Is the iteration protocol used when indexing a range object?












5















Since the range object produces values on demand, does it mean that anytime a range is indexed, the iteration protocol is invoked up to that index?



What I mean is when:



>>> R = range(1,11)

>>> print(R[5])

6


Since R[5] isn't stored in memory, is it calculated every time by creating a new iterator? If not, how is it possible to index a range object?






python python-3.x indexing python-internals







share|improve this question





























    5















    Since the range object produces values on demand, does it mean that anytime a range is indexed, the iteration protocol is invoked up to that index?



    What I mean is when:



    >>> R = range(1,11)
    
>>> print(R[5])
    
6


    Since R[5] isn't stored in memory, is it calculated every time by creating a new iterator? If not, how is it possible to index a range object?






    python python-3.x indexing python-internals







    share|improve this question



























      5












      5








      5


      0






      Since the range object produces values on demand, does it mean that anytime a range is indexed, the iteration protocol is invoked up to that index?



      What I mean is when:



      >>> R = range(1,11)
      
>>> print(R[5])
      
6


      Since R[5] isn't stored in memory, is it calculated every time by creating a new iterator? If not, how is it possible to index a range object?






      python python-3.x indexing python-internals







      share|improve this question
















      Since the range object produces values on demand, does it mean that anytime a range is indexed, the iteration protocol is invoked up to that index?



      What I mean is when:



      >>> R = range(1,11)
      
>>> print(R[5])
      
6


      Since R[5] isn't stored in memory, is it calculated every time by creating a new iterator? If not, how is it possible to index a range object?






      python python-3.x indexing python-internals




      python python-3.x indexing python-internals






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited Nov 22 '18 at 11:30









      Alex Riley

      82.6k26163166




      82.6k26163166










      asked Nov 21 '18 at 22:45









      masiewpaomasiewpao

      84110




      84110
























          2 Answers
          2






          active

          oldest

          votes


















          5














          No iterator is created here, and no iteration occurs. The range object is implemented such that Python computes the value of R[5] on-demand in constant time.1



          If the index i is not negative, the computation boils down to:



          i * step + start


          So in the case of your code R[5], this would be 5*1 + 1 which is 6.



          If the index i is negative, the length of R is added to i first, and then the value is computed as before:



          (i + len(R)) * step + start



          Python-internals



          When you write R[5], this Python syntax is eventually translated into a call to PyObject_GetItem, which inspects the object R to see how it should proceed to find the item at index 5.



          PyObject_GetItem first checks the tp_as_mapping slot of the range type. This is not null; it holds a reference to a struct called range_as_mapping. PyObject_GetItem then checks to see what is in the mp_subscript field of this struct:



          static PyMappingMethods range_as_mapping = {
          
        (lenfunc)range_length,       /* mp_length */
          
        (binaryfunc)range_subscript, /* mp_subscript */
          
        (objobjargproc)0,            /* mp_ass_subscript */
          
};


          As you can see in the snippet above, it finds the range_subscript function occupying the mp_subscript field.2



          Now range_subscript inspects the arguments it was passed (R and 5) to decide if a single index or a slice was requested. The integer 5 means that just a single index is needed and so the function delegates the computation of the value to compute_range_item. This function performs the computation to return the integer 6 as outlined in the first part of this answer.




          1 I have assumed you are using CPython: other Python implementations may implement the range object differently.



          2 If you were to call len(R), you can see the internal function in mp_length that is called to compute the length of R (cf. Why is "1000000000000000 in range(1000000000000001)" so fast in Python 3?).






          share|improve this answer


























          • In addition to these C-level functions, it might be worth mentioning the Python-level __getitem__ and __len__, as well as the sequence protocol.

            – lvc
            Nov 22 '18 at 11:43



















          -1














          Nope. It is not.



          However range supports both iteration protocol and indexing (through getitem)






          share|improve this answer



















          • 1





            Hi thanks for the response! I had a look at the docs for getitem but I am not sure I understand it. If it equates to using self[key] for an object, isn't there still the problem of knowing the value in a virtual sequence (since it's not stored in memory)?

            – masiewpao
            Nov 22 '18 at 9:50












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          2 Answers
          2






          active

          oldest

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          2 Answers
          2






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          5














          No iterator is created here, and no iteration occurs. The range object is implemented such that Python computes the value of R[5] on-demand in constant time.1



          If the index i is not negative, the computation boils down to:



          i * step + start


          So in the case of your code R[5], this would be 5*1 + 1 which is 6.



          If the index i is negative, the length of R is added to i first, and then the value is computed as before:



          (i + len(R)) * step + start



          Python-internals



          When you write R[5], this Python syntax is eventually translated into a call to PyObject_GetItem, which inspects the object R to see how it should proceed to find the item at index 5.



          PyObject_GetItem first checks the tp_as_mapping slot of the range type. This is not null; it holds a reference to a struct called range_as_mapping. PyObject_GetItem then checks to see what is in the mp_subscript field of this struct:



          static PyMappingMethods range_as_mapping = {
          
        (lenfunc)range_length,       /* mp_length */
          
        (binaryfunc)range_subscript, /* mp_subscript */
          
        (objobjargproc)0,            /* mp_ass_subscript */
          
};


          As you can see in the snippet above, it finds the range_subscript function occupying the mp_subscript field.2



          Now range_subscript inspects the arguments it was passed (R and 5) to decide if a single index or a slice was requested. The integer 5 means that just a single index is needed and so the function delegates the computation of the value to compute_range_item. This function performs the computation to return the integer 6 as outlined in the first part of this answer.




          1 I have assumed you are using CPython: other Python implementations may implement the range object differently.



          2 If you were to call len(R), you can see the internal function in mp_length that is called to compute the length of R (cf. Why is "1000000000000000 in range(1000000000000001)" so fast in Python 3?).






          share|improve this answer


























          • In addition to these C-level functions, it might be worth mentioning the Python-level __getitem__ and __len__, as well as the sequence protocol.

            – lvc
            Nov 22 '18 at 11:43
















          5














          No iterator is created here, and no iteration occurs. The range object is implemented such that Python computes the value of R[5] on-demand in constant time.1



          If the index i is not negative, the computation boils down to:



          i * step + start


          So in the case of your code R[5], this would be 5*1 + 1 which is 6.



          If the index i is negative, the length of R is added to i first, and then the value is computed as before:



          (i + len(R)) * step + start



          Python-internals



          When you write R[5], this Python syntax is eventually translated into a call to PyObject_GetItem, which inspects the object R to see how it should proceed to find the item at index 5.



          PyObject_GetItem first checks the tp_as_mapping slot of the range type. This is not null; it holds a reference to a struct called range_as_mapping. PyObject_GetItem then checks to see what is in the mp_subscript field of this struct:



          static PyMappingMethods range_as_mapping = {
          
        (lenfunc)range_length,       /* mp_length */
          
        (binaryfunc)range_subscript, /* mp_subscript */
          
        (objobjargproc)0,            /* mp_ass_subscript */
          
};


          As you can see in the snippet above, it finds the range_subscript function occupying the mp_subscript field.2



          Now range_subscript inspects the arguments it was passed (R and 5) to decide if a single index or a slice was requested. The integer 5 means that just a single index is needed and so the function delegates the computation of the value to compute_range_item. This function performs the computation to return the integer 6 as outlined in the first part of this answer.




          1 I have assumed you are using CPython: other Python implementations may implement the range object differently.



          2 If you were to call len(R), you can see the internal function in mp_length that is called to compute the length of R (cf. Why is "1000000000000000 in range(1000000000000001)" so fast in Python 3?).






          share|improve this answer


























          • In addition to these C-level functions, it might be worth mentioning the Python-level __getitem__ and __len__, as well as the sequence protocol.

            – lvc
            Nov 22 '18 at 11:43














          5












          5








          5







          No iterator is created here, and no iteration occurs. The range object is implemented such that Python computes the value of R[5] on-demand in constant time.1



          If the index i is not negative, the computation boils down to:



          i * step + start


          So in the case of your code R[5], this would be 5*1 + 1 which is 6.



          If the index i is negative, the length of R is added to i first, and then the value is computed as before:



          (i + len(R)) * step + start



          Python-internals



          When you write R[5], this Python syntax is eventually translated into a call to PyObject_GetItem, which inspects the object R to see how it should proceed to find the item at index 5.



          PyObject_GetItem first checks the tp_as_mapping slot of the range type. This is not null; it holds a reference to a struct called range_as_mapping. PyObject_GetItem then checks to see what is in the mp_subscript field of this struct:



          static PyMappingMethods range_as_mapping = {
          
        (lenfunc)range_length,       /* mp_length */
          
        (binaryfunc)range_subscript, /* mp_subscript */
          
        (objobjargproc)0,            /* mp_ass_subscript */
          
};


          As you can see in the snippet above, it finds the range_subscript function occupying the mp_subscript field.2



          Now range_subscript inspects the arguments it was passed (R and 5) to decide if a single index or a slice was requested. The integer 5 means that just a single index is needed and so the function delegates the computation of the value to compute_range_item. This function performs the computation to return the integer 6 as outlined in the first part of this answer.




          1 I have assumed you are using CPython: other Python implementations may implement the range object differently.



          2 If you were to call len(R), you can see the internal function in mp_length that is called to compute the length of R (cf. Why is "1000000000000000 in range(1000000000000001)" so fast in Python 3?).






          share|improve this answer















          No iterator is created here, and no iteration occurs. The range object is implemented such that Python computes the value of R[5] on-demand in constant time.1



          If the index i is not negative, the computation boils down to:



          i * step + start


          So in the case of your code R[5], this would be 5*1 + 1 which is 6.



          If the index i is negative, the length of R is added to i first, and then the value is computed as before:



          (i + len(R)) * step + start



          Python-internals



          When you write R[5], this Python syntax is eventually translated into a call to PyObject_GetItem, which inspects the object R to see how it should proceed to find the item at index 5.



          PyObject_GetItem first checks the tp_as_mapping slot of the range type. This is not null; it holds a reference to a struct called range_as_mapping. PyObject_GetItem then checks to see what is in the mp_subscript field of this struct:



          static PyMappingMethods range_as_mapping = {
          
        (lenfunc)range_length,       /* mp_length */
          
        (binaryfunc)range_subscript, /* mp_subscript */
          
        (objobjargproc)0,            /* mp_ass_subscript */
          
};


          As you can see in the snippet above, it finds the range_subscript function occupying the mp_subscript field.2



          Now range_subscript inspects the arguments it was passed (R and 5) to decide if a single index or a slice was requested. The integer 5 means that just a single index is needed and so the function delegates the computation of the value to compute_range_item. This function performs the computation to return the integer 6 as outlined in the first part of this answer.




          1 I have assumed you are using CPython: other Python implementations may implement the range object differently.



          2 If you were to call len(R), you can see the internal function in mp_length that is called to compute the length of R (cf. Why is "1000000000000000 in range(1000000000000001)" so fast in Python 3?).







          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited Nov 22 '18 at 11:36

























          answered Nov 22 '18 at 11:29









          Alex RileyAlex Riley

          82.6k26163166




          82.6k26163166













          • In addition to these C-level functions, it might be worth mentioning the Python-level __getitem__ and __len__, as well as the sequence protocol.

            – lvc
            Nov 22 '18 at 11:43



















          • In addition to these C-level functions, it might be worth mentioning the Python-level __getitem__ and __len__, as well as the sequence protocol.

            – lvc
            Nov 22 '18 at 11:43

















          In addition to these C-level functions, it might be worth mentioning the Python-level __getitem__ and __len__, as well as the sequence protocol.

          – lvc
          Nov 22 '18 at 11:43





          In addition to these C-level functions, it might be worth mentioning the Python-level __getitem__ and __len__, as well as the sequence protocol.

          – lvc
          Nov 22 '18 at 11:43













          -1














          Nope. It is not.



          However range supports both iteration protocol and indexing (through getitem)






          share|improve this answer



















          • 1





            Hi thanks for the response! I had a look at the docs for getitem but I am not sure I understand it. If it equates to using self[key] for an object, isn't there still the problem of knowing the value in a virtual sequence (since it's not stored in memory)?

            – masiewpao
            Nov 22 '18 at 9:50
















          -1














          Nope. It is not.



          However range supports both iteration protocol and indexing (through getitem)






          share|improve this answer



















          • 1





            Hi thanks for the response! I had a look at the docs for getitem but I am not sure I understand it. If it equates to using self[key] for an object, isn't there still the problem of knowing the value in a virtual sequence (since it's not stored in memory)?

            – masiewpao
            Nov 22 '18 at 9:50














          -1












          -1








          -1







          Nope. It is not.



          However range supports both iteration protocol and indexing (through getitem)






          share|improve this answer













          Nope. It is not.



          However range supports both iteration protocol and indexing (through getitem)







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered Nov 22 '18 at 2:01









          ealfieealfie

          1




          1








          • 1





            Hi thanks for the response! I had a look at the docs for getitem but I am not sure I understand it. If it equates to using self[key] for an object, isn't there still the problem of knowing the value in a virtual sequence (since it's not stored in memory)?

            – masiewpao
            Nov 22 '18 at 9:50














          • 1





            Hi thanks for the response! I had a look at the docs for getitem but I am not sure I understand it. If it equates to using self[key] for an object, isn't there still the problem of knowing the value in a virtual sequence (since it's not stored in memory)?

            – masiewpao
            Nov 22 '18 at 9:50








          1




          1





          Hi thanks for the response! I had a look at the docs for getitem but I am not sure I understand it. If it equates to using self[key] for an object, isn't there still the problem of knowing the value in a virtual sequence (since it's not stored in memory)?

          – masiewpao
          Nov 22 '18 at 9:50





          Hi thanks for the response! I had a look at the docs for getitem but I am not sure I understand it. If it equates to using self[key] for an object, isn't there still the problem of knowing the value in a virtual sequence (since it's not stored in memory)?

          – masiewpao
          Nov 22 '18 at 9:50


















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