在Python生成器中使用for…else

Question

I'm a big fan of Python's for...else syntax - it's surprising how often it's applicable, and how effectively it can simplify code.

However, I've not figured out a nice way to use it in a generator, for example:

def iterate(i):
    for value in i:
        yield value
    else:
        print 'i is empty'

In the above example, I'd like the print statement to be executed only if i is empty. However, as else only respects break and return , it is always executed, regardless of the length of i .

If it's impossible to use for...else in this way, what's the best approach to this so that the print statement is only executed when nothing is yielded?

Answer 1

You're breaking the definition of a generator, which should throw a StopIteration exception when iteration is complete (which is automatically handled by a return statement in a generator function)

So:

def iterate(i):
    for value in i:
        yield value
    return

Best to let the calling code handle the case of an empty iterator:

count = 0
for value in iterate(range([])):
    print value
    count += 1
else:
    if count == 0:
        print "list was empty"

Might be a cleaner way of doing the above, but that ought to work fine, and doesn't fall into any of the common 'treating an iterator like a list' traps below.

Answer 2

There are a couple ways of doing this. You could always use the Iterator directly:

def iterate(i):
    try:
        i_iter = iter(i)
        next = i_iter.next()
    except StopIteration:
        print 'i is empty'
        return

    while True:
        yield next
        next = i_iter.next()

But if you know more about what to expect from the argument i , you can be more concise:

def iterate(i):
    if i:  # or if len(i) == 0
        for next in i:
            yield next
    else:
        print 'i is empty'
        raise StopIteration()

Answer 3

As you note, for..else only detects a break . So it's only applicable when you look for something and then stop .

It's not applicable to your purpose not because it's a generator, but because you want to process all elements, without stopping (because you want to yield them all, but that's not the point).

So generator or not, you really need a boolean, as in Ber's solution.

Answer 4

Summing up some of the earlier answers, it could be solved like this:

def iterate(i):
    empty = True
    for value in i:
        yield value
        empty = False

    if empty:
        print "empty"

so there really is no "else" clause involved.

Answer 5

If it's impossible to use for...else in this way, what's the best approach to this so that the print statement is only executed when nothing is yielded?

Maximum i can think of:

>>> empty = True
>>> for i in [1,2]:
...     empty = False
... if empty:
...     print 'empty'
...
>>>
>>>
>>> empty = True
>>> for i in []:
...     empty = False
... if empty:
...    print 'empty'
...
empty
>>>

Answer 6

What about simple if-else?

def iterate(i):
    if len(i) == 0: print 'i is empty'
    else:
        for value in i:
            yield value