Does range() really create lists?
In Python 2.x, range
returns a list, but in Python 3.x range
returns an immutable sequence, of type range
.
Python 2.x:
>>> type(range(10))
<type 'list'>
>>> type(xrange(10))
<type 'xrange'>
Python 3.x:
>>> type(range(10))
<class 'range'>
In Python 2.x, if you want to get an iterable object, like in Python 3.x, you can use xrange
function, which returns an immutable sequence of type xrange
.
Advantage of xrange
over range
in Python 2.x:
The advantage of
xrange()
overrange()
is minimal (sincexrange()
still has to create the values when asked for them) except when a very large range is used on a memory-starved machine or when all of the range’s elements are never used (such as when the loop is usually terminated with break).
Note:
Furthermore, the only apparent way to access the integers created by
range()
is to iterate through them,
Nope. Since range
objects in Python 3 are immutable sequences, they support indexing as well. Quoting from the range
function documentation,
Ranges implement all of the common sequence operations except concatenation and repetition
...
Range objects implement the
collections.abc.Sequence
ABC, and provide features such as containment tests, element index lookup, slicing and support for negative indices.
For example,
>>> range(10, 20)[5]
15
>>> range(10, 20)[2:5]
range(12, 15)
>>> list(range(10, 20)[2:5])
[12, 13, 14]
>>> list(range(10, 20, 2))
[10, 12, 14, 16, 18]
>>> 18 in range(10, 20)
True
>>> 100 in range(10, 20)
False
All these are possible with that immutable range
sequence.
Recently, I faced a problem and I think it would be appropriate to include here. Consider this Python 3.x code
from itertools import islice
numbers = range(100)
items = list(islice(numbers, 10))
while items:
items = list(islice(numbers, 10))
print(items)
One would expect this code to print every ten numbers as a list, till 99. But, it would run infinitely. Can you reason why?
Solution
Because the
range
returns an immutable sequence, not an iterator object. So, wheneverislice
is done on arange
object, it always starts from the beginning. Think of it as a drop-in replacement for an immutable list. Now the question comes, how will you fix it? Its simple, you just have to get an iterator out of it. Simply changenumbers = range(100)
tonumbers = iter(range(100))
Now,numbers
is an iterator object and it remembers how long it has been iterated before. So, when theislice
iterates it, it just starts from the place where it previously ended.
range creates a list if the python version used is 2.x . In this scenario range is to be used only if its referenced more than once otherwise use xrange which creates a generator there by redusing the memory usage and sometimes time as it has lazy approach.
xrange is not there in python 3.x rather range stands for what xrange is for python 2.x
refer to question What is the difference between range and xrange functions in Python 2.X?
It depends.
In python-2.x, range
actually creates a list (which is also a sequence) whereas xrange
creates an xrange
object that can be used to iterate through the values.
On the other hand, in python-3.x, range
creates an iterable (or more specifically, a sequence)