"dictionary changed size during iteration" error in Python 3 but not in Python 2

[Chris Green]

I have a (fairly) simple little program that removes old mail messages
from my junk folder.  I have just tried to upgrade it from Python 2 to
Python 3 and now, when it finds any message[s] to delete it produces
the error:-

    RuntimeError: dictionary changed size during iteration

I can sort of see why I'm getting the error but have two questions: 

1 - Why doesn't it error in Python 2?

2 - How do I fix it?


Here is the program:-

#!/usr/bin/python3
#
#
# Remove old mail from mbox files
#
import email
import mailbox
import os
import sys
import time
#
#
# Constants/configuration
#
junkdir = "/home/chris/Mail/Ju"         # directory to clear old mail from
days = 7                                # remove mail older than this
#
#
#
#
for f in os.listdir(junkdir):
    mbxPath = os.path.join(junkdir, f)
    mbx = mailbox.mbox(mbxPath, factory=None)
    try:
        mbx.lock()
        for k, msg in mbx.iteritems():
            subject = msg['subject']
            if msg['date'] is None:
                continue
            parsedDate = email.utils.parsedate(msg['date'])
            if parsedDate is None:
                print (subject + '\n - Bad date format:- "' + msg['date'] + '"')
                continue
            msgDate = time.mktime(parsedDate)
            if (time.time() - msgDate) / (24 * 60 * 60) > days:
                mbx.remove(k)
        mbx.flush()
        mbx.unlock()
    except mailbox.ExternalClashError:
        print ("Failed to lock: " + mbxPath)


-- 
Chris Green
·