Understanding builtin next() function

Question

I read the documentation on next() and I understand it abstractly. From what I understand, next() is used as a reference to an iterable object and makes python cycle to the next iterable object sequentially. Makes sense! My question is, how is this useful outside the context of the builtin for loop? When would someone ever need to use next() directly? Can someone provide a simplistic example? Thanks mates!

Answer 1

There are many places where we can use next, for eg.

Drop the header while reading a file.

with open(filename) as f:
    next(f)  #drop the first line
    #now do something with rest of the lines

Iterator based implementation of zip(seq, seq[1:])(from pairwise recipe iterools):

from itertools import tee, izip
it1, it2 = tee(seq)
next(it2)
izip(it1, it2)

Get the first item that satisfies a condition:

next(x for x in seq if x % 100)

Creating a dictionary using adjacent items as key-value:

>>> it = iter(['a', 1, 'b', 2, 'c', '3'])
>>> {k: next(it) for k in it}
{'a': 1, 'c': '3', 'b': 2}

Answer 2

As luck would have it, I wrote one yesterday:

def skip_letters(f, skip=" "):
    """Wrapper function to skip specified characters when encrypting."""
    def func(plain, *args, **kwargs):
        gen = f(p for p in plain if p not in skip, *args, **kwargs)              
        for p in plain:
            if p in skip:
                yield p
            else:
                yield next(gen)
    return func

This uses next to get the return values from the generator function f, but interspersed with other values. This allows some values to be passed through the generator, but others to be yielded straight out.

Answer 3

next is useful in many different ways, even outside of a for-loop. For example, if you have an iterable of objects and you want the first that meets a condition, you can give it a generator expression like so:

>>> lst = [1, 2, 'a', 'b']
>>> # Get the first item in lst that is a string
>>> next(x for x in lst if isinstance(x, str))
'a'
>>> # Get the fist item in lst that != 1
>>> lst = [1, 1, 1, 2, 1, 1, 3]
>>> next(x for x in lst if x != 1)
2
>>>