Python 2.7.2 doesn't properly iterate through logger's handlers

Question

the following code is very simple, but for some reason, for-loop does not iterate through all handlers of the logger. however, for-loop does iterate through all handlers if we remove my_logger.removeHandler( handler ) in the else clause. any idea if I'm doing anything wrong?

import logging
import sys

stdf = logging.Formatter( "%(message)s" )
filef = logging.Formatter( "%(message)s" )
my_logger = logging.getLogger( "file std logger" )
stdh = logging.StreamHandler( sys.stdout )
stdh.setFormatter( stdf )
my_logger.addHandler( stdh )
fileh = logging.FileHandler( "run.log", mode = 'w', encoding = "utf-16-le", delay = True )
fileh.setLevel( logging.DEBUG )
fileh.setFormatter( filef )
my_logger.addHandler( fileh )

my_logger.info("wow1")
my_logger.info("test string1")
my_logger.info("wow2")
my_logger.info("test string2")
my_logger.info("wow3")
my_logger.info("test string3")

for handler in my_logger.handlers:
    print(handler)
    if handler.__class__.__name__ == "FileHandler":
        handler.flush()
        handler.close()
        my_logger.removeHandler( handler )
    else:
        handler.flush()
        my_logger.removeHandler( handler )

my_logger.handlers

Answer 1

This is a classic destructive iteration. What you are doing is:

>>> l= [0, 1, 2, 3, 4]
>>> for n in l:
...     l.remove(n)
... 
>>> l
[1, 3]

In this example only every second element is removed. Why? Well the for...in syntax obscures what's actually happening, which is the same as in a traditional C-style for-index loop:

>>> l= [0, 1, 2, 3, 4]
>>> i= 0
>>> while i<len(l):
...     del l[i]
...     i+= 1
...
>>> l
[1, 3]

So first time round the loop i is 0. Item 0 gets removed, moving items 1–4 down one place to be the new items 0–3. Next time round the loop i is 1 so the current item 1, which is 2, is removed. The original 1 has been jumped and remains in the loop.

A simple workaround to avoid destructive iteration is to take a copy of the list and iterate the copy whilst changing the original:

for handler in list(my_logger.handlers):

Using filter or list comprehensions is typically a simpler route when you are trying to remove certain items from a list.