python - best way to implement custom pretty-printers

Question

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.

Answer 1

My solution was to replace pprint.PrettyPrinter with a simple wrapper that formats any floats it finds before calling the original printer.

from __future__ import division
import pprint
if not hasattr(pprint,'old_printer'):
    pprint.old_printer=pprint.PrettyPrinter

class MyPrettyPrinter(pprint.old_printer):
    def _format(self,obj,*args,**kwargs):
        if isinstance(obj,float):
            obj=round(obj,4)
        return pprint.old_printer._format(self,obj,*args,**kwargs)
pprint.PrettyPrinter=MyPrettyPrinter

def pp(obj):
    pprint.pprint(obj)

if __name__=='__main__':
    x=[1,2,4,6,457,3,8,3,4]
    x=[_/17 for _ in x]
    pp(x)