Commands

One of the most appealing aspect of the command extension is how easy it is to define commands and how you can arbitrarily nest groups and commands to have a rich sub-command system.

Commands are defined by attaching it to a regular Python function. The command is then invoked by the user using a similar signature to the Python function.

For example, in the given command definition:

@bot.command()
async def foo(ctx, arg):
    await ctx.send(arg)

With the following prefix ($), it would be invoked by the user via:

$foo abc

A command must always have at least one parameter, ctx, which is the Context as the first one.

There are two ways of registering a command. The first one is by using Bot.command() decorator, as seen in the example above. The second is using the command() decorator followed by Bot.add_command() on the instance.

Essentially, these two are equivalent:

from discord.ext import commands

bot = commands.Bot(command_prefix='$')

@bot.command()
async def test(ctx):
    pass

# or:

@commands.command()
async def test(ctx):
    pass

bot.add_command(test)

Since the Bot.command() decorator is shorter and easier to comprehend, it will be the one used throughout the documentation here.

Any parameter that is accepted by the Command constructor can be passed into the decorator. For example, to change the name to something other than the function would be as simple as doing this:

@bot.command(name='list')
async def _list(ctx, arg):
    pass

Parameters

Since we define commands by making Python functions, we also define the argument passing behaviour by the function parameters.

Certain parameter types do different things in the user side and most forms of parameter types are supported.

Positional

The most basic form of parameter passing is the positional parameter. This is where we pass a parameter as-is:

@bot.command()
async def test(ctx, arg):
    await ctx.send(arg)

On the bot using side, you can provide positional arguments by just passing a regular string:

../../_images/positional1.png

To make use of a word with spaces in between, you should quote it:

../../_images/positional2.png

As a note of warning, if you omit the quotes, you will only get the first word:

../../_images/positional3.png

Since positional arguments are just regular Python arguments, you can have as many as you want:

@bot.command()
async def test(ctx, arg1, arg2):
    await ctx.send('You passed {} and {}'.format(arg1, arg2))

Variable

Sometimes you want users to pass in an undetermined number of parameters. The library supports this similar to how variable list parameters are done in Python:

@bot.command()
async def test(ctx, *args):
    await ctx.send('{} arguments: {}'.format(len(args), ', '.join(args)))

This allows our user to accept either one or many arguments as they please. This works similar to positional arguments, so multi-word parameters should be quoted.

For example, on the bot side:

../../_images/variable1.png

If the user wants to input a multi-word argument, they have to quote it like earlier:

../../_images/variable2.png

Do note that similar to the Python function behaviour, a user can technically pass no arguments at all:

../../_images/variable3.png

Since the args variable is a tuple, you can do anything you would usually do with one.

Keyword-Only Arguments

When you want to handle parsing of the argument yourself or do not feel like you want to wrap multi-word user input into quotes, you can ask the library to give you the rest as a single argument. We do this by using a keyword-only argument, seen below:

@bot.command()
async def test(ctx, *, arg):
    await ctx.send(arg)

Warning

You can only have one keyword-only argument due to parsing ambiguities.

On the bot side, we do not need to quote input with spaces:

../../_images/keyword1.png

Do keep in mind that wrapping it in quotes leaves it as-is:

../../_images/keyword2.png

By default, the keyword-only arguments are stripped of white space to make it easier to work with. This behaviour can be toggled by the Command.rest_is_raw argument in the decorator.

Invocation Context

As seen earlier, every command must take at least a single parameter, called the Context.

This parameter gives you access to something called the “invocation context”. Essentially all the information you need to know how the command was executed. It contains a lot of useful information:

The context implements the abc.Messageable interface, so anything you can do on a abc.Messageable you can do on the Context.

Converters

Adding bot arguments with function parameters is only the first step in defining your bot’s command interface. To actually make use of the arguments, we usually want to convert the data into a target type. We call these Converters.

Converters come in a few flavours:

  • A regular callable object that takes an argument as a sole parameter and returns a different type.

    • These range from your own function, to something like bool or int.

  • A custom class that inherits from Converter.

Basic Converters

At its core, a basic converter is a callable that takes in an argument and turns it into something else.

For example, if we wanted to add two numbers together, we could request that they are turned into integers for us by specifying the converter:

@bot.command()
async def add(ctx, a: int, b: int):
    await ctx.send(a + b)

We specify converters by using something called a function annotation. This is a Python 3 exclusive feature that was introduced in PEP 3107.

This works with any callable, such as a function that would convert a string to all upper-case:

def to_upper(argument):
    return argument.upper()

@bot.command()
async def up(ctx, *, content: to_upper):
    await ctx.send(content)

bool

Unlike the other basic converters, the bool converter is treated slightly different. Instead of casting directly to the bool type, which would result in any non-empty argument returning True, it instead evaluates the argument as True or False based on it’s given content:

if lowered in ('yes', 'y', 'true', 't', '1', 'enable', 'on'):
    return True
elif lowered in ('no', 'n', 'false', 'f', '0', 'disable', 'off'):
    return False

Advanced Converters

Sometimes a basic converter doesn’t have enough information that we need. For example, sometimes we want to get some information from the Message that called the command or we want to do some asynchronous processing.

For this, the library provides the Converter interface. This allows you to have access to the Context and have the callable be asynchronous. Defining a custom converter using this interface requires overriding a single method, Converter.convert().

An example converter:

import random

class Slapper(commands.Converter):
    async def convert(self, ctx, argument):
        to_slap = random.choice(ctx.guild.members)
        return '{0.author} slapped {1} because *{2}*'.format(ctx, to_slap, argument)

@bot.command()
async def slap(ctx, *, reason: Slapper):
    await ctx.send(reason)

The converter provided can either be constructed or not. Essentially these two are equivalent:

@bot.command()
async def slap(ctx, *, reason: Slapper):
    await ctx.send(reason)

# is the same as...

@bot.command()
async def slap(ctx, *, reason: Slapper()):
    await ctx.send(reason)

Having the possibility of the converter be constructed allows you to set up some state in the converter’s __init__ for fine tuning the converter. An example of this is actually in the library, clean_content.

@bot.command()
async def clean(ctx, *, content: commands.clean_content):
    await ctx.send(content)

# or for fine-tuning

@bot.command()
async def clean(ctx, *, content: commands.clean_content(use_nicknames=False)):
    await ctx.send(content)

If a converter fails to convert an argument to its designated target type, the BadArgument exception must be raised.

Inline Advanced Converters

If we don’t want to inherit from Converter, we can still provide a converter that has the advanced functionalities of an advanced converter and save us from specifying two types.

For example, a common idiom would be to have a class and a converter for that class:

class JoinDistance:
    def __init__(self, joined, created):
        self.joined = joined
        self.created = created

    @property
    def delta(self):
        return self.joined - self.created

class JoinDistanceConverter(commands.MemberConverter):
    async def convert(self, ctx, argument):
        member = await super().convert(ctx, argument)
        return JoinDistance(member.joined_at, member.created_at)

@bot.command()
async def delta(ctx, *, member: JoinDistanceConverter):
    is_new = member.delta.days < 100
    if is_new:
        await ctx.send("Hey you're pretty new!")
    else:
        await ctx.send("Hm you're not so new.")

This can get tedious, so an inline advanced converter is possible through a classmethod inside the type:

class JoinDistance:
    def __init__(self, joined, created):
        self.joined = joined
        self.created = created

    @classmethod
    async def convert(cls, ctx, argument):
        member = await commands.MemberConverter().convert(ctx, argument)
        return cls(member.joined_at, member.created_at)

    @property
    def delta(self):
        return self.joined - self.created

@bot.command()
async def delta(ctx, *, member: JoinDistance):
    is_new = member.delta.days < 100
    if is_new:
        await ctx.send("Hey you're pretty new!")
    else:
        await ctx.send("Hm you're not so new.")

Discord Converters

Working with Discord Models is a fairly common thing when defining commands, as a result the library makes working with them easy.

For example, to receive a Member, you can just pass it as a converter:

@bot.command()
async def joined(ctx, *, member: discord.Member):
    await ctx.send('{0} joined on {0.joined_at}'.format(member))

When this command is executed, it attempts to convert the string given into a Member and then passes it as a parameter for the function. This works by checking if the string is a mention, an ID, a nickname, a username + discriminator, or just a regular username. The default set of converters have been written to be as easy to use as possible.

A lot of discord models work out of the gate as a parameter:

Having any of these set as the converter will intelligently convert the argument to the appropriate target type you specify.

Under the hood, these are implemented by the Advanced Converters interface. A table of the equivalent converter is given below:

Discord Class

Converter

Member

MemberConverter

User

UserConverter

TextChannel

TextChannelConverter

VoiceChannel

VoiceChannelConverter

VoiceChannel

CategoryChannelConverter

Role

RoleConverter

Invite

InviteConverter

Game

GameConverter

Emoji

EmojiConverter

PartialEmoji

PartialEmojiConverter

Colour

ColourConverter

By providing the converter it allows us to use them as building blocks for another converter:

class MemberRoles(commands.MemberConverter):
    async def convert(self, ctx, argument):
        member = await super().convert(ctx, argument)
        return member.roles

@bot.command()
async def roles(ctx, *, member: MemberRoles):
    """Tells you a member's roles."""
    await ctx.send('I see the following roles: ' + ', '.join(member))

Special Converters

The command extension also has support for certain converters to allow for more advanced and intricate use cases that go beyond the generic linear parsing. These converters allow you to introduce some more relaxed and dynamic grammar to your commands in an easy to use manner.

typing.Union

A typing.Union is a special type hint that allows for the command to take in any of the specific types instead of a singular type. For example, given the following:

import typing

@bot.command()
async def union(ctx, what: typing.Union[discord.TextChannel, discord.Member]):
    await ctx.send(what)

The what parameter would either take a discord.TextChannel converter or a discord.Member converter. The way this works is through a left-to-right order. It first attempts to convert the input to a discord.TextChannel, and if it fails it tries to convert it to a discord.Member. If all converters fail, then a special error is raised, BadUnionArgument.

Note that any valid converter discussed above can be passed in to the argument list of a typing.Union.

typing.Optional

A typing.Optional is a special type hint that allows for “back-referencing” behaviour. If the converter fails to parse into the specified type, the parser will skip the parameter and then either None or the specified default will be passed into the parameter instead. The parser will then continue on to the next parameters and converters, if any.

Consider the following example:

import typing

@bot.command()
async def bottles(ctx, amount: typing.Optional[int] = 99, *, liquid="beer"):
    await ctx.send('{} bottles of {} on the wall!'.format(amount, liquid))
../../_images/optional1.png

In this example, since the argument could not be converted into an int, the default of 99 is passed and the parser resumes handling, which in this case would be to pass it into the liquid parameter.

Note

This converter only works in regular positional parameters, not variable parameters or keyword-only parameters.

Greedy

The Greedy converter is a generalisation of the typing.Optional converter, except applied to a list of arguments. In simple terms, this means that it tries to convert as much as it can until it can’t convert any further.

Consider the following example:

@bot.command()
async def slap(ctx, members: commands.Greedy[discord.Member], *, reason='no reason'):
    slapped = ", ".join(x.name for x in members)
    await ctx.send('{} just got slapped for {}'.format(slapped, reason))

When invoked, it allows for any number of members to be passed in:

../../_images/greedy1.png

The type passed when using this converter depends on the parameter type that it is being attached to:

  • Positional parameter types will receive either the default parameter or a list of the converted values.

  • Variable parameter types will be a tuple as usual.

  • Keyword-only parameter types will be the same as if Greedy was not passed at all.

Greedy parameters can also be made optional by specifying an optional value.

When mixed with the typing.Optional converter you can provide simple and expressive command invocation syntaxes:

import typing

@bot.command()
async def ban(ctx, members: commands.Greedy[discord.Member],
                   delete_days: typing.Optional[int] = 0, *,
                   reason: str):
    """Mass bans members with an optional delete_days parameter"""
    for member in members:
        await member.ban(delete_message_days=delete_days, reason=reason)

This command can be invoked any of the following ways:

$ban @Member @Member2 spam bot
$ban @Member @Member2 7 spam bot
$ban @Member spam

Warning

The usage of Greedy and typing.Optional are powerful and useful, however as a price, they open you up to some parsing ambiguities that might surprise some people.

For example, a signature expecting a typing.Optional of a discord.Member followed by a int could catch a member named after a number due to the different ways a MemberConverter decides to fetch members. You should take care to not introduce unintended parsing ambiguities in your code. One technique would be to clamp down the expected syntaxes allowed through custom converters or reordering the parameters to minimise clashes.

To help aid with some parsing ambiguities, str, None and Greedy are forbidden as parameters for the Greedy converter.

Error Handling

When our commands fail to either parse we will, by default, receive a noisy error in stderr of our console that tells us that an error has happened and has been silently ignored.

In order to handle our errors, we must use something called an error handler. There is a global error handler, called on_command_error() which works like any other event in the Event Reference. This global error handler is called for every error reached.

Most of the time however, we want to handle an error local to the command itself. Luckily, commands come with local error handlers that allow us to do just that. First we decorate an error handler function with Command.error():

@bot.command()
async def info(ctx, *, member: discord.Member):
    """Tells you some info about the member."""
    fmt = '{0} joined on {0.joined_at} and has {1} roles.'
    await ctx.send(fmt.format(member, len(member.roles)))

@info.error
async def info_error(ctx, error):
    if isinstance(error, commands.BadArgument):
        await ctx.send('I could not find that member...')

The first parameter of the error handler is the Context while the second one is an exception that is derived from CommandError. A list of errors is found in the Errors page of the documentation.

Checks

There are cases when we don’t want a user to use our commands. They don’t have permissions to do so or maybe we blocked them from using our bot earlier. The commands extension comes with full support for these things in a concept called a Checks.

A check is a basic predicate that can take in a Context as its sole parameter. Within it, you have the following options:

  • Return True to signal that the person can run the command.

  • Return False to signal that the person cannot run the command.

  • Raise a CommandError derived exception to signal the person cannot run the command.

    • This allows you to have custom error messages for you to handle in the error handlers.

To register a check for a command, we would have two ways of doing so. The first is using the check() decorator. For example:

async def is_owner(ctx):
    return ctx.author.id == 316026178463072268

@bot.command(name='eval')
@commands.check(is_owner)
async def _eval(ctx, *, code):
    """A bad example of an eval command"""
    await ctx.send(eval(code))

This would only evaluate the command if the function is_owner returns True. Sometimes we re-use a check often and want to split it into its own decorator. To do that we can just add another level of depth:

def is_owner():
    async def predicate(ctx):
        return ctx.author.id == 316026178463072268
    return commands.check(predicate)

@bot.command(name='eval')
@is_owner()
async def _eval(ctx, *, code):
    """A bad example of an eval command"""
    await ctx.send(eval(code))

Since an owner check is so common, the library provides it for you (is_owner()):

@bot.command(name='eval')
@commands.is_owner()
async def _eval(ctx, *, code):
    """A bad example of an eval command"""
    await ctx.send(eval(code))

When multiple checks are specified, all of them must be True:

def is_in_guild(guild_id):
    async def predicate(ctx):
        return ctx.guild and ctx.guild.id == guild_id
    return commands.check(predicate)

@bot.command()
@is_in_guild(41771983423143937)
async def secretguilddata(ctx):
    """super secret stuff"""
    await ctx.send('secret stuff')

If any of those checks fail in the example above, then the command will not be run.

When an error happens, the error is propagated to the error handlers. If you do not raise a custom CommandError derived exception, then it will get wrapped up into a CheckFailure exception as so:

@bot.command()
@is_in_guild(41771983423143937)
async def secretguilddata(ctx):
    """super secret stuff"""
    await ctx.send('secret stuff')

@secretguilddata.error
async def secretguilddata_error(ctx, error):
    if isinstance(error, commands.CheckFailure):
        await ctx.send('nothing to see here comrade.')

If you want a more robust error system, you can derive from the exception and raise it instead of returning False:

class NoPrivateMessages(commands.CheckFailure):
    pass

def guild_only():
    async def predicate(ctx):
        if ctx.guild is None:
            raise NoPrivateMessages('Hey no DMs!')
        return True
    return commands.check(predicate)

@guild_only()
async def test(ctx):
    await ctx.send('Hey this is not a DM! Nice.')

@test.error
async def test_error(ctx, error):
    if isinstance(error, NoPrivateMessages):
        await ctx.send(error)

Note

Since having a guild_only decorator is pretty common, it comes built-in via guild_only().

Global Checks

Sometimes we want to apply a check to every command, not just certain commands. The library supports this as well using the global check concept.

Global checks work similarly to regular checks except they are registered with the Bot.check() decorator.

For example, to block all DMs we could do the following:

@bot.check
async def globally_block_dms(ctx):
    return ctx.guild is not None

Warning

Be careful on how you write your global checks, as it could also lock you out of your own bot.