How can I make my class pretty printable in Python?

Question

Python has a pretty printer (pprint(...)). I would like to make my classes pretty printable. Will pretty print print my instances in a better way, if I provide a certain interface?

The Python documentation in section 8.11 shows different examples, but no example how to make a user defined class pretty printable.

So what interface need my classes to provide?
Is there any other (maybe better) formatter?

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Use Case:

I want to pretty print the content of ConfigParser, for which I have create an extended version called ExtendenConfigParser. So I have the possibility to add more functionality or add a matching pretty print interface.

Answer 1

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.

Answer 2

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

pprint(obj.dict())

Answer 3

The subclass solution by Martijn Pieters is working for me, and I made it more general by not hard coding foo and bar.

Take out:

    self._format(object.foo, stream, indent, allowance + 1,
                 context, level)
    self._format(object.bar, stream, indent, allowance + 1,
                 context, level)

Substitute (put in):

    for s in vars(object):
        stream.write( '\n%s: ' % s )
        self._format( object.__dict__[s],
                      stream, indent, allowance + 1, context, level )

Answer 4

If you're going to go to all that effort, you might as well superclass pprint to accept a hook, then you'll only have to write all that code once.

It will also work better in cases where your class is defined after you've instantiated a pprint helper with pp = pprint.PrettyPrinter(indent=4).pprint (a bad habit of mine).

Then you can opt-in with any class by using one of these methods [pun not intended]

[edit]: after some self-use, I realised a much easier alternate solution, __pprint_repr__. Rather than attempt to create your own pprint function, simply define the __pprint_repr__ method and return a standard python object. You can group multiple objects inside a dict if you have a complex class.

[edit #2]: I also realised that that it can be useful to have all the _format variables passed to the __pprint_repr__ function, because that allows you to do really cool things -- like show a compacted output if your item is in a list (indent > 0) vs a full output if it's the only object (indent == 0)

This also means this solution is compatibled with Python 2.7, not just Python ~> 3.3

class my_object(object):

    # produce pprint compatible object, easy as pie!
    def __pprint_repr__(self, **kwargs):
        return self.__dict__
    
    # make a multi-level object, easy as cheese-cake!
    def __pprint_repr__(self, **kwargs):
        _indent = kwargs['indent']
        if _indent:
            return self._toText()
        return { self._toText(): self.__dict__ }

    # to take total control (python 3) (requires __repr__ be defined)
    def __pprint__(self, object, stream, indent, allowance, context, level):
        stream.write('my_object(\n')
        self._format(object._data, stream, indent, allowance + 1, context, level)
        stream.write(')')
        pass

The sub-classing is simplicity itself -- tested in Python 3.7 and 2.7:

        if _pprint_repr:
            return PrettyPrinter._format(self, _pprint_repr(object, stream=stream, 
                indent=indent, allowance=allowance, context=context, level=level), 
                    stream, indent, allowance, context, level)

        # else check for alternate _pprint method (if supported ~ python 3.3)
        if getattr(PrettyPrinter, '_dispatch', None):
            _repr = type(object).__repr__
            _pprint = getattr(type(object), '__pprint__', None)
            _exists = self._dispatch.get(_repr, None)
            if not _exists and _pprint:
                self._dispatch[_repr] = _pprint

        return PrettyPrinter._format(self, object, stream, indent, allowance, context, level)