How can I make my class pretty printable in Python?

It is not really a solution, but I usually just make objects serializable and pretty print them like this:

pprint(obj.dict())

pprint does not look for any hooks. The pprint.PrettyPrinter uses a dispatch pattern instead; a series of methods on the class that are keyed on class.__repr__ references.

You can subclass pprint.PrettyPrinter to teach it about your class:

class YourPrettyPrinter(pprint.PrettyPrinter):
    _dispatch = pprint.PrettyPrinter._dispatch.copy()

    def _pprint_yourtype(self, object, stream, indent, allowance, context, level):
        stream.write('YourType(')
        self._format(object.foo, stream, indent, allowance + 1,
                     context, level)
        self._format(object.bar, stream, indent, allowance + 1,
                     context, level)
        stream.write(')')

    _dispatch[YourType.__repr__] = _pprint_yourtype

then use the class directly to pretty print data containing YourType instances. Note that this is contingent on the type having their own custom __repr__ method!

You can also plug functions directly into the PrettyPrinter._dispatch dictionary; self is passed in explicitly. This is probably the better option for a 3rd-party library:

from pprint import PrettyPrinter

if isinstance(getattr(PrettyPrinter, '_dispatch'), dict):
     # assume the dispatch table method still works
     def pprint_ExtendedConfigParser(printer, object, stream, indent, allowance, context, level):
         # pretty print it!
     PrettyPrinter._dispactch[ExtendedConfigParser.__repr__] = pprint_ExtendedConfigParser

See the pprint module source code for how the other dispatch methods are written.

As always, single-underscore names like _dispatch are internal implementation details that can be altered in a future version. However, it is the best option you have here. The dispatch table was added in Python 3.5 and is present in at least Python 3.5 - 3.9 alpha.