Use kwargs as key in dictionary

Question

I have a method which retrieves kwargs and future. I would like to save a dictionary of kwargs to future for later processing future result with kwargs.

class ThreadPoolExecutorImproved(object):

def __init__(self, max_workers):
    self._executor = ThreadPoolExecutor(max_workers)
    self._futures = {}

def __enter__(self):
    return self

def __exit__(self, exc_type, exc_val, exc_tb):
    kwargs_to_exception = {}
    for kwargs, future in self._futures.iteritems():
        if future.exception():
            kwargs_to_exception[kwargs] = future.exception

    if kwargs_to_exception:
        raise ThreadPoolException(kwargs_to_exception)

def submit(self, fn, *args, **kwargs):
    future = self._executor.submit(fn, *args, **kwargs)
    key = tuple(kwargs.items())
    self._futures[key] = future
    return future

However, I get an error on the line self.futures[key] = future:

TypeError: unhashable type: 'dict' for python

why is it? I created a tuple from kwargs!

Answer 1

The typical workaround for this situation is to use future as a key; Your code

key = tuple(kwargs.items())
self._futures[key] = future

Migrate to

self._futures[future] = tuple(kwargs.items())

And when you want to process self._futures, you can just iterate this

for future, kw in self._futures:
    # check kw
    # do your stuff