Concatenate errors during parsing

[peverwhee]

@mwaxmonsky you can ignore this (for now!); opening this PR to get some feedback from @nusbaume on how to do this... better.

[mwaxmonsky]

So one thing that I thought about far after the fact of looking at this was it might make sense to use an observer pattern (https://www.geeksforgeeks.org/observer-method-python-design-patterns/) similar to how logging is handled in python (https://docs.python.org/3/howto/logging.html#configuring-logging)

So we might be able to implement something that allows for:

h = handler.setup('parse_errors')
# Call parsing code
if not h.empty()
  # print all errors

then in the called code:

# parse code
# if error detected:
  h = handler.global('parse_errors')
  h.addError(...)
  # determine exit patter

[mwaxmonsky]

Okay, I think we have a proof of concept that we can try. In the main python file where we start processing files, we can have:

import logging
import logging.handlers
import queue

...

#in processing function
  message_queue = queue.Queue(-1)
  queue_handler = logging.handlers.QueueHandler(message_queue)
  stream_handler = logging.StreamHandler()
  queue_listener = logging.handlers.QueueListener(message_queue, stream_handler)
  metadata_processing_logger = logging.getLogger("metadata_processing_logger")
  metadata_processing_logger.addHandler(queue_handler)

  #call processing functions

  if message_queue.qsize() > 0:
    #determine how to handle errors
    
    #These will start printing messages to stdout
    queue_listener.start()
    queue_listener.stop()

The in the different parsing code segments, we can have:

import logging

# in analyze() function
  metadata_processing_logger = logging.getLogger("metadata_processing_logger")
  metadata_processing_logger.error(our_error_message)

Just let me know if you need any help testing this out and I'd be happy to help!

[nusbaume]

This is exactly what I was looking for, thanks @mwaxmonsky! @peverwhee let me know if you have any problems implementing this solution, or if it ends up being more complex than anticipated.

Thanks again!

[peverwhee]

closing - preempted by another solution (related to #15)