Iterable VS Iterator

Today, during the interview, I discovered that the interviewer has a big conflict between iterator and iterable. So, I decided to write this post.

Firstable, let’s start by presenting the context, the question was to locate the error in the below code

>>> l = range(3)
>>> l [2] = 10
>>> l = (1, 3, 4)

The interviewer think that there is two errors in the last two lines:

  • he think that range returns a non mutable object so we can not modify its value
  • he think that we can not assign l to another object

However, In Python 2.x, range returns a list, and not an iterator, So the second line is true in Python 2.x. In the case of the third line, It’s acceptable in all python versions.

>>> l = xrange(3)
>>> l [2] = 10
>>> l = (1, 3, 4)

For the precedent code, the second line raise a TypeError exception, because xrange returns an iterator object not an iterable object

Conclusion:

They are iterable, but they are not iterators. They can be passed to iter() to get an iterator for them either implicitly (e.g. via for) or explicitly, but they are not iterators in and of themselves. An iterator is an object representing a stream of data. It implements the iterator protocol:

  • __iter__ method
  • next method

On the other hand, Iterable object implements the __iter__ method that returns an Iterator object.

 

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