popping an iterator

[Skaperen]

i can easily extract the last item from a list with mylist.pop(). is there an easy way to do that with an iterator?

[Yoriz]

import time.TimeMachine , use the future method to get the item at the end of the iterator.

[bowlofred]

An iterator isn't a specific type of object. Lists are iterators. Some iterators have an accessible last item and some do not.

You can iterate over an object like itertools.count which has no fixed length and no "end". So the most general answer is no.

[Skaperen]

import time.TimeMachine , use the future method to get the item at the end of the iterator.

i guess this must run the iterator since it could be a generator or such.

[Skaperen]

import time.TimeMachine , use the future method to get the item at the end of the iterator.

i don't seem to have time.TimeMachine. i'm still on Python 3.6. i hope to be on 3.9 soon.

[ndc85430]

Somehow I don't think that was a serious suggestion..

[ndc85430]

Also, what would you think should happen if the iterator was infinitely long?

It sounds like you've already come up with the solution to a problem, but it might help if you told us what the problem really is. Why do you want to do that? What are you trying to achieve, at a high level?

[DeaD_EyE]

Write your custom iterable/iterator

from collections.abc import Sequence


class SeekableIterator:
    def __init__(self, sequence):
        if not isinstance(sequence, Sequence):
            raise ValueError("No!")
            
        self.sequence = sequence
        self.index = 0

    def seek(self, position):
        if 0 <= position <= len(self):
            self.index = position
        else:
            raise ValueError("Position out of range")
                
    def seek_relative(self, position):
        if 0 <= (new_index := self.index + position) <= len(self):
            self.index = new_index
        else:
            raise ValueError("Position out of range")
        
    @property
    def first(self):
        if self.sequence:
            return self.sequence[0]
    
    @property
    def last(self):
        if self.sequence:
            return self.sequence[-1]    

    def __reversed__(self):
        """
        Returns a new instance of SeekableIterator
        and the sequence is a reversed copy of the old
        """
        return self.__class__(self.sequence[::-1])

    def __len__(self):
        return len(self.sequence)
        
    def __iter__(self):
        return self.__class__(self.sequence)
    
    def __next__(self):
        if self.index <= len(self) - 1:
            value = self.sequence[self.index]
            self.index += 1
            return value
        else:
            raise StopIteration



iterator = SeekableIterator(["a", "b", "c"])

[Skaperen]

i did find something called time_machine for CPython on Unix. it was a "PRE_LOAD" binary library that allowed setting fake clock values. it probably intercepted syscalls that get the time and added/subtracted a time offset. i think Linux can already do that in containers which would not let the same Python script control it (you'd have to create containers from Python and run something in them).

[Skaperen]

does reversed() work on iterators? ... nope!

Output:
lt2a/phil /home/phil 16> box try_rev.py
+----<try_rev.py>----+
| a=[1,2,3,4,5,6]    |
| b=iter(a)          |
| c=reversed(b)      |
| print(c[0])        |
+--------------------+
lt2a/phil /home/phil 17> py try_rev.py
Traceback (most recent call last):
  File "try_rev.py", line 3, in <module>
    c=reversed(b)
TypeError: 'list_iterator' object is not reversible
lt2a/phil /home/phil 18> 

i think it's time for me to create a function named revit.