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From: Tim Chase <python.list@tim.thechases.com>
Newsgroups: comp.lang.python
Subject: Re: Iteration, while loop, and for loop
Date: Thu, 30 Jun 2016 05:44:59 -0500
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On 2016-06-30 09:59, Steven D'Aprano wrote:
> But there's no need to go to such effort for a mutable iterator.
> This is much simpler:
>   
> py> mi = list('bananas')
> py> for char in mi:    
> ...     if char == 'a':
> ...             mi.extend(' yum')
> ...     print(char, end='')
> ... else:  # oh no, the feared for...else!
> ...     # needed to prevent the prompt overwriting the output
> ...     print()
> ...
> bananas yum yum yum  
> py>     
> 
> 
> This example shows two things:
> 
> (1) There's no need for a MutableIterator, we have list;  

Convenient to know.  I was fairly certain that this had failed for me
in past versions, but I went back to the oldest I have (2.4) and it
still works there.  Might have to revisit some queuing code I have.

That said, it's not consistent across iterable container types.  With

  mi = set('bananas')
  for char in mi:
    if char == 'a':
      mi.add('X')
    print(char)
  else:
    print()

I get

  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  RuntimeError: Set changed size during iteration

If the list() meets your needs, then you're in luck.  But just as
frequently, I want to use a set() or a dict() and have to write my
own wrapper around it.

-tkc