





    Code to "make it work"



    That depends on what you mean by "make it work". If you want f to be a filtered list, then use a list, not a generator:



    f = [x for x in array if array.count(x) == 2] # Filters original





    share|improve this answer


























    • I somewhat understand. Could you show some code to make it work, because I need to re-assign the same list again in the main code.

      – Suraj Kothari
      2 hours ago



















    0














    Generators are lazy and your newly defined array is used when you exhaust your generator after redefining. Therefore, the output is correct. A quick fix is to use a list comprehension by replacing parentheses () by brackets .



    Moving on to how better to write your logic, counting a value in a loop has quadratic complexity. For an algorithm that works in linear time, you can use collections.Counter to count values, and keep a copy of your original list:



    from collections import Counter

    array = [1, 2, 2, 4, 5] # original array
    counts = Counter(array) # count each value in array
    old_array = array.copy() # make copy
    array = [5, 6, 1, 2, 9] # updates array

    # order relevant
    res = [x for x in old_array if counts[x] >= 2]
    print(res)
    # [2, 2]

    # order irrelevant
    from itertools import chain
    res = list(chain.from_iterable([x]*count for x, count in counts.items() if count >= 2))
    print(res)
    # [2, 2]


    Notice the second version doesn't even require old_array and is useful if there is no need to maintain ordering of values in your original array.






    share|improve this answer























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      5 Answers
      5






      active

      oldest

      votes








      5 Answers
      5






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      6














      As others have mentioned Python generators are lazy. When this line is run:



      f = (x for x in array if array.count(x) == 2) # Filters original


      nothing actually happens yet. You've just declared how the generator function f will work. Array is not looked at yet. Then, you create a new array that replaces the first one, and finally when you call



      print(list(f)) # Outputs filtered


      the generator now needs the actual values and starts pulling them from the generator f. But at this point, array already refers to the second one, so you get an empty list.



      If you need to reassign the list, and can't use a different variable to hold it, consider creating the list instead of a generator in the second line:



      f = [x for x in array if array.count(x) == 2] # Filters original
      ...
      print(f)





      share|improve this answer






























        6














        As others have mentioned Python generators are lazy. When this line is run:



        f = (x for x in array if array.count(x) == 2) # Filters original


        nothing actually happens yet. You've just declared how the generator function f will work. Array is not looked at yet. Then, you create a new array that replaces the first one, and finally when you call



        print(list(f)) # Outputs filtered


        the generator now needs the actual values and starts pulling them from the generator f. But at this point, array already refers to the second one, so you get an empty list.



        If you need to reassign the list, and can't use a different variable to hold it, consider creating the list instead of a generator in the second line:



        f = [x for x in array if array.count(x) == 2] # Filters original
        ...
        print(f)





        share|improve this answer




























          6












          6








          6







          As others have mentioned Python generators are lazy. When this line is run:



          f = (x for x in array if array.count(x) == 2) # Filters original


          nothing actually happens yet. You've just declared how the generator function f will work. Array is not looked at yet. Then, you create a new array that replaces the first one, and finally when you call



          print(list(f)) # Outputs filtered


          the generator now needs the actual values and starts pulling them from the generator f. But at this point, array already refers to the second one, so you get an empty list.



          If you need to reassign the list, and can't use a different variable to hold it, consider creating the list instead of a generator in the second line:



          f = [x for x in array if array.count(x) == 2] # Filters original
          ...
          print(f)





          share|improve this answer















          As others have mentioned Python generators are lazy. When this line is run:



          f = (x for x in array if array.count(x) == 2) # Filters original


          nothing actually happens yet. You've just declared how the generator function f will work. Array is not looked at yet. Then, you create a new array that replaces the first one, and finally when you call



          print(list(f)) # Outputs filtered


          the generator now needs the actual values and starts pulling them from the generator f. But at this point, array already refers to the second one, so you get an empty list.



          If you need to reassign the list, and can't use a different variable to hold it, consider creating the list instead of a generator in the second line:



          f = [x for x in array if array.count(x) == 2] # Filters original
          ...
          print(f)






          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited 2 hours ago

























          answered 2 hours ago









          StevenSteven

          767




          767

























              2














              Generators are lazy, they won't be evaluated until you iterate through them. In this case that's at the point you create the list with the generator as input, at the print.






              share|improve this answer


























              • When am I iterating through them. Am I meant to?

                – Suraj Kothari
                3 hours ago











              • @SurajKothari when you create the list it will iterate for you without you needing to do it explicitly.

                – Mark Ransom
                3 hours ago











              • Also which list? When I declare the first one, or re-assign the second?

                – Suraj Kothari
                2 hours ago











              • What first & second? You define only one list, at the final line of your code.

                – Prune
                2 hours ago











              • @SurajKothari I updated the answer to be more clear.

                – Mark Ransom
                2 hours ago
















              2














              Generators are lazy, they won't be evaluated until you iterate through them. In this case that's at the point you create the list with the generator as input, at the print.






              share|improve this answer


























              • When am I iterating through them. Am I meant to?

                – Suraj Kothari
                3 hours ago











              • @SurajKothari when you create the list it will iterate for you without you needing to do it explicitly.

                – Mark Ransom
                3 hours ago











              • Also which list? When I declare the first one, or re-assign the second?

                – Suraj Kothari
                2 hours ago











              • What first & second? You define only one list, at the final line of your code.

                – Prune
                2 hours ago











              • @SurajKothari I updated the answer to be more clear.

                – Mark Ransom
                2 hours ago














              2












              2








              2







              Generators are lazy, they won't be evaluated until you iterate through them. In this case that's at the point you create the list with the generator as input, at the print.






              share|improve this answer















              Generators are lazy, they won't be evaluated until you iterate through them. In this case that's at the point you create the list with the generator as input, at the print.







              share|improve this answer














              share|improve this answer



              share|improve this answer








              edited 2 hours ago

























              answered 3 hours ago









              Mark RansomMark Ransom

              223k29281508




              223k29281508













              • When am I iterating through them. Am I meant to?

                – Suraj Kothari
                3 hours ago











              • @SurajKothari when you create the list it will iterate for you without you needing to do it explicitly.

                – Mark Ransom
                3 hours ago











              • Also which list? When I declare the first one, or re-assign the second?

                – Suraj Kothari
                2 hours ago











              • What first & second? You define only one list, at the final line of your code.

                – Prune
                2 hours ago











              • @SurajKothari I updated the answer to be more clear.

                – Mark Ransom
                2 hours ago



















              • When am I iterating through them. Am I meant to?

                – Suraj Kothari
                3 hours ago











              • @SurajKothari when you create the list it will iterate for you without you needing to do it explicitly.

                – Mark Ransom
                3 hours ago











              • Also which list? When I declare the first one, or re-assign the second?

                – Suraj Kothari
                2 hours ago











              • What first & second? You define only one list, at the final line of your code.

                – Prune
                2 hours ago











              • @SurajKothari I updated the answer to be more clear.

                – Mark Ransom
                2 hours ago

















              When am I iterating through them. Am I meant to?

              – Suraj Kothari
              3 hours ago





              When am I iterating through them. Am I meant to?

              – Suraj Kothari
              3 hours ago













              @SurajKothari when you create the list it will iterate for you without you needing to do it explicitly.

              – Mark Ransom
              3 hours ago





              @SurajKothari when you create the list it will iterate for you without you needing to do it explicitly.

              – Mark Ransom
              3 hours ago













              Also which list? When I declare the first one, or re-assign the second?

              – Suraj Kothari
              2 hours ago





              Also which list? When I declare the first one, or re-assign the second?

              – Suraj Kothari
              2 hours ago













              What first & second? You define only one list, at the final line of your code.

              – Prune
              2 hours ago





              What first & second? You define only one list, at the final line of your code.

              – Prune
              2 hours ago













              @SurajKothari I updated the answer to be more clear.

              – Mark Ransom
              2 hours ago





              @SurajKothari I updated the answer to be more clear.

              – Mark Ransom
              2 hours ago











              1














              You are not using a generator correctly if this is the primary use of this code. Use a list comprehension instead of a generator comprehension. Just replace the parentheses with brackets. It evaluates to a list if you don't know.



              array = [1, 2, 2, 4, 5]
              f = [x for x in array if array.count(x) == 2]
              array = [5, 6, 1, 2, 9]

              print(f)
              #[2, 2]


              You are getting this response because of the nature of a generator. You're calling the generator when it't contents will evaluate to






              share|improve this answer


























              • Thank you. I seem to have used the wrong brackets. But in general using a generator comprehension seems odd.

                – Suraj Kothari
                2 hours ago











              • With your change, list(f) becomes redundant.

                – Mark Ransom
                2 hours ago











              • Lol @Mark Ransom, copy paste got me, I edited.

                – Jaba
                2 hours ago













              • @SurajKothari It is not odd, it's a great tool! It just takes some time to wrap the ole brain around. Do some research you'll find that generators are amazing!

                – Jaba
                2 hours ago
















              1














              You are not using a generator correctly if this is the primary use of this code. Use a list comprehension instead of a generator comprehension. Just replace the parentheses with brackets. It evaluates to a list if you don't know.



              array = [1, 2, 2, 4, 5]
              f = [x for x in array if array.count(x) == 2]
              array = [5, 6, 1, 2, 9]

              print(f)
              #[2, 2]


              You are getting this response because of the nature of a generator. You're calling the generator when it't contents will evaluate to






              share|improve this answer


























              • Thank you. I seem to have used the wrong brackets. But in general using a generator comprehension seems odd.

                – Suraj Kothari
                2 hours ago











              • With your change, list(f) becomes redundant.

                – Mark Ransom
                2 hours ago











              • Lol @Mark Ransom, copy paste got me, I edited.

                – Jaba
                2 hours ago













              • @SurajKothari It is not odd, it's a great tool! It just takes some time to wrap the ole brain around. Do some research you'll find that generators are amazing!

                – Jaba
                2 hours ago














              1












              1








              1







              You are not using a generator correctly if this is the primary use of this code. Use a list comprehension instead of a generator comprehension. Just replace the parentheses with brackets. It evaluates to a list if you don't know.



              array = [1, 2, 2, 4, 5]
              f = [x for x in array if array.count(x) == 2]
              array = [5, 6, 1, 2, 9]

              print(f)
              #[2, 2]


              You are getting this response because of the nature of a generator. You're calling the generator when it't contents will evaluate to






              share|improve this answer















              You are not using a generator correctly if this is the primary use of this code. Use a list comprehension instead of a generator comprehension. Just replace the parentheses with brackets. It evaluates to a list if you don't know.



              array = [1, 2, 2, 4, 5]
              f = [x for x in array if array.count(x) == 2]
              array = [5, 6, 1, 2, 9]

              print(f)
              #[2, 2]


              You are getting this response because of the nature of a generator. You're calling the generator when it't contents will evaluate to







              share|improve this answer














              share|improve this answer



              share|improve this answer








              edited 2 hours ago

























              answered 2 hours ago









              JabaJaba

              6,949175394




              6,949175394













              • Thank you. I seem to have used the wrong brackets. But in general using a generator comprehension seems odd.

                – Suraj Kothari
                2 hours ago











              • With your change, list(f) becomes redundant.

                – Mark Ransom
                2 hours ago











              • Lol @Mark Ransom, copy paste got me, I edited.

                – Jaba
                2 hours ago













              • @SurajKothari It is not odd, it's a great tool! It just takes some time to wrap the ole brain around. Do some research you'll find that generators are amazing!

                – Jaba
                2 hours ago



















              • Thank you. I seem to have used the wrong brackets. But in general using a generator comprehension seems odd.

                – Suraj Kothari
                2 hours ago











              • With your change, list(f) becomes redundant.

                – Mark Ransom
                2 hours ago











              • Lol @Mark Ransom, copy paste got me, I edited.

                – Jaba
                2 hours ago













              • @SurajKothari It is not odd, it's a great tool! It just takes some time to wrap the ole brain around. Do some research you'll find that generators are amazing!

                – Jaba
                2 hours ago

















              Thank you. I seem to have used the wrong brackets. But in general using a generator comprehension seems odd.

              – Suraj Kothari
              2 hours ago





              Thank you. I seem to have used the wrong brackets. But in general using a generator comprehension seems odd.

              – Suraj Kothari
              2 hours ago













              With your change, list(f) becomes redundant.

              – Mark Ransom
              2 hours ago





              With your change, list(f) becomes redundant.

              – Mark Ransom
              2 hours ago













              Lol @Mark Ransom, copy paste got me, I edited.

              – Jaba
              2 hours ago







              Lol @Mark Ransom, copy paste got me, I edited.

              – Jaba
              2 hours ago















              @SurajKothari It is not odd, it's a great tool! It just takes some time to wrap the ole brain around. Do some research you'll find that generators are amazing!

              – Jaba
              2 hours ago





              @SurajKothari It is not odd, it's a great tool! It just takes some time to wrap the ole brain around. Do some research you'll find that generators are amazing!

              – Jaba
              2 hours ago











              0














              Generator evaluation is "lazy" -- it doesn't get executed until you actualize it with a proper reference. With your line:



              Look again at your output with the type of f: that object is a generator, not a sequence. It's waiting to be used, an iterator of sorts.



              Your generator isn't evaluated until you start requiring values from it. At that point, it uses the available values at that point, not the point at which it was defined.





              Code to "make it work"



              That depends on what you mean by "make it work". If you want f to be a filtered list, then use a list, not a generator:



              f = [x for x in array if array.count(x) == 2] # Filters original





              share|improve this answer


























              • I somewhat understand. Could you show some code to make it work, because I need to re-assign the same list again in the main code.

                – Suraj Kothari
                2 hours ago
















              0














              Generator evaluation is "lazy" -- it doesn't get executed until you actualize it with a proper reference. With your line:



              Look again at your output with the type of f: that object is a generator, not a sequence. It's waiting to be used, an iterator of sorts.



              Your generator isn't evaluated until you start requiring values from it. At that point, it uses the available values at that point, not the point at which it was defined.





              Code to "make it work"



              That depends on what you mean by "make it work". If you want f to be a filtered list, then use a list, not a generator:



              f = [x for x in array if array.count(x) == 2] # Filters original





              share|improve this answer


























              • I somewhat understand. Could you show some code to make it work, because I need to re-assign the same list again in the main code.

                – Suraj Kothari
                2 hours ago














              0












              0








              0







              Generator evaluation is "lazy" -- it doesn't get executed until you actualize it with a proper reference. With your line:



              Look again at your output with the type of f: that object is a generator, not a sequence. It's waiting to be used, an iterator of sorts.



              Your generator isn't evaluated until you start requiring values from it. At that point, it uses the available values at that point, not the point at which it was defined.





              Code to "make it work"



              That depends on what you mean by "make it work". If you want f to be a filtered list, then use a list, not a generator:



              f = [x for x in array if array.count(x) == 2] # Filters original





              share|improve this answer















              Generator evaluation is "lazy" -- it doesn't get executed until you actualize it with a proper reference. With your line:



              Look again at your output with the type of f: that object is a generator, not a sequence. It's waiting to be used, an iterator of sorts.



              Your generator isn't evaluated until you start requiring values from it. At that point, it uses the available values at that point, not the point at which it was defined.





              Code to "make it work"



              That depends on what you mean by "make it work". If you want f to be a filtered list, then use a list, not a generator:



              f = [x for x in array if array.count(x) == 2] # Filters original






              share|improve this answer














              share|improve this answer



              share|improve this answer








              edited 2 hours ago

























              answered 2 hours ago









              PrunePrune

              43.2k143456




              43.2k143456













              • I somewhat understand. Could you show some code to make it work, because I need to re-assign the same list again in the main code.

                – Suraj Kothari
                2 hours ago



















              • I somewhat understand. Could you show some code to make it work, because I need to re-assign the same list again in the main code.

                – Suraj Kothari
                2 hours ago

















              I somewhat understand. Could you show some code to make it work, because I need to re-assign the same list again in the main code.

              – Suraj Kothari
              2 hours ago





              I somewhat understand. Could you show some code to make it work, because I need to re-assign the same list again in the main code.

              – Suraj Kothari
              2 hours ago











              0














              Generators are lazy and your newly defined array is used when you exhaust your generator after redefining. Therefore, the output is correct. A quick fix is to use a list comprehension by replacing parentheses () by brackets .



              Moving on to how better to write your logic, counting a value in a loop has quadratic complexity. For an algorithm that works in linear time, you can use collections.Counter to count values, and keep a copy of your original list:



              from collections import Counter

              array = [1, 2, 2, 4, 5] # original array
              counts = Counter(array) # count each value in array
              old_array = array.copy() # make copy
              array = [5, 6, 1, 2, 9] # updates array

              # order relevant
              res = [x for x in old_array if counts[x] >= 2]
              print(res)
              # [2, 2]

              # order irrelevant
              from itertools import chain
              res = list(chain.from_iterable([x]*count for x, count in counts.items() if count >= 2))
              print(res)
              # [2, 2]


              Notice the second version doesn't even require old_array and is useful if there is no need to maintain ordering of values in your original array.






              share|improve this answer




























                0














                Generators are lazy and your newly defined array is used when you exhaust your generator after redefining. Therefore, the output is correct. A quick fix is to use a list comprehension by replacing parentheses () by brackets .



                Moving on to how better to write your logic, counting a value in a loop has quadratic complexity. For an algorithm that works in linear time, you can use collections.Counter to count values, and keep a copy of your original list:



                from collections import Counter

                array = [1, 2, 2, 4, 5] # original array
                counts = Counter(array) # count each value in array
                old_array = array.copy() # make copy
                array = [5, 6, 1, 2, 9] # updates array

                # order relevant
                res = [x for x in old_array if counts[x] >= 2]
                print(res)
                # [2, 2]

                # order irrelevant
                from itertools import chain
                res = list(chain.from_iterable([x]*count for x, count in counts.items() if count >= 2))
                print(res)
                # [2, 2]


                Notice the second version doesn't even require old_array and is useful if there is no need to maintain ordering of values in your original array.






                share|improve this answer


























                  0












                  0








                  0







                  Generators are lazy and your newly defined array is used when you exhaust your generator after redefining. Therefore, the output is correct. A quick fix is to use a list comprehension by replacing parentheses () by brackets .



                  Moving on to how better to write your logic, counting a value in a loop has quadratic complexity. For an algorithm that works in linear time, you can use collections.Counter to count values, and keep a copy of your original list:



                  from collections import Counter

                  array = [1, 2, 2, 4, 5] # original array
                  counts = Counter(array) # count each value in array
                  old_array = array.copy() # make copy
                  array = [5, 6, 1, 2, 9] # updates array

                  # order relevant
                  res = [x for x in old_array if counts[x] >= 2]
                  print(res)
                  # [2, 2]

                  # order irrelevant
                  from itertools import chain
                  res = list(chain.from_iterable([x]*count for x, count in counts.items() if count >= 2))
                  print(res)
                  # [2, 2]


                  Notice the second version doesn't even require old_array and is useful if there is no need to maintain ordering of values in your original array.






                  share|improve this answer













                  Generators are lazy and your newly defined array is used when you exhaust your generator after redefining. Therefore, the output is correct. A quick fix is to use a list comprehension by replacing parentheses () by brackets .



                  Moving on to how better to write your logic, counting a value in a loop has quadratic complexity. For an algorithm that works in linear time, you can use collections.Counter to count values, and keep a copy of your original list:



                  from collections import Counter

                  array = [1, 2, 2, 4, 5] # original array
                  counts = Counter(array) # count each value in array
                  old_array = array.copy() # make copy
                  array = [5, 6, 1, 2, 9] # updates array

                  # order relevant
                  res = [x for x in old_array if counts[x] >= 2]
                  print(res)
                  # [2, 2]

                  # order irrelevant
                  from itertools import chain
                  res = list(chain.from_iterable([x]*count for x, count in counts.items() if count >= 2))
                  print(res)
                  # [2, 2]


                  Notice the second version doesn't even require old_array and is useful if there is no need to maintain ordering of values in your original array.







                  share|improve this answer












                  share|improve this answer



                  share|improve this answer










                  answered 44 mins ago









                  jppjpp

                  95.7k2157109




                  95.7k2157109






























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