This issue was previously addressed and fixed here:

http://bugs.python.org/issue1351692

When subclassing PrettyPrinter, overriding the format() method should allow users to define custom pretty-printers.

However, for objects whose repr is short, format() is not called for the individual members.

Example code that reproduces the issue is as follows:

import pprint
import sys

pprint.pprint(sys.version_info)

class MyPrettyPrinter(pprint.PrettyPrinter):
    def format(self, object, context, maxlevels, level):
        if isinstance(object, int):
            return hex(object), True, False
        else:
            return pprint.PrettyPrinter.format(self, object, context, maxlevels, level)

MyPrettyPrinter().pprint(10)
MyPrettyPrinter().pprint([10])

When run with different versions of Python:

sys.version_info(major=2, minor=7, micro=11, releaselevel='final', serial=0)
0xa
[0xa]

(3, 0, 1, 'final', 0)
0xa
[10]

sys.version_info(major=3, minor=2, micro=5, releaselevel='final', serial=0)
0xa
[10]

sys.version_info(major=3, minor=4, micro=4, releaselevel='final', serial=0)
0xa
[10]

sys.version_info(major=3, minor=6, micro=0, releaselevel='beta', serial=4)
0xa
[10]

You'll notice that the regression exists in all versions >= 3.0,
even though the commit that fixed the issue is still in the log (2008-01-20):
https://hg.python.org/cpython/log/3.0/Lib/pprint.py

I have not had the time to look at the source of the issue or provide a fix; I might do so tonight.

This issue only impacts container objects where the len(repr(o)) is less than width. If the length is greater than width, containers are handled by a different code path which is covered by a unit test and works correctly.

For this case, indeed it works in Python 2 but not in Python 3. The PrettyPrinter._format code has been refactored quite a lot with respect to handling of containers.

In both Python 2 and 3 this happens:

PrettyPrinter.pprint([10]) calls
PrettyPrinter._format([10]) which calls
PrettyPrinter._repr([10]) which calls
MyPrettyPrinter.format([10]) which calls
PrettyPrinter.format([10]) which calls
_safe_repr([10]) which calls
_safe_repr(10)
returns 10
returns [10]
returns [10]
returns [10]
returns [10]

But then they diverge - in Python 3 the [10] is returned as the result, but in Python 2 there is another piece of code (starting here

	since issubclass(type([10]), list):
		\# ignore the [10] calculated above, now call
		self.\_format(10) which calls
			PrettyPrinter.\_repr(10) which calls
				MyPrettyPrinter.format(10)
				returns 0xa
			returns 0xa
		returns 0xa
	returns [0xa]

This explains the difference between Python 2 and 3.

As to why the first calculation returns [10] and not [0xa]:
This is because _safe_repr is defined at module scope, so once it is called we are no longer on the PrettyPrinter instance, and the format() override is no longer accessible. When _safe_repr needs to recurse on container contents, it calls itself.

I think the solution is to make _safe_repr a method of PrettyPrinter, and make it call self.format() when it needs to recurse. The default self.format() just calls _safe_repr, so when there is no override the result is the same.

The PR's diff looks substantial, but that's mostly due to the indentation of _safe_repr code. To make it easier to review, I split it into several commits, where the indentation is done in one commit as a noop change. The other commits have much smaller diffs.

New changeset ff420f0 by Irit Katriel in branch 'master':
bpo-28850: Fix PrettyPrinter.format overrides ignored for contents of small containers (GH-22120)
ff420f0

Thanks for the report, Michael!

Thanks for the fix, Irit!