Load bots.
        Import each bot module.
        It is thread-safe and idempotent, but not re-entrant.

def populate(self, installed_bots=None):
        """
        Load bots.
        Import each bot module.
        It is thread-safe and idempotent, but not re-entrant.
        """
        if self.ready:
            return

        # populate() might be called by two threads in parallel on servers
        # that create threads before initializing the WSGI callable.
        with self._lock:
            if self.ready:
                return

            # An RLock prevents other threads from entering this section. The
            # compare and set operation below is atomic.
            if self.loading:
                # Prevent re-entrant calls to avoid running AppConfig.ready()
                # methods twice.
                raise RuntimeError("populate() isn't re-entrant")
            self.loading = True

            # Phase 1: Initialize bots
            for entry in installed_bots or {}:
                if isinstance(entry, Bot):
                    cls = entry
                    entry = '.'.join([cls.__module__, cls.__name__])
                bot_reg = BotRegistry.create(entry)
                if bot_reg.label in self.bots:
                    raise ImproperlyConfigured(
                        "Bot labels aren't unique, "
                        "duplicates: %s" % bot_reg.label)

                self.bots[bot_reg.label] = bot_reg
                bot_reg.bots = self

            # Check for duplicate bot names.
            counts = Counter(
                bot_reg.name for bot_reg in self.bots.values())
            duplicates = [
                name for name, count in counts.most_common() if count > 1]
            if duplicates:
                raise ImproperlyConfigured(
                    "Bot names aren't unique, "
                    "duplicates: %s" % ", ".join(duplicates))

            self.bots_ready = True

            # Phase 2: import config files
            for bot in self.bots.values():
                bot.import_configs()

            self.configs_ready = True

            self.ready = True