recognizing empty iterators

[Chris Perkins]

mis6 at pitt.edu (Michele Simionato) wrote in message 

> I realized that I don't know a satisfactory way to check if an
> iterator is empty. In other words I am looking for an
> "isempty" function to use in "if" statements such as
> 
> if isempty(iterator):
>    do_something()
> 
> without side effects.
> 


In general, it is impossible to test whether an iterator is exhausted.
For example, how would you test the following:

import random
def am_i_empty():
	while random.choice((True,False)):
		yield 42
	return

So you have to call "next" - there's no way around it. You just need a
wrapper to cache the value:

class checkable_iter:
	def __init__(self,it):
		self.it = iter(it)
		self.cached = []
	def isempty(self):
		if self.cached:
			return False
		try:
			self.cached.append(self.it.next())
			return False
		except StopIteration:
			return True
	def next(self):
		if self.cached:
			return self.cached.pop()
		return self.it.next()


>>> it = checkable_iter(am_i_empty())
>>> it.isempty()
False
>>> it.next()
42
>>> it.isempty()
True
>>> it.next()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
StopIteration

Chris