Is the iteration protocol used when indexing a range object?
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
edited Nov 22 '18 at 11:30
Alex Riley
82.6k26163166
asked Nov 21 '18 at 22:45
masiewpaomasiewpao
84110
add a comment |
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
edited Nov 22 '18 at 11:30
Alex Riley
82.6k26163166
asked Nov 21 '18 at 22:45
masiewpaomasiewpao
84110
add a comment |
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
edited Nov 22 '18 at 11:30
Alex Riley
82.6k26163166
asked Nov 21 '18 at 22:45
masiewpaomasiewpao
84110
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
edited Nov 22 '18 at 11:30
Alex Riley
82.6k26163166
asked Nov 21 '18 at 22:45
masiewpaomasiewpao
84110
edited Nov 22 '18 at 11:30
Alex Riley
82.6k26163166
asked Nov 21 '18 at 22:45
masiewpaomasiewpao
84110
edited Nov 22 '18 at 11:30
Alex Riley
82.6k26163166
edited Nov 22 '18 at 11:30
Alex Riley
82.6k26163166
edited Nov 22 '18 at 11:30
Alex Riley
82.6k26163166
82.6k26163166
asked Nov 21 '18 at 22:45
masiewpaomasiewpao
84110
asked Nov 21 '18 at 22:45
masiewpaomasiewpao
84110
asked Nov 21 '18 at 22:45
masiewpaomasiewpao
84110
84110
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
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?).
answered Nov 22 '18 at 11:29
Alex RileyAlex Riley
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
add a comment |
Nope. It is not.
However range supports both iteration protocol and indexing (through getitem)
answered Nov 22 '18 at 2:01
ealfieealfie
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
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
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?).
answered Nov 22 '18 at 11:29
Alex RileyAlex Riley
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
add a comment |
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?).
answered Nov 22 '18 at 11:29
Alex RileyAlex Riley
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
add a comment |
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?).
answered Nov 22 '18 at 11:29
Alex RileyAlex Riley
82.6k26163166
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?).
answered Nov 22 '18 at 11:29
Alex RileyAlex Riley
82.6k26163166
edited Nov 22 '18 at 11:36
answered Nov 22 '18 at 11:29
Alex RileyAlex Riley
82.6k26163166
answered Nov 22 '18 at 11:29
Alex RileyAlex Riley
82.6k26163166
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
add a comment |
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
add a comment |
Nope. It is not.
However range supports both iteration protocol and indexing (through getitem)
answered Nov 22 '18 at 2:01
ealfieealfie
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
add a comment |
Nope. It is not.
However range supports both iteration protocol and indexing (through getitem)
answered Nov 22 '18 at 2:01
ealfieealfie
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
add a comment |
Nope. It is not.
However range supports both iteration protocol and indexing (through getitem)
answered Nov 22 '18 at 2:01
ealfieealfie
1
Nope. It is not.
However range supports both iteration protocol and indexing (through getitem)
answered Nov 22 '18 at 2:01
ealfieealfie
1
answered Nov 22 '18 at 2:01
ealfieealfie
1
answered Nov 22 '18 at 2:01
ealfieealfie
1
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
add a comment |
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
add a comment |
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