单行检查迭代器是否产生至少一个元素？

Currently I’m doing this:

try:
    something = iterator.next()
    # ...
except StopIteration:
    # ...

But I would like an expression that I can place inside a simple if statement. Is there anything built-in which would make this code look less clumsy?

any() returns False if an iterable is empty, but it will potentially iterate over all the items if it’s not. I only need it to check the first item.

Someone asks what I’m trying to do. I have written a function which executes an SQL query and yields its results. Sometimes when I call this function I just want to know if the query returned anything and make a decision based on that.

any won’t go beyond the first element if it’s True. In case the iterator yields something false-ish you can write any(True for _ in iterator).

In Python 2.6+, if name sentinel is bound to a value which the iterator can’t possibly yield,

if next(iterator, sentinel) is sentinel:
    print('iterator was empty')

If you have no idea of what the iterator might possibly yield, make your own sentinel (e.g. at the top of your module) with

sentinel = object()

Otherwise, you could use, in the sentinel role, any value which you “know” (based on application considerations) that the iterator can’t possibly yield.

This isn’t really cleaner, but it shows a way to package it in a function losslessly:

def has_elements(iter):
  from itertools import tee
  iter, any_check = tee(iter)
  try:
    any_check.next()
    return True, iter
  except StopIteration:
    return False, iter

has_el, iter = has_elements(iter)
if has_el:
  # not empty

This isn’t really pythonic, and for particular cases, there are probably better (but less general) solutions, like the next default.

first = next(iter, None)
if first:
  # Do something

This isn’t general because None can be a valid element in many iterables.

you can use:

if zip([None], iterator):
    # ...
else:
    # ...

but it’s a bit nonexplanatory for the code reader

The best way to do that is with a peekable from more_itertools.

from more_itertools import peekable
iterator = peekable(iterator)
if iterator:
    # Iterator is non-empty.
else:
    # Iterator is empty.

Just beware if you kept refs to the old iterator, that iterator will get advanced. You have to use the new peekable iterator from then on. Really, though, peekable expects to be the only bit of code modifying that old iterator, so you shouldn’t be keeping refs to the old iterator lying around anyway.

What about:

In [1]: i=iter([])

In [2]: bool(next(i,False))
Out[2]: False

In [3]: i=iter([1])

In [4]: bool(next(i,False))
Out[4]: True

__length_hint__ estimates the length of list(it) – it’s private method, though:

x = iter( (1, 2, 3) )
help(x.__length_hint__)
      1 Help on built-in function __length_hint__:
      2 
      3 __length_hint__(...)
      4     Private method returning an estimate of len(list(it)).

This is an overkill iterator wrapper that generally allows to check whether there’s a next item (via conversion to boolean). Of course pretty inefficient.

class LookaheadIterator ():

    def __init__(self, iterator):
        self.__iterator = iterator
        try:
            self.__next      = next (iterator)
            self.__have_next = True
        except StopIteration:
            self.__have_next = False

    def __iter__(self):
        return self

    def next (self):
        if self.__have_next:
            result = self.__next
            try:
                self.__next      = next (self.__iterator)
                self.__have_next = True
            except StopIteration:
                self.__have_next = False

            return result

        else:
            raise StopIteration

    def __nonzero__(self):
        return self.__have_next

x = LookaheadIterator (iter ([]))
print bool (x)
print list (x)

x = LookaheadIterator (iter ([1, 2, 3]))
print bool (x)
print list (x)

Output:

False
[]
True
[1, 2, 3]

A little late, but… You could turn the iterator into a list and then work with that list:

# Create a list of objects but runs out the iterator.
l = [_ for _ in iterator]

# If the list is not empty then the iterator had elements; else it was empty.
if l :
    pass # Use the elements of the list (i.e. from the iterator)
else :
    pass # Iterator was empty, thus list is empty.