A pretty simplified way to implement custom commands into your bot.

custom_commands.py

from discord.ext import commands


# The check to ensure this is the guild used where the command was made
def guild_check(_custom_commands):
    async def predicate(ctx):
        return _custom_commands.get(ctx.command.qualified_name) and ctx.guild.id in _custom_commands.get(ctx.command.qualified_name)
    return commands.check(predicate)

class CustomCommands(commands.Cog):

    """ Each entry in _custom_commands will look like this:
    {
        "command_name": {
            guild_id: "This guild's output",
            guild_id2: "This other guild's output",
        }
    }
    """
    _custom_commands = {}

    @commands.command()
    async def add_command(self, ctx, name, *, output):
        # First check if there's a custom command with that name already
        existing_command = self._custom_commands.get(name)
        # Check if there's a built in command, we don't want to override that
        if existing_command is None and ctx.bot.get_command(name):
            return await ctx.send(f"A built in command with the name {name} is already registered")

        # Now, if the command already exists then we just need to add/override the message for this guild
        if existing_command:
            self._custom_commands[name][ctx.guild.id] = output
        # Otherwise, we need to create the command object
        else:
            @commands.command(name=name, help=f"Custom command: Outputs your custom provided output")
            @guild_check(self._custom_commands)
            async def cmd(self, ctx):
                await ctx.send(self._custom_commands[ctx.invoked_with][ctx.guild.id])

            cmd.cog = self
            # And add it to the cog and the bot
            self.__cog_commands__ = self.__cog_commands__ + (cmd,)
            ctx.bot.add_command(cmd)
            # Now add it to our list of custom commands
            self._custom_commands[name] = {ctx.guild.id: output}
        await ctx.send(f"Added a command called {name}")

    @commands.command()
    async def remove_command(self, ctx, name):
        # Make sure it's actually a custom command, to avoid removing a real command
        if name not in self._custom_commands or ctx.guild.id not in self._custom_commands[name]:
            return await ctx.send(f"There is no custom command called {name}")
        # All that technically has to be removed, is our guild from the dict for the command
        del self._custom_commands[name][ctx.guild.id]
        await ctx.send(f"Removed a command called {name}")

def setup(bot):
    bot.add_cog(CustomCommands(bot))

I'm sorry, but how i can get list of all custom commands for help command?

I'm sorry, but how i can get list of all custom commands for help command?

By default this implementation has the commands already show up in the "custom command" category in the help command (that's actually the entire purpose of this slightly complicated implementation - to have them automatically work with the help command). There should be nothing special you have to do. If you're doing a help command from scratch then that's not the recommended way. I'd recommend going over this walk through.

If for some reason though you are doing something abnormal with the help command, and still need the custom commands directly - then the ._custom_commands attribute on the cog instance can be used.

im having a error whenever i run the cmd: discord.ext.commands.errors.CommandNotFound: Command "add" is not found

im having a error whenever i run the cmd: discord.ext.commands.errors.CommandNotFound: Command "add" is not found

There is no command called "add" in this gist. It's called "add_command" just as an example, if you want to name it something different you'll have to understand how the commands extension works. Documentation can be found here:

https://discordpy.readthedocs.io/en/stable/ext/commands/api.html
"Subcommands" probably make sense for this, but this gist is purely an example - you're meant to customize it to your liking

This is no longer compatible with discord.py 2.0 and later. See this forum post in the discord.py server for details.