How to make a discord.py bot private/direct message someone who's not the author?

Question

Say I want to make a bot with a "poke" feature (aka sends a pm to a user saying "Boop" when someone says "!poke @user#0000"), how would I do this? It works perfectly when I do this:

@bot.command(pass_context=True)
async def poke(ctx, message):
    await client.send_message(ctx.message.author, 'boop')

but only if I want to poke the author of the message. I want to poke whoever's being @'d.

I know the discord.py documents say I can use this:

start_private_message(user)

but I don't know what to put in place of user.

Answer 1

It's actually simpler than that

@bot.command(pass_context=True)
async def poke(ctx, member: discord.Member):
    await bot.send_message(member, 'boop')

send_message contains logic for private messages, so you don't have to use start_private_message yourself. The : discord.Member is called a converter, and is described in the documentation here

Answer 2

I might be necroing this post a bit, but the recent version of this would look like this:

@bot.command():
async def poke(ctx, user: discord.Member=None):

    if user is None:
        await ctx.send("Incorrect Syntax:\nUsage: `!poke [user]`")

    await user.send("boop")

The if user is None: block is optional, but is useful if you want to have an error message if a command isn't used correctly.