On Tue, Jun 28, 2016 at 11:58 AM, Grant Edwards
<grant.b.edwa...@gmail.com> wrote:
> On 2016-06-28, Tim Chase <python.l...@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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