I have used many discord API wrappers, but as an experienced python developer, unfortunately I somehow still do not understand how a command gets called!

@client.command()
async demo(ctx):
    channel = ctx.channel
    await channel.send(f'Demonstration')

Above a command has been created (function) and it is placed after its decorator @client.command() To my understanding, the decorator is in a way, a "check" performed before running the function (demo) but I do not understand how the discord.py library seemingly "calls" the demo function.....?? Is there some form of short/long polling system in the local imported discord.py library which polls the discord API and receives a list of jobs/messages and checks these against the functions the user has created?

I would love to know how this works as I dont understand what "calls" the functions that the user makes, and this would allow me to make my own wrapper for another similar social media platform! Many thanks in advance.

I am trying to work out how functions created by the user are seemingly "called" by the discord.py library. I have worked with the discord.py wrapper and other API wrappers before.

CodePudding user response：

(See source code attached at the bottom of the answer)

The @bot.command() decorator adds a command to the internal lists/mappings of commands stored in the Bot instance.

Whenever a message is received, this runs through Bot.process_commands. It can then look through every command stored to check if the message starts with one of them (prefix is checked beforehand). If it finds a match, then it can invoke it (the underlying callback is stored in the Command instance).

If you've ever overridden an on_message event and your commands stopped working, then this is why: that method is no longer being called, so it no longer tries to look through your commands to find a match.

This uses a dictionary to make it far more efficient - instead of having to iterate over every single command & alias available, it only has to check if the first letters of the message match anything at all.

The commands.Command() decorator used in Cogs works slightly different. This turns your function into a Command instance, and when adding a cog (using Bot.add_cog()) the library checks every attribute to see if any of them are Command instances.

References to source code

GroupMixin.command() (called when you use @client.command()): https://github.com/Rapptz/discord.py/blob/24bdb44d54686448a336ea6d72b1bf8600ef7220/discord/ext/commands/core.py#L1493

As you can see, it calls add_command() internally to add it to the list of commands.

Adding commands (GroupMixin.add_command()): https://github.com/Rapptz/discord.py/blob/24bdb44d54686448a336ea6d72b1bf8600ef7220/discord/ext/commands/core.py#L1315

Bot.process_commands(): https://github.com/Rapptz/discord.py/blob/master/discord/ext/commands/bot.py#L1360

You'll have to follow the chain - most of the processing actually happens in get_context which tries to create a Context instance out of the message: https://github.com/Rapptz/discord.py/blob/24bdb44d54686448a336ea6d72b1bf8600ef7220/discord/ext/commands/bot.py#L1231

commands.Command(): https://github.com/Rapptz/discord.py/blob/master/discord/ext/commands/core.py#L1745