Iteration, while loop, and for loop

Ian Kelly ian.g.kelly at gmail.com
Wed Jun 29 11:29:18 EDT 2016


On Tue, Jun 28, 2016 at 11:58 AM, Grant Edwards
<grant.b.edwards at gmail.com> wrote:
> On 2016-06-28, Tim Chase <python.list at tim.thechases.com> wrote:
>> On 2016-06-29 01:20, Steven D'Aprano wrote:
>>> While loops are great for loops where you don't know how many
>>> iterations there will be but you do know that you want to keep
>>> going while some condition applies:
>>>
>>> while there is still work to be done:
>>>     do some more work
>>
>> I find this particularly the case when the thing being iterated over
>> can be changed, such as a queue of things to process:
>>
>>   items = deque()
>>   items.append(root_node)
>>   while items:
>>     item = items.popleft()
>>     process(item)
>>     items.extend(item.children)
>
> Yep, I often do something similar when processing a block of data
> bytes comprising a sequence of "things" of varying number of bytes.
>
>     data = read_a_blob_of_bytes()
>     while data:
>         #figure out how long the first "thing" is
>         len = <some expression typically involving the first few bytes of 'data'>
>         handle_thing(data[:len])
>         data = data[len:]
>> But then, if you wrap up your "while" loop as a generator that yields
>> things, you can then use it in a "for" loop which seems to me like
>> the Pythonic way to do things. :-)
>
> Yea, I keep telling myself that, but I never actually do it.

Here you go:

import collections

class MutableIterator:

  def __init__(self, iterable):
    self._stopped = False
    self.replace(iterable)

  def __iter__(self):
    return self

  def __next__(self):
    if self._stopped:
      raise StopIteration
    while self._iterator or self._iterables:
      if self._iterator:
        try:
          return next(self._iterator)
        except StopIteration:
          self._iterator = None
      if self._iterables:
        self._iterator = iter(self._iterables.popleft())
    self._stopped = True
    raise StopIteration

  def clear():
    self._iterables.clear()
    self._iterator = None

  def replace(self, iterable):
    self._check_stopped()
    self._iterables = collections.deque([iterable])
    self._iterator = None

  def append(self, item):
    self.extend([item])

  def extend(self, iterable):
    self._check_stopped()
    self._iterables.append(iterable)

  def _check_stopped(self):
    if self._stopped:
      raise ValueError('Tried to mutate a stopped iterator')


# Example:

>>> mi = MutableIterator('bananas')
>>> for char in mi:
...   if char == 'a':
...     mi.extend(' yum')
...   print(char, end='')
...
bananas yum yum yum


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