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pythonrange

Does range() really create lists?

Both my professor and this guy claim that range creates a list of values.

"Note: The range function simply returns a list containing the numbers from x to y-1. For example, range(5, 10) returns the list [5, 6, 7, 8, 9]."

I believe this is to be inaccurate because:

type(range(5, 10))
<class 'range'>

Furthermore, the only apparent way to access the integers created by range is to iterate through them, which leads me to believe that labeling range as a lists is incorrect.

Solution

  • 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() over range() is minimal (since xrange() 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, whenever islice is done on a range 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 change

    numbers = range(100)

    to

    numbers = iter(range(100))

    Now, numbers is an iterator object and it remembers how long it has been iterated before. So, when the islice iterates it, it just starts from the place where it previously ended.