Why does Python allow out-of-range slice indexes for sequences?












11















So I just came across what seems to me like a strange Python feature and wanted some clarification about it.



The following array manipulation somewhat makes sense:



p = [1,2,3]
p[3:] = 4
p = [1,2,3,4]


I imagine it is actually just appending this value to the end, correct?

Why can I do this, however?



p[20:22] = [5,6]
p = [1,2,3,4,5,6]


And even more so this:



p[20:100] = [7,8]
p = [1,2,3,4,5,6,7,8]


This just seems like wrong logic. It seems like this should throw an error!



Any explanation?

-Is it just a weird thing Python does?

-Is there a purpose to it?

-Or am I thinking about this the wrong way?










share|improve this question





























    11















    So I just came across what seems to me like a strange Python feature and wanted some clarification about it.



    The following array manipulation somewhat makes sense:



    p = [1,2,3]
    p[3:] = 4
    p = [1,2,3,4]


    I imagine it is actually just appending this value to the end, correct?

    Why can I do this, however?



    p[20:22] = [5,6]
    p = [1,2,3,4,5,6]


    And even more so this:



    p[20:100] = [7,8]
    p = [1,2,3,4,5,6,7,8]


    This just seems like wrong logic. It seems like this should throw an error!



    Any explanation?

    -Is it just a weird thing Python does?

    -Is there a purpose to it?

    -Or am I thinking about this the wrong way?










    share|improve this question



























      11












      11








      11


      1






      So I just came across what seems to me like a strange Python feature and wanted some clarification about it.



      The following array manipulation somewhat makes sense:



      p = [1,2,3]
      p[3:] = 4
      p = [1,2,3,4]


      I imagine it is actually just appending this value to the end, correct?

      Why can I do this, however?



      p[20:22] = [5,6]
      p = [1,2,3,4,5,6]


      And even more so this:



      p[20:100] = [7,8]
      p = [1,2,3,4,5,6,7,8]


      This just seems like wrong logic. It seems like this should throw an error!



      Any explanation?

      -Is it just a weird thing Python does?

      -Is there a purpose to it?

      -Or am I thinking about this the wrong way?










      share|improve this question
















      So I just came across what seems to me like a strange Python feature and wanted some clarification about it.



      The following array manipulation somewhat makes sense:



      p = [1,2,3]
      p[3:] = 4
      p = [1,2,3,4]


      I imagine it is actually just appending this value to the end, correct?

      Why can I do this, however?



      p[20:22] = [5,6]
      p = [1,2,3,4,5,6]


      And even more so this:



      p[20:100] = [7,8]
      p = [1,2,3,4,5,6,7,8]


      This just seems like wrong logic. It seems like this should throw an error!



      Any explanation?

      -Is it just a weird thing Python does?

      -Is there a purpose to it?

      -Or am I thinking about this the wrong way?







      python python-3.x sequence slice range-checking






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited 16 mins ago









      Raymond Hettinger

      133k39254348




      133k39254348










      asked 48 mins ago









      Akaisteph7Akaisteph7

      655




      655
























          4 Answers
          4






          active

          oldest

          votes


















          5














          Slice logic automatically clips the indices to the length of the sequence.



          Allowing slice indices to extend past end points was done for convenience. It would be a pain to have to range check every expression and then adjust the limits manually, so Python does it for you.



          Consider the use case of wanting to display no more than the first 50 characters of a text message.



          The easy way (what Python does now):



          preview = msg[:50]


          Or the hard way (do the limit checks yourself):



          n = len(msg)
          preview = msg[:50] if n > 50 else msg


          Manually implementing that logic for adjustment of end points would be easy to forget, would be easy to get wrong (updating the 50 in two places), would be wordy, and would be slow. Python moves that logic to its internals where it is succint, automatic, fast, and correct. This is one of the reasons I love Python :-)






          share|improve this answer

































            3














            The documentation has your answer:




            s[i:j]: slice of s from i to j (note (4))



            (4) The slice of s from i to j is defined as the sequence of items
            with index k such that i <= k < j. If i or j is greater than
            len(s), use len(s). If i is omitted or None, use 0. If j
            is omitted or None, use len(s). If i is greater than or equal to
            j, the slice is empty.




            The documentation of IndexError confirms this behavior:




            exception IndexError



            Raised when a sequence subscript is out of range. (Slice indices are silently truncated to fall in the allowed range; if an index is
            not an integer, TypeError is raised.)




            Essentially, stuff like p[20:100] is being reduced to p[len(p):len(p]. p[len(p):len(p] is an empty slice at the end of the list, and assigning a list to it will modify the end of the list to contain said list. Thus, it works like appending/extending the original list.



            This behavior is the same as what happens when you assign a list to an empty slice anywhere in the original list. For example:



            In [1]: p = [1, 2, 3, 4]

            In [2]: p[2:2] = [42, 42, 42]

            In [3]: p
            Out[3]: [1, 2, 42, 42, 42, 3, 4]





            share|improve this answer





















            • 1





              I don't think OP is asking how slicing works, he's asking for the rationale behind the design choice.

              – Primusa
              30 mins ago











            • @Primusa - I believe they're asking both. This explains the how, which is good to know because it explains why the behavior isn't broken. The why is probably buried in the depths of one of the mailing lists somewhere.

              – g.d.d.c
              26 mins ago











            • Good answer but this doesn't explain why the new numbers get appended to the end of the list.

              – Atirag
              26 mins ago






            • 1





              @Atirag I added a small blurb about it for completeness.

              – Tomothy32
              18 mins ago











            • It is a bit confusing though that p[len(p):len(p)] is empty but p[len(p)] is out of range. Following the logic from the former I would assume p[len(p)] =[c,d] would also append the values but it won't of course.

              – Atirag
              11 mins ago



















            0














            [20:22]--> this way is called slicing in python
            Look when you insert the value in array of python at index 20 to 22 in above case python has created indexes till to 22 but when you require it the output you will get the all numbers with series but it has created the space in array
            **for example



            arr = [1,2,3]
            arr[9:11]=[10,11]
            arr[3:8]=[4,5,6,7,8,9]
            your output array will be
            [1,2,3,4,5,6,7,8,9,10,11]**





            share|improve this answer








            New contributor




            Muhammad Moid Shams is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
            Check out our Code of Conduct.




























              0














              Presumably, you missed the around p[3:] = 4 (should be p[3:] = [4]), but that aside...



              p[20:22] adds the values to the 20th, 21st and 22nd elements of the list, or the end if it is shorter.



              Similarly, p[20:100] adds the values 7 and 8 at the end of your list because your list is shorter than the specified amount.






              share|improve this answer























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                4 Answers
                4






                active

                oldest

                votes








                4 Answers
                4






                active

                oldest

                votes









                active

                oldest

                votes






                active

                oldest

                votes









                5














                Slice logic automatically clips the indices to the length of the sequence.



                Allowing slice indices to extend past end points was done for convenience. It would be a pain to have to range check every expression and then adjust the limits manually, so Python does it for you.



                Consider the use case of wanting to display no more than the first 50 characters of a text message.



                The easy way (what Python does now):



                preview = msg[:50]


                Or the hard way (do the limit checks yourself):



                n = len(msg)
                preview = msg[:50] if n > 50 else msg


                Manually implementing that logic for adjustment of end points would be easy to forget, would be easy to get wrong (updating the 50 in two places), would be wordy, and would be slow. Python moves that logic to its internals where it is succint, automatic, fast, and correct. This is one of the reasons I love Python :-)






                share|improve this answer






























                  5














                  Slice logic automatically clips the indices to the length of the sequence.



                  Allowing slice indices to extend past end points was done for convenience. It would be a pain to have to range check every expression and then adjust the limits manually, so Python does it for you.



                  Consider the use case of wanting to display no more than the first 50 characters of a text message.



                  The easy way (what Python does now):



                  preview = msg[:50]


                  Or the hard way (do the limit checks yourself):



                  n = len(msg)
                  preview = msg[:50] if n > 50 else msg


                  Manually implementing that logic for adjustment of end points would be easy to forget, would be easy to get wrong (updating the 50 in two places), would be wordy, and would be slow. Python moves that logic to its internals where it is succint, automatic, fast, and correct. This is one of the reasons I love Python :-)






                  share|improve this answer




























                    5












                    5








                    5







                    Slice logic automatically clips the indices to the length of the sequence.



                    Allowing slice indices to extend past end points was done for convenience. It would be a pain to have to range check every expression and then adjust the limits manually, so Python does it for you.



                    Consider the use case of wanting to display no more than the first 50 characters of a text message.



                    The easy way (what Python does now):



                    preview = msg[:50]


                    Or the hard way (do the limit checks yourself):



                    n = len(msg)
                    preview = msg[:50] if n > 50 else msg


                    Manually implementing that logic for adjustment of end points would be easy to forget, would be easy to get wrong (updating the 50 in two places), would be wordy, and would be slow. Python moves that logic to its internals where it is succint, automatic, fast, and correct. This is one of the reasons I love Python :-)






                    share|improve this answer















                    Slice logic automatically clips the indices to the length of the sequence.



                    Allowing slice indices to extend past end points was done for convenience. It would be a pain to have to range check every expression and then adjust the limits manually, so Python does it for you.



                    Consider the use case of wanting to display no more than the first 50 characters of a text message.



                    The easy way (what Python does now):



                    preview = msg[:50]


                    Or the hard way (do the limit checks yourself):



                    n = len(msg)
                    preview = msg[:50] if n > 50 else msg


                    Manually implementing that logic for adjustment of end points would be easy to forget, would be easy to get wrong (updating the 50 in two places), would be wordy, and would be slow. Python moves that logic to its internals where it is succint, automatic, fast, and correct. This is one of the reasons I love Python :-)







                    share|improve this answer














                    share|improve this answer



                    share|improve this answer








                    edited 18 mins ago

























                    answered 24 mins ago









                    Raymond HettingerRaymond Hettinger

                    133k39254348




                    133k39254348

























                        3














                        The documentation has your answer:




                        s[i:j]: slice of s from i to j (note (4))



                        (4) The slice of s from i to j is defined as the sequence of items
                        with index k such that i <= k < j. If i or j is greater than
                        len(s), use len(s). If i is omitted or None, use 0. If j
                        is omitted or None, use len(s). If i is greater than or equal to
                        j, the slice is empty.




                        The documentation of IndexError confirms this behavior:




                        exception IndexError



                        Raised when a sequence subscript is out of range. (Slice indices are silently truncated to fall in the allowed range; if an index is
                        not an integer, TypeError is raised.)




                        Essentially, stuff like p[20:100] is being reduced to p[len(p):len(p]. p[len(p):len(p] is an empty slice at the end of the list, and assigning a list to it will modify the end of the list to contain said list. Thus, it works like appending/extending the original list.



                        This behavior is the same as what happens when you assign a list to an empty slice anywhere in the original list. For example:



                        In [1]: p = [1, 2, 3, 4]

                        In [2]: p[2:2] = [42, 42, 42]

                        In [3]: p
                        Out[3]: [1, 2, 42, 42, 42, 3, 4]





                        share|improve this answer





















                        • 1





                          I don't think OP is asking how slicing works, he's asking for the rationale behind the design choice.

                          – Primusa
                          30 mins ago











                        • @Primusa - I believe they're asking both. This explains the how, which is good to know because it explains why the behavior isn't broken. The why is probably buried in the depths of one of the mailing lists somewhere.

                          – g.d.d.c
                          26 mins ago











                        • Good answer but this doesn't explain why the new numbers get appended to the end of the list.

                          – Atirag
                          26 mins ago






                        • 1





                          @Atirag I added a small blurb about it for completeness.

                          – Tomothy32
                          18 mins ago











                        • It is a bit confusing though that p[len(p):len(p)] is empty but p[len(p)] is out of range. Following the logic from the former I would assume p[len(p)] =[c,d] would also append the values but it won't of course.

                          – Atirag
                          11 mins ago
















                        3














                        The documentation has your answer:




                        s[i:j]: slice of s from i to j (note (4))



                        (4) The slice of s from i to j is defined as the sequence of items
                        with index k such that i <= k < j. If i or j is greater than
                        len(s), use len(s). If i is omitted or None, use 0. If j
                        is omitted or None, use len(s). If i is greater than or equal to
                        j, the slice is empty.




                        The documentation of IndexError confirms this behavior:




                        exception IndexError



                        Raised when a sequence subscript is out of range. (Slice indices are silently truncated to fall in the allowed range; if an index is
                        not an integer, TypeError is raised.)




                        Essentially, stuff like p[20:100] is being reduced to p[len(p):len(p]. p[len(p):len(p] is an empty slice at the end of the list, and assigning a list to it will modify the end of the list to contain said list. Thus, it works like appending/extending the original list.



                        This behavior is the same as what happens when you assign a list to an empty slice anywhere in the original list. For example:



                        In [1]: p = [1, 2, 3, 4]

                        In [2]: p[2:2] = [42, 42, 42]

                        In [3]: p
                        Out[3]: [1, 2, 42, 42, 42, 3, 4]





                        share|improve this answer





















                        • 1





                          I don't think OP is asking how slicing works, he's asking for the rationale behind the design choice.

                          – Primusa
                          30 mins ago











                        • @Primusa - I believe they're asking both. This explains the how, which is good to know because it explains why the behavior isn't broken. The why is probably buried in the depths of one of the mailing lists somewhere.

                          – g.d.d.c
                          26 mins ago











                        • Good answer but this doesn't explain why the new numbers get appended to the end of the list.

                          – Atirag
                          26 mins ago






                        • 1





                          @Atirag I added a small blurb about it for completeness.

                          – Tomothy32
                          18 mins ago











                        • It is a bit confusing though that p[len(p):len(p)] is empty but p[len(p)] is out of range. Following the logic from the former I would assume p[len(p)] =[c,d] would also append the values but it won't of course.

                          – Atirag
                          11 mins ago














                        3












                        3








                        3







                        The documentation has your answer:




                        s[i:j]: slice of s from i to j (note (4))



                        (4) The slice of s from i to j is defined as the sequence of items
                        with index k such that i <= k < j. If i or j is greater than
                        len(s), use len(s). If i is omitted or None, use 0. If j
                        is omitted or None, use len(s). If i is greater than or equal to
                        j, the slice is empty.




                        The documentation of IndexError confirms this behavior:




                        exception IndexError



                        Raised when a sequence subscript is out of range. (Slice indices are silently truncated to fall in the allowed range; if an index is
                        not an integer, TypeError is raised.)




                        Essentially, stuff like p[20:100] is being reduced to p[len(p):len(p]. p[len(p):len(p] is an empty slice at the end of the list, and assigning a list to it will modify the end of the list to contain said list. Thus, it works like appending/extending the original list.



                        This behavior is the same as what happens when you assign a list to an empty slice anywhere in the original list. For example:



                        In [1]: p = [1, 2, 3, 4]

                        In [2]: p[2:2] = [42, 42, 42]

                        In [3]: p
                        Out[3]: [1, 2, 42, 42, 42, 3, 4]





                        share|improve this answer















                        The documentation has your answer:




                        s[i:j]: slice of s from i to j (note (4))



                        (4) The slice of s from i to j is defined as the sequence of items
                        with index k such that i <= k < j. If i or j is greater than
                        len(s), use len(s). If i is omitted or None, use 0. If j
                        is omitted or None, use len(s). If i is greater than or equal to
                        j, the slice is empty.




                        The documentation of IndexError confirms this behavior:




                        exception IndexError



                        Raised when a sequence subscript is out of range. (Slice indices are silently truncated to fall in the allowed range; if an index is
                        not an integer, TypeError is raised.)




                        Essentially, stuff like p[20:100] is being reduced to p[len(p):len(p]. p[len(p):len(p] is an empty slice at the end of the list, and assigning a list to it will modify the end of the list to contain said list. Thus, it works like appending/extending the original list.



                        This behavior is the same as what happens when you assign a list to an empty slice anywhere in the original list. For example:



                        In [1]: p = [1, 2, 3, 4]

                        In [2]: p[2:2] = [42, 42, 42]

                        In [3]: p
                        Out[3]: [1, 2, 42, 42, 42, 3, 4]






                        share|improve this answer














                        share|improve this answer



                        share|improve this answer








                        edited 15 mins ago

























                        answered 31 mins ago









                        Tomothy32Tomothy32

                        5,7501425




                        5,7501425








                        • 1





                          I don't think OP is asking how slicing works, he's asking for the rationale behind the design choice.

                          – Primusa
                          30 mins ago











                        • @Primusa - I believe they're asking both. This explains the how, which is good to know because it explains why the behavior isn't broken. The why is probably buried in the depths of one of the mailing lists somewhere.

                          – g.d.d.c
                          26 mins ago











                        • Good answer but this doesn't explain why the new numbers get appended to the end of the list.

                          – Atirag
                          26 mins ago






                        • 1





                          @Atirag I added a small blurb about it for completeness.

                          – Tomothy32
                          18 mins ago











                        • It is a bit confusing though that p[len(p):len(p)] is empty but p[len(p)] is out of range. Following the logic from the former I would assume p[len(p)] =[c,d] would also append the values but it won't of course.

                          – Atirag
                          11 mins ago














                        • 1





                          I don't think OP is asking how slicing works, he's asking for the rationale behind the design choice.

                          – Primusa
                          30 mins ago











                        • @Primusa - I believe they're asking both. This explains the how, which is good to know because it explains why the behavior isn't broken. The why is probably buried in the depths of one of the mailing lists somewhere.

                          – g.d.d.c
                          26 mins ago











                        • Good answer but this doesn't explain why the new numbers get appended to the end of the list.

                          – Atirag
                          26 mins ago






                        • 1





                          @Atirag I added a small blurb about it for completeness.

                          – Tomothy32
                          18 mins ago











                        • It is a bit confusing though that p[len(p):len(p)] is empty but p[len(p)] is out of range. Following the logic from the former I would assume p[len(p)] =[c,d] would also append the values but it won't of course.

                          – Atirag
                          11 mins ago








                        1




                        1





                        I don't think OP is asking how slicing works, he's asking for the rationale behind the design choice.

                        – Primusa
                        30 mins ago





                        I don't think OP is asking how slicing works, he's asking for the rationale behind the design choice.

                        – Primusa
                        30 mins ago













                        @Primusa - I believe they're asking both. This explains the how, which is good to know because it explains why the behavior isn't broken. The why is probably buried in the depths of one of the mailing lists somewhere.

                        – g.d.d.c
                        26 mins ago





                        @Primusa - I believe they're asking both. This explains the how, which is good to know because it explains why the behavior isn't broken. The why is probably buried in the depths of one of the mailing lists somewhere.

                        – g.d.d.c
                        26 mins ago













                        Good answer but this doesn't explain why the new numbers get appended to the end of the list.

                        – Atirag
                        26 mins ago





                        Good answer but this doesn't explain why the new numbers get appended to the end of the list.

                        – Atirag
                        26 mins ago




                        1




                        1





                        @Atirag I added a small blurb about it for completeness.

                        – Tomothy32
                        18 mins ago





                        @Atirag I added a small blurb about it for completeness.

                        – Tomothy32
                        18 mins ago













                        It is a bit confusing though that p[len(p):len(p)] is empty but p[len(p)] is out of range. Following the logic from the former I would assume p[len(p)] =[c,d] would also append the values but it won't of course.

                        – Atirag
                        11 mins ago





                        It is a bit confusing though that p[len(p):len(p)] is empty but p[len(p)] is out of range. Following the logic from the former I would assume p[len(p)] =[c,d] would also append the values but it won't of course.

                        – Atirag
                        11 mins ago











                        0














                        [20:22]--> this way is called slicing in python
                        Look when you insert the value in array of python at index 20 to 22 in above case python has created indexes till to 22 but when you require it the output you will get the all numbers with series but it has created the space in array
                        **for example



                        arr = [1,2,3]
                        arr[9:11]=[10,11]
                        arr[3:8]=[4,5,6,7,8,9]
                        your output array will be
                        [1,2,3,4,5,6,7,8,9,10,11]**





                        share|improve this answer








                        New contributor




                        Muhammad Moid Shams is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                        Check out our Code of Conduct.

























                          0














                          [20:22]--> this way is called slicing in python
                          Look when you insert the value in array of python at index 20 to 22 in above case python has created indexes till to 22 but when you require it the output you will get the all numbers with series but it has created the space in array
                          **for example



                          arr = [1,2,3]
                          arr[9:11]=[10,11]
                          arr[3:8]=[4,5,6,7,8,9]
                          your output array will be
                          [1,2,3,4,5,6,7,8,9,10,11]**





                          share|improve this answer








                          New contributor




                          Muhammad Moid Shams is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                          Check out our Code of Conduct.























                            0












                            0








                            0







                            [20:22]--> this way is called slicing in python
                            Look when you insert the value in array of python at index 20 to 22 in above case python has created indexes till to 22 but when you require it the output you will get the all numbers with series but it has created the space in array
                            **for example



                            arr = [1,2,3]
                            arr[9:11]=[10,11]
                            arr[3:8]=[4,5,6,7,8,9]
                            your output array will be
                            [1,2,3,4,5,6,7,8,9,10,11]**





                            share|improve this answer








                            New contributor




                            Muhammad Moid Shams is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                            Check out our Code of Conduct.










                            [20:22]--> this way is called slicing in python
                            Look when you insert the value in array of python at index 20 to 22 in above case python has created indexes till to 22 but when you require it the output you will get the all numbers with series but it has created the space in array
                            **for example



                            arr = [1,2,3]
                            arr[9:11]=[10,11]
                            arr[3:8]=[4,5,6,7,8,9]
                            your output array will be
                            [1,2,3,4,5,6,7,8,9,10,11]**






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                            answered 31 mins ago









                            Muhammad Moid ShamsMuhammad Moid Shams

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                                0














                                Presumably, you missed the around p[3:] = 4 (should be p[3:] = [4]), but that aside...



                                p[20:22] adds the values to the 20th, 21st and 22nd elements of the list, or the end if it is shorter.



                                Similarly, p[20:100] adds the values 7 and 8 at the end of your list because your list is shorter than the specified amount.






                                share|improve this answer




























                                  0














                                  Presumably, you missed the around p[3:] = 4 (should be p[3:] = [4]), but that aside...



                                  p[20:22] adds the values to the 20th, 21st and 22nd elements of the list, or the end if it is shorter.



                                  Similarly, p[20:100] adds the values 7 and 8 at the end of your list because your list is shorter than the specified amount.






                                  share|improve this answer


























                                    0












                                    0








                                    0







                                    Presumably, you missed the around p[3:] = 4 (should be p[3:] = [4]), but that aside...



                                    p[20:22] adds the values to the 20th, 21st and 22nd elements of the list, or the end if it is shorter.



                                    Similarly, p[20:100] adds the values 7 and 8 at the end of your list because your list is shorter than the specified amount.






                                    share|improve this answer













                                    Presumably, you missed the around p[3:] = 4 (should be p[3:] = [4]), but that aside...



                                    p[20:22] adds the values to the 20th, 21st and 22nd elements of the list, or the end if it is shorter.



                                    Similarly, p[20:100] adds the values 7 and 8 at the end of your list because your list is shorter than the specified amount.







                                    share|improve this answer












                                    share|improve this answer



                                    share|improve this answer










                                    answered 19 mins ago









                                    hd1hd1

                                    24.5k35568




                                    24.5k35568






























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