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

Apparently, this now easier to do thanks to the PrettyPrint class (documentation). You need to subclass the pprint.PrettyPrinter, implement the format method, and then you can construct an instance and call pformat or pprint. There's a few important details to read about in the documentation about that format method:

Returns three values: the formatted version of object as a string, a flag indicating whether the result is readable, and a flag indicating whether recursion was detected. The first argument is the object to be presented. The second is a dictionary which contains the id() of objects that are part of the current presentation context (direct and indirect containers for object that are affecting the presentation) as the keys; if an object needs to be presented which is already represented in context, the third return value should be True. Recursive calls to the format() method should add additional entries for containers to this dictionary. The third argument, maxlevels, gives the requested limit to recursion; this will be 0 if there is no requested limit. This argument should be passed unmodified to recursive calls. The fourth argument, level, gives the current level; recursive calls should be passed a value less than that of the current call.

Here's an example of how I was using it to handle a custom repr for Pillow images (some extraneous details removed):

class CustomPrettyPrinter(pprint.PrettyPrinter):
    def format(self, object, context, maxlevels, level):
        if isinstance(object, Image):
            # new string representation, was readable, is not recursive
            return repr_pil_image(object), True, False
        # Otherwise, use default behavior (could be super?)
        return pprint.PrettyPrinter.format(self, object, context, maxlevels, level)

def format_page_content(content, indent, width=80):
    custom_pretty_printer = CustomPrettyPrinter(indent=indent, width=width)
    return custom_pretty_printer.pformat(content)

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

Consider using the pretty module:

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

Answer 4

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 5

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.