best way to implement custom pretty-printers

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Customizing pprint.PrettyPrinter

The documentation for the pprint module mentions that the method PrettyPrinter.format is intended to make it possible to customize formatting.

I gather that it's possible to override this method in a subclass, but this doesn't seem to provide a way to have the base class methods apply line wrapping and indentation.

  • Am I missing something here?
  • Is there a better way to do this (e.g. another module)?

Alternatives?

I've checked out the pretty module, which looks interesting, but doesn't seem to provide a way to customize formatting of classes from other modules without modifying those modules.

I think what I'm looking for is something that would allow me to provide a mapping of types (or maybe functions) that identify types to routines that process a node. The routines that process a node would take a node and return the string representation it, along with a list of child nodes. And so on.

Why I’m looking into pretty-printing

My end goal is to compactly print custom-formatted sections of a DocBook-formatted xml.etree.ElementTree.

(I was surprised to not find more Python support for DocBook. Maybe I missed something there.)

I built some basic functionality into a client called xmlearn that uses lxml. For example, to dump a Docbook file, you could:

xmlearn -i docbook_file.xml dump -f docbook -r book

It's pretty half-ass, but it got me the info I was looking for.

xmlearn has other features too, like the ability to build a graph image and do dumps showing the relationships between tags in an XML document. These are pretty much totally unrelated to this question.

You can also perform a dump to an arbitrary depth, or specify an XPath as a set of starting points. The XPath stuff sort of obsoleted the docbook-specific format, so that isn't really well-developed.

This still isn't really an answer for the question. I'm still hoping that there's a readily customizable pretty printer out there somewhere.

docbook pretty-print subclass xml python
4 Answers

If you would like to modify the default pretty printer without subclassing, you can use the internal _dispatch table on the pprint.PrettyPrinter class. You can see how examples of how dispatching is added for internal types like dictionaries and lists in the source.

Here is how I added a custom pretty printer for MatchPy's Operation type:

import pprint
import matchpy

def _pprint_operation(self, object, stream, indent, allowance, context, level):
    """
    Modified from pprint dict https://github.com/python/cpython/blob/3.7/Lib/pprint.py#L194
    """
    operands = object.operands
    if not operands:
        stream.write(repr(object))
        return
    cls = object.__class__
    stream.write(cls.__name__ + "(")
    self._format_items(
        operands, stream, indent + len(cls.__name__), allowance + 1, context, level
    )
    stream.write(")")


pprint.PrettyPrinter._dispatch[matchpy.Operation.__repr__] = _pprint_operation

Now if I use pprint.pprint on any object that has the same __repr__ as matchpy.Operation, it will use this method to pretty print it. This works on subclasses as well, as long as they don't override the __repr__, which makes some sense! If you have the same __repr__ you have the same pretty printing behavior.

Here is an example of the pretty printing some MatchPy operations now:

ReshapeVector(Vector(Scalar('1')),
              Vector(Index(Vector(Scalar('0')),
                           If(Scalar('True'),
                              Scalar("ReshapeVector(Vector(Scalar('2'), Scalar('2')), Iota(Scalar('10')))"),
                              Scalar("ReshapeVector(Vector(Scalar('2'), Scalar('2')), Ravel(Iota(Scalar('10'))))")))))
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