RHS not consulted in `str % subclass_of_str` case.

[mjpieters]

BPO	28598
Nosy	@mjpieters, @berkerpeksag, @serhiy-storchaka, @zhangyangyu
PRs	bpo-28598: Support __rmod__ for RHS subclasses of str in % string formatting operations #51 [3.5] bpo-28598: Support __rmod__ for RHS subclasses of str in % string formatting operations #94 [backport to 3.6] bpo-28598: Support __rmod__ for RHS subclasses of str in % string formatting operations #95 [2.7] bpo-28598: Support __rmod__ for RHS subclasses of str in % string formatting operations #366 [backport to 2.7] bpo-28598: Support __rmod__ for RHS subclasses of str in % string formatting operations #382 [Do Not Merge] Sample of CPython life with blurb. #703 [Do Not Merge] Convert Misc/NEWS so that it is managed by towncrier #552
Files	issue28598.patch: Proposed patch, v1

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2017-03-01.06:29:40.530>
created_at = <Date 2016-11-03.13:36:04.717>
labels = ['interpreter-core', 'type-bug']
title = 'RHS not consulted in `str % subclass_of_str` case.'
updated_at = <Date 2017-03-31.16:36:38.525>
user = 'https://github.com/mjpieters'

bugs.python.org fields:

activity = <Date 2017-03-31.16:36:38.525>
actor = 'dstufft'
assignee = 'none'
closed = True
closed_date = <Date 2017-03-01.06:29:40.530>
closer = 'xiang.zhang'
components = ['Interpreter Core']
creation = <Date 2016-11-03.13:36:04.717>
creator = 'mjpieters'
dependencies = []
files = ['45338']
hgrepos = []
issue_num = 28598
keywords = ['patch']
message_count = 9.0
messages = ['279993', '280001', '287638', '288695', '288717', '290356', '290364', '290365', '290428']
nosy_count = 4.0
nosy_names = ['mjpieters', 'berker.peksag', 'serhiy.storchaka', 'xiang.zhang']
pr_nums = ['51', '94', '95', '366', '382', '703', '552']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue28598'
versions = ['Python 2.7']

[mjpieters]

The BINARY_MODULO operator hardcodes a test for PyUnicode:

        TARGET(BINARY_MODULO) {
            PyObject *divisor = POP();
            PyObject *dividend = TOP();
            PyObject *res = PyUnicode_CheckExact(dividend) ?
                PyUnicode_Format(dividend, divisor) :
                PyNumber_Remainder(dividend, divisor);

This means that a RHS subclass of str can't override the operator:

>>> class Foo(str):
...     def __rmod__(self, other):
...         return self % other
...
>>> "Bar: %s" % Foo("Foo: %s")
'Bar: Foo %s'

The expected output there is "Foo: Bar %s".

This works correctly for bytes:

>>> class FooBytes(bytes):
...     def __rmod__(self, other):
...         return self % other
...
>>> b"Bar: %s" % FooBytes(b"Foo: %s")
b'Foo: Bar: %s'

and for all other types where the RHS is a subclass.

Perhaps there should be a test to see if divisor is a subclass, and in that case take the slow path?

[mjpieters]

Here's a proposed patch for tip; what versions would it be worth backporting this to?

(Note, there's no NEWS update in this patch).

[mjpieters]

I'm not sure if issues are linked automatically yet. I put the patch up as a pull request on GitHub: #51

[serhiy-storchaka]

Is 2.7 free from this bug?

[mjpieters]

Is 2.7 free from this bug?

No, 2.7 is affected too:

>>> class SubclassedStr(str):
...     def __rmod__(self, other):
...         return 'Success, self.__rmod__({!r}) was called'.format(other)
...
>>> 'lhs %% %r' % SubclassedStr('rhs')
"lhs % 'rhs'"

Expected output is "Success, self.__rmod__('lhs %% %r') was called"

On the plus side, unicode is not affected:

>>> class SubclassedUnicode(unicode):
...     def __rmod__(self, other):
...         return u'Success, self.__rmod__({!r}) was called'.format(other)
...
>>> u'lhs %% %r' % SubclassedUnicode(u'rhs')
u"Success, self.__rmod__(u'lhs %% %r') was called"

[zhangyangyu]

New changeset b4f0e98 by Xiang Zhang in branch '2.7':
bpo-28598: Support __rmod__ for RHS subclasses of str in % string formatting operations (GH-366)
b4f0e98

[berkerpeksag]

New changeset bc144f0 by Berker Peksag (Martijn Pieters) in branch '3.5':
bpo-28598: Support __rmod__ for RHS subclasses of str in % string formatting operations (#94)
bc144f0

[berkerpeksag]

New changeset 53039ad by Berker Peksag (Martijn Pieters) in branch '3.6':
bpo-28598: Support __rmod__ for RHS subclasses of str in % string formatting operations (#95)
53039ad