removing items from a list or group within a for loop.

[perfringo]

Typing >>> help('for') into interactive interpreter will give pretty comprehensive description about for-loop behaviour. For example:

There is a subtlety when the sequence is being modified by the
loop (this can only occur for mutable sequences, e.g. lists). An
internal counter is used to keep track of which item is used next,
and this is incremented on each iteration. When this counter has
reached the length of the sequence the loop terminates. This means
that if the suite deletes the current (or a previous) item from the
sequence, the next item will be skipped (since it gets the index of
the current item which has already been treated). Likewise, if the
suite inserts an item in the sequence before the current item, the
current item will be treated again the next time through the loop.
This can lead to nasty bugs that can be avoided by making a
temporary copy using a slice of the whole sequence, e.g.,

     for x in a[:]:
         if x < 0: a.remove(x)