sorted, list.sort reject non-boolean objects with __bool__() as `reverse` parameter

[nodakai]

BPO	37427
Nosy	@tim-one, @rhettinger, @terryjreedy, @mdickinson, @serhiy-storchaka, @nodakai

^{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 2019-06-28.22:04:26.230>
created_at = <Date 2019-06-27.09:51:45.573>
labels = ['interpreter-core', 'type-bug', 'invalid', '3.9']
title = 'sorted, list.sort reject non-boolean objects with __bool__() as `reverse` parameter'
updated_at = <Date 2019-08-29.17:04:27.073>
user = 'https://github.com/nodakai'

bugs.python.org fields:

activity = <Date 2019-08-29.17:04:27.073>
actor = 'mark.dickinson'
assignee = 'none'
closed = True
closed_date = <Date 2019-06-28.22:04:26.230>
closer = 'terry.reedy'
components = ['Interpreter Core']
creation = <Date 2019-06-27.09:51:45.573>
creator = 'nodakai'
dependencies = []
files = []
hgrepos = []
issue_num = 37427
keywords = []
message_count = 6.0
messages = ['346722', '346730', '346731', '346742', '346870', '350808']
nosy_count = 6.0
nosy_names = ['tim.peters', 'rhettinger', 'terry.reedy', 'mark.dickinson', 'serhiy.storchaka', 'nodakai']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue37427'
versions = ['Python 3.9']

[nodakai]

class X:
    def __bool__(self):
        return True

x = [1, 2, 3]
x.sort(reverse=X())
print(x)
print(sorted([1, 2, 3], reverse=X()))

TypeError: an integer is required (got type X)

We can always still do

x.sort(reverse=bool(X()))
print(sorted([1, 2, 3], reverse=bool(X())))

but that isn't cool

[mdickinson]

I haven't looked at the source yet, but from experimentation I'm finding it rather hard to guess what the rules are for what works and what doesn't:

Python 3.7.3 (default, Mar 30 2019, 03:37:43) 
[Clang 10.0.0 (clang-1000.11.45.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import decimal, fractions, numpy
>>> sorted([1, 2, 3], reverse=0.5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: integer argument expected, got float
>>> sorted([1, 2, 3], reverse=decimal.Decimal(0.5))
[1, 2, 3]
>>> sorted([1, 2, 3], reverse=fractions.Fraction(0.5))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: an integer is required (got type Fraction)
>>> sorted([1, 2, 3], reverse=numpy.int64(1))
[3, 2, 1]
>>> sorted([1, 2, 3], reverse=numpy.bool_(True))
[3, 2, 1]

[mdickinson]

I'm finding it rather hard to guess what the rules are for what works and what doesn't

So now that I've looked at the source: anything with an __int__ method works, except that float instances are explicitly excluded. So this explains for example:

>>> sorted([1, 2, 3], reverse=numpy.float64(0.5))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: integer argument expected, got float
>>> sorted([1, 2, 3], reverse=numpy.float32(0.5))
[1, 2, 3]

[serhiy-storchaka]

See bpo-15999.

[terryjreedy]

Not a bug. Both function docs say "reverse is a boolean value."
https://docs.python.org/3/library/stdtypes.html#boolean-values
says "Boolean values are the two constant objects False and True." These should be used in new code.

For compatibility with old code, CPython generally allows 0 and 1. In this case, it allows any 'integral value', but I would not necessarily expect this of other implementations.

[In general, the *Python language* docs do not document extra latitude allowed by the *CPython implementation*. So the lack thereof here is also not a bug.]

All objects without fancy trickery have a __bool__ method, either inherited from *object* or overridden. This in no way makes all such objects, including Xs, into boolean values.

[mdickinson]

See also bpo-37980. I'd support a change to have the reversed argument to sort and sorted be automatically interpreted in boolean context.