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)

Answer 2

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'))))")))))

Answer 3

This question may be a duplicate of:

• Any way to properly pretty-print ordered dictionaries in Python?

---

Using pprint.PrettyPrinter

I looked through the source of pprint. It seems to suggest that, in order to enhance pprint(), you’d need to:

• subclass PrettyPrinter
• override _format()
• test for issubclass(),
• and (if it's not your class), pass back to _format()

Alternative

I think a better approach would be just to have your own pprint(), which defers to pprint.pformat when it doesn't know what's up.

For example:

'''Extending pprint'''

from pprint import pformat

class CrazyClass: pass

def prettyformat(obj):
    if isinstance(obj, CrazyClass):
        return "^CrazyFoSho^"
    else:
        return pformat(obj)

def prettyp(obj):
    print(prettyformat(obj))

# test
prettyp([1]*100)
prettyp(CrazyClass())

The big upside here is that you don't depend on pprint internals. It’s explicit and concise.

The downside is that you’ll have to take care of indentation manually.

Answer 4

Consider using the pretty module:

• http://pypi.python.org/pypi/pretty/0.1