Create a ExecutionContext to run a transaction against.

        Args:
            state_hash: (str): Merkle root to base state on.
            base_contexts (list of str): Context ids of contexts that will
                have their state applied to make this context.
            inputs (list of str): Addresses that can be read from.
            outputs (list of str): Addresses that can be written to.
        Returns:
            context_id (str): the unique context_id of the session

def create_context(self, state_hash, base_contexts, inputs, outputs):
        """Create a ExecutionContext to run a transaction against.

        Args:
            state_hash: (str): Merkle root to base state on.
            base_contexts (list of str): Context ids of contexts that will
                have their state applied to make this context.
            inputs (list of str): Addresses that can be read from.
            outputs (list of str): Addresses that can be written to.
        Returns:
            context_id (str): the unique context_id of the session
        """

        for address in inputs:
            if not self.namespace_is_valid(address):
                raise CreateContextException(
                    "Address or namespace {} listed in inputs is not "
                    "valid".format(address))
        for address in outputs:
            if not self.namespace_is_valid(address):
                raise CreateContextException(
                    "Address or namespace {} listed in outputs is not "
                    "valid".format(address))

        addresses_to_find = [add for add in inputs if len(add) == 70]

        address_values, reads = self._find_address_values_in_chain(
            base_contexts=base_contexts,
            addresses_to_find=addresses_to_find)

        context = ExecutionContext(
            state_hash=state_hash,
            read_list=inputs,
            write_list=outputs,
            base_context_ids=base_contexts)

        contexts_asked_not_found = [cid for cid in base_contexts
                                    if cid not in self._contexts]
        if contexts_asked_not_found:
            raise KeyError(
                "Basing a new context off of context ids {} "
                "that are not in context manager".format(
                    contexts_asked_not_found))

        context.create_initial(address_values)

        self._contexts[context.session_id] = context

        if reads:
            context.create_prefetch(reads)
            self._address_queue.put_nowait(
                (context.session_id, state_hash, reads))
        return context.session_id