I don't think there is anything specifically defined for a placeholder.  However, looking at the current urllib.request documentation, there are 2 places that use the ``<name>_`` method for a placeholder - <protocol>.proxy and <scheme>.proxy, with the latter explaining what <scheme> means.

> By default, ProxyHandler uses the environment variables named <scheme>_proxy, where <scheme> is the URL scheme involved.

That line was added by Georg Brandl (granted it was part of converting all the docs), which would lead me to believe that is the accepted way to do it.

Also, the argument clinic how-to has the following:
> The length of the string will be passed in to the impl function, just after the string parameter, as a parameter named ``<parameter_name>_length``.

I'll make a comment on the PR too.