带有检查空条件的 python 生成器

Question

在大多数情况下，python 生成器是列表的良好替代品，除了我想检查空条件的地方，这对于普通生成器来说是不可能的。我正在尝试编写一个包装器，它允许检查空条件但仍然很懒惰并提供生成器的好处。

class mygen:
  def __init__(self,iterable):
    self.iterable = (x for x in iterable)
    self.peeked = False
    self.peek = None
  def __iter__(self):
    if self.peeked:
      yield self.peek
      self.peeked = False
    for val in self.iterable:
      if self.peeked:
        yield self.peek
        self.peeked = False
      yield val
    if self.peeked:
      yield self.peek
      self.peeked = False
  def __nonzero__(self):
    if self.peeked:
      return True
    try:
      self.peek = self.iterable.next()
      self.peeked = True
      return True
    except:
      return False

1. 我认为它的行为与普通生成器一样正确。有没有任何角落案例我迷路了？
2. 这看起来不太优雅。是否有更好的更 pythonic 方式来做同样的事情？

示例用法:

def get_odd(l):
    return mygen(x for x in l if x%2)

def print_odd(odd_nums):
  if odd_nums:
      print "odd numbers found",list(odd_nums)
  else:
      print "No odd numbers found"

print_odd(get_odd([2,4,6,8]))
print_odd(get_odd([2,4,6,8,7]))

Answer 1

我通常不会实现这种的发电机。有一种惯用的方法来测试迭代器是否 it筋疲力尽:

try:
    next_item = next(it)
except StopIteration:
    # exhausted, handle this case

用一些特定于项目的 LBYL 惯用语替换这个 EAFP 惯用语似乎令人困惑且毫无益处。

也就是说，如果我真的想要的话，我将如何实现它:

class MyIterator(object):
    def __init__(self, iterable):
        self._iterable = iter(iterable)
        self._exhausted = False
        self._cache_next_item()
    def _cache_next_item(self):
        try:
            self._next_item = next(self._iterable)
        except StopIteration:
            self._exhausted = True
    def __iter__(self):
        return self
    def next(self):
        if self._exhausted:
            raise StopIteration
        next_item = self._next_item
        self._cache_next_item()
        return next_item
    def __nonzero__(self):
        return not self._exhausted