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I am confused about why different looping constructs behave so differently when used with a simple generator. Consider the following code:

example_list = [1, 2, 3, 4, 5] for_count = 0 next_count = 0 while_count = 0 def consumer(iterable): for item in iterable: yield return for item in consumer(example_list): print("For Count: {0}".format(for_count)) for_count += 1 # First while loop while consumer(example_list).next(): print("Next Count: {0}".format(next_count)) next_count += 1 # Second while loop while consumer(example_list): print("While Count: {0}".format(while_count)) while_count += 1 if while_count > 10: # add contrived limit for infinite loop break

The output for this code is:

For Count: 0
For Count: 1
For Count: 2
For Count: 3
For Count: 4
While Count: 0
While Count: 1
While Count: 2
While Count: 3
While Count: 4
While Count: 5
While Count: 6
While Count: 7
While Count: 8
While Count: 9
While Count: 10

I would appreciate help understanding the following:

  1. How does the for loop know when to end but not the second while loop? I expected both while loops to exit immediately since None is yielded.
  2. Why doesn't using .next() raise an exception? The consumer routine isn't a class with a __next__() method defined, so does it magically appear when you use the yield keyword?
  3. Why is it that if I change consumer to yield item, the first while loop become infinite like the second one?
  4. If I change consumer to simply return instead of yielding anything, the second while loop exits immediate instead of becoming infinite. I've been under the impression that a yield is essentially a return that you can resume from. Why are they treated differently by a while loop?
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2 Answers 2

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6

The for loop

Your first for loop works as expected. Update: Mark Ransom noted that your yield is not accompanied by the expected item, so it just returns [None, None, None, None, None] rather than [1, 2, 3, 4, 5] - but it still iterates over the list.

The first while loop

The very same commentator also noticed that the first while loop never starts because 0 is a False-equivalent in Python.

The second while loop

In the second while loop, you are testing the value of consumer(example_list). This is the generator object itself, not the values return by its next(). The object itself never equals None, or any other False equivalent - so your loop never ends.

This can be seen by printing the value of consumer(example_list), your while condition, within the loop:

>>> while_count=0 >>> while consumer(example_list): ... print while_count, consumer(example_list) ... while_count += 1 ... if while_count > 10: ... break

Giving:

0 <generator object consumer at 0x1044a1b90> 1 <generator object consumer at 0x1044a1b90> 2 <generator object consumer at 0x1044a1b90> 3 <generator object consumer at 0x1044a1b90> 4 <generator object consumer at 0x1044a1b90> 5 <generator object consumer at 0x1044a1b90> 6 <generator object consumer at 0x1044a1b90> 7 <generator object consumer at 0x1044a1b90> 8 <generator object consumer at 0x1044a1b90> 9 <generator object consumer at 0x1044a1b90> 10 <generator object consumer at 0x1044a1b90>

The second item is the object, which never equals None.

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4 Comments

And the middle example returns 0 the first time, which does evaluate as False thus terminating that loop immediately.
@MarkRansom True, good point. Didn't event look at that loop.
Actually I was mistaken, since there's nothing after the yield it returns None not 0, which also evaluates as False. So I was right but for the wrong reason.
@MarkRansom - Splendid. BTW, working with breakpoints and a debugger would have made it clear.
1

Answering only a subset of your question:

Your misconception with the while loops is, that a while loop does not iterate over a generator object by itself, as a for item in generator loop does.

The consumer(example_list) in your second while loop always returns a generator, which evaluates as a boolean True, hence the loops runs forever.

In the first while loop, you're testing the first yield value from that generator, which is None. Hence, the loop doesn't even start.

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