[issue28850] Regression in Python 3: Subclassing PrettyPrinter.format doesn't work anymore

[Tal Einat]

Tal Einat  added the comment:

Thanks for the report, Michael!

Thanks for the fix, Irit!

--
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed
versions:  -Python 3.8, Python 3.9

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[issue28850] Regression in Python 3: Subclassing PrettyPrinter.format doesn't work anymore

[Tal Einat]

Tal Einat  added the comment:


New changeset ff420f0e08a2443339da0df7ace95e14177bac53 by Irit Katriel in 
branch 'master':
bpo-28850: Fix PrettyPrinter.format overrides ignored for contents of small 
containers (GH-22120)
https://github.com/python/cpython/commit/ff420f0e08a2443339da0df7ace95e14177bac53


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nosy: +taleinat

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[issue28850] Regression in Python 3: Subclassing PrettyPrinter.format doesn't work anymore

[Georg Brandl]

Change by Georg Brandl :


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nosy:  -georg.brandl

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[issue28850] Regression in Python 3: Subclassing PrettyPrinter.format doesn't work anymore

[Irit Katriel]

Change by Irit Katriel :


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nosy: +serhiy.storchaka

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[issue28850] Regression in Python 3: Subclassing PrettyPrinter.format doesn't work anymore

[Irit Katriel]

Irit Katriel  added the comment:

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 
https://github.com/python/cpython/blob/fdda200195f9747e411d3491aae0806bc1fcd919/Lib/pprint.py#L179)
 which overrides this result and recalculates the representation for 
containers. In our case it does:

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.

--
versions: +Python 3.10, Python 3.8, Python 3.9 -Python 3.5, Python 3.6, Python 
3.7

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[issue28850] Regression in Python 3: Subclassing PrettyPrinter.format doesn't work anymore

[Irit Katriel]

Change by Irit Katriel :


--
keywords: +patch
nosy: +iritkatriel
nosy_count: 7.0 -> 8.0
pull_requests: +21205
stage: needs patch -> patch review
pull_request: https://github.com/python/cpython/pull/22120

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[issue28850] Regression in Python 3: Subclassing PrettyPrinter.format doesn't work anymore

[Xiang Zhang]

Changes by Xiang Zhang :


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nosy: +xiang.zhang
stage:  -> needs patch
versions: +Python 3.7 -Python 3.3, Python 3.4

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[issue28850] Regression in Python 3: Subclassing PrettyPrinter.format doesn't work anymore

[Guido van Rossum]

Changes by Guido van Rossum :


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[issue28850] Regression in Python 3: Subclassing PrettyPrinter.format doesn't work anymore

[Michael Enßlin]

New submission from Michael Enßlin:

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.

--
components: Library (Lib)
messages: 282159
nosy: anthonybaxter, doerwalter, georg.brandl, gvanrossum, markhirota, mic_e, 
rhettinger
priority: normal
severity: normal
status: open
title: Regression in Python 3: Subclassing PrettyPrinter.format doesn't work 
anymore
type: behavior
versions: Python 3.3, Python 3.4, Python 3.5, Python 3.6

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