> Consider applications that need to validate addresses (or networks,
> but not both) supplied as user input:
> 
> address = ipaddr.IP(input)

If that is a frequent need, it would be reasonable to add an API

address = ipaddr.IP(input, allow_mask=False)

which would raise an exception if a mask was specified (as an
old-style bit mask, or in CIDR form).

> if isinstance(address, ipaddr.IPv4):
>    if address.prefixlen != 32:
>        raise TypeError("Expecting IP address, not network")
> elif isinstance(address, ipaddr.IPv6):
>    if address.prefixlen != 128:
>        raise TypeError("Expecting IP address, not network")

With the current API, you don't need to write it in such a quirky
way. Instead

if address.numhosts != 1:
   raise TypeError("Expecting IP address, not network")

would do as well.

> Given its myriad quirks

Well, you deliberately make it appear more quirky than it actually is.