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Close #10042: functools.total_ordering now handles NotImplemented 

(Patch by Katie Miller)
This commit is contained in:
Nick Coghlan 2013-10-02 00:02:03 +10:00
parent e6f4631f08
commit f05d981f58
5 changed files with 207 additions and 19 deletions
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19
Doc/library/functools.rst
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@ -134,15 +134,34 @@ The :mod:`functools` module defines the following functions:
@total_ordering
class Student:
def _is_valid_operand(self, other):
return (hasattr(other, "lastname") and
hasattr(other, "firstname"))
def __eq__(self, other):
if not self._is_valid_operand(other):
return NotImplemented
return ((self.lastname.lower(), self.firstname.lower()) ==
(other.lastname.lower(), other.firstname.lower()))
def __lt__(self, other):
if not self._is_valid_operand(other):
return NotImplemented
return ((self.lastname.lower(), self.firstname.lower()) <
(other.lastname.lower(), other.firstname.lower()))
.. note::
While this decorator makes it easy to create well behaved totally
ordered types, it *does* come at the cost of slower execution and
more complex stack traces for the derived comparison methods. If
performance benchmarking indicates this is a bottleneck for a given
application, implementing all six rich comparison methods instead is
likely to provide an easy speed boost.
.. versionadded:: 3.2
.. versionchanged:: 3.4
Returning NotImplemented from the underlying comparison function for
unrecognised types is now supported.
.. function:: partial(func, *args, **keywords)

94
Lib/functools.py
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@ -89,21 +89,91 @@ def wraps(wrapped,
### total_ordering class decorator
################################################################################
# The correct way to indicate that a comparison operation doesn't
# recognise the other type is to return NotImplemented and let the
# interpreter handle raising TypeError if both operands return
# NotImplemented from their respective comparison methods
#
# This makes the implementation of total_ordering more complicated, since
# we need to be careful not to trigger infinite recursion when two
# different types that both use this decorator encounter each other.
#
# For example, if a type implements __lt__, it's natural to define
# __gt__ as something like:
#
# lambda self, other: not self < other and not self == other
#
# However, using the operator syntax like that ends up invoking the full
# type checking machinery again and means we can end up bouncing back and
# forth between the two operands until we run out of stack space.
#
# The solution is to define helper functions that invoke the appropriate
# magic methods directly, ensuring we only try each operand once, and
# return NotImplemented immediately if it is returned from the
# underlying user provided method. Using this scheme, the __gt__ derived
# from a user provided __lt__ becomes:
#
# lambda self, other: _not_op_and_not_eq(self.__lt__, self, other))
def _not_op(op, other):
# "not a < b" handles "a >= b"
# "not a <= b" handles "a > b"
# "not a >= b" handles "a < b"
# "not a > b" handles "a <= b"
op_result = op(other)
if op_result is NotImplemented:
return NotImplemented
return not op_result
def _op_or_eq(op, self, other):
# "a < b or a == b" handles "a <= b"
# "a > b or a == b" handles "a >= b"
op_result = op(other)
if op_result is NotImplemented:
return NotImplemented
return op_result or self == other
def _not_op_and_not_eq(op, self, other):
# "not (a < b or a == b)" handles "a > b"
# "not a < b and a != b" is equivalent
# "not (a > b or a == b)" handles "a < b"
# "not a > b and a != b" is equivalent
op_result = op(other)
if op_result is NotImplemented:
return NotImplemented
return not op_result and self != other
def _not_op_or_eq(op, self, other):
# "not a <= b or a == b" handles "a >= b"
# "not a >= b or a == b" handles "a <= b"
op_result = op(other)
if op_result is NotImplemented:
return NotImplemented
return not op_result or self == other
def _op_and_not_eq(op, self, other):
# "a <= b and not a == b" handles "a < b"
# "a >= b and not a == b" handles "a > b"
op_result = op(other)
if op_result is NotImplemented:
return NotImplemented
return op_result and self != other
def total_ordering(cls):
"""Class decorator that fills in missing ordering methods"""
convert = {
'__lt__': [('__gt__', lambda self, other: not (self < other or self == other)),
('__le__', lambda self, other: self < other or self == other),
('__ge__', lambda self, other: not self < other)],
'__le__': [('__ge__', lambda self, other: not self <= other or self == other),
('__lt__', lambda self, other: self <= other and not self == other),
('__gt__', lambda self, other: not self <= other)],
'__gt__': [('__lt__', lambda self, other: not (self > other or self == other)),
('__ge__', lambda self, other: self > other or self == other),
('__le__', lambda self, other: not self > other)],
'__ge__': [('__le__', lambda self, other: (not self >= other) or self == other),
('__gt__', lambda self, other: self >= other and not self == other),
('__lt__', lambda self, other: not self >= other)]
'__lt__': [('__gt__', lambda self, other: _not_op_and_not_eq(self.__lt__, self, other)),
('__le__', lambda self, other: _op_or_eq(self.__lt__, self, other)),
('__ge__', lambda self, other: _not_op(self.__lt__, other))],
'__le__': [('__ge__', lambda self, other: _not_op_or_eq(self.__le__, self, other)),
('__lt__', lambda self, other: _op_and_not_eq(self.__le__, self, other)),
('__gt__', lambda self, other: _not_op(self.__le__, other))],
'__gt__': [('__lt__', lambda self, other: _not_op_and_not_eq(self.__gt__, self, other)),
('__ge__', lambda self, other: _op_or_eq(self.__gt__, self, other)),
('__le__', lambda self, other: _not_op(self.__gt__, other))],
'__ge__': [('__le__', lambda self, other: _not_op_or_eq(self.__ge__, self, other)),
('__gt__', lambda self, other: _op_and_not_eq(self.__ge__, self, other)),
('__lt__', lambda self, other: _not_op(self.__ge__, other))]
}
# Find user-defined comparisons (not those inherited from object).
roots = [op for op in convert if getattr(cls, op, None) is not getattr(object, op, None)]

108
Lib/test/test_functools.py
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@ -584,6 +584,7 @@ class TestTotalOrdering(unittest.TestCase):
self.assertTrue(A(2) >= A(1))
self.assertTrue(A(2) <= A(2))
self.assertTrue(A(2) >= A(2))
self.assertFalse(A(1) > A(2))
def test_total_ordering_le(self):
@functools.total_ordering
@ -600,6 +601,7 @@ class TestTotalOrdering(unittest.TestCase):
self.assertTrue(A(2) >= A(1))
self.assertTrue(A(2) <= A(2))
self.assertTrue(A(2) >= A(2))
self.assertFalse(A(1) >= A(2))
def test_total_ordering_gt(self):
@functools.total_ordering
@ -616,6 +618,7 @@ class TestTotalOrdering(unittest.TestCase):
self.assertTrue(A(2) >= A(1))
self.assertTrue(A(2) <= A(2))
self.assertTrue(A(2) >= A(2))
self.assertFalse(A(2) < A(1))
def test_total_ordering_ge(self):
@functools.total_ordering
@ -632,6 +635,7 @@ class TestTotalOrdering(unittest.TestCase):
self.assertTrue(A(2) >= A(1))
self.assertTrue(A(2) <= A(2))
self.assertTrue(A(2) >= A(2))
self.assertFalse(A(2) <= A(1))
def test_total_ordering_no_overwrite(self):
# new methods should not overwrite existing
@ -651,22 +655,112 @@ class TestTotalOrdering(unittest.TestCase):
class A:
pass
def test_bug_10042(self):
def test_type_error_when_not_implemented(self):
# bug 10042; ensure stack overflow does not occur
# when decorated types return NotImplemented
@functools.total_ordering
class TestTO:
class ImplementsLessThan:
def __init__(self, value):
self.value = value
def __eq__(self, other):
if isinstance(other, TestTO):
if isinstance(other, ImplementsLessThan):
return self.value == other.value
return False
def __lt__(self, other):
if isinstance(other, TestTO):
if isinstance(other, ImplementsLessThan):
return self.value < other.value
raise TypeError
with self.assertRaises(TypeError):
TestTO(8) <= ()
return NotImplemented
@functools.total_ordering
class ImplementsGreaterThan:
def __init__(self, value):
self.value = value
def __eq__(self, other):
if isinstance(other, ImplementsGreaterThan):
return self.value == other.value
return False
def __gt__(self, other):
if isinstance(other, ImplementsGreaterThan):
return self.value > other.value
return NotImplemented
@functools.total_ordering
class ImplementsLessThanEqualTo:
def __init__(self, value):
self.value = value
def __eq__(self, other):
if isinstance(other, ImplementsLessThanEqualTo):
return self.value == other.value
return False
def __le__(self, other):
if isinstance(other, ImplementsLessThanEqualTo):
return self.value <= other.value
return NotImplemented
@functools.total_ordering
class ImplementsGreaterThanEqualTo:
def __init__(self, value):
self.value = value
def __eq__(self, other):
if isinstance(other, ImplementsGreaterThanEqualTo):
return self.value == other.value
return False
def __ge__(self, other):
if isinstance(other, ImplementsGreaterThanEqualTo):
return self.value >= other.value
return NotImplemented
@functools.total_ordering
class ComparatorNotImplemented:
def __init__(self, value):
self.value = value
def __eq__(self, other):
if isinstance(other, ComparatorNotImplemented):
return self.value == other.value
return False
def __lt__(self, other):
return NotImplemented
with self.subTest("LT < 1"), self.assertRaises(TypeError):
ImplementsLessThan(-1) < 1
with self.subTest("LT < LE"), self.assertRaises(TypeError):
ImplementsLessThan(0) < ImplementsLessThanEqualTo(0)
with self.subTest("LT < GT"), self.assertRaises(TypeError):
ImplementsLessThan(1) < ImplementsGreaterThan(1)
with self.subTest("LE <= LT"), self.assertRaises(TypeError):
ImplementsLessThanEqualTo(2) <= ImplementsLessThan(2)
with self.subTest("LE <= GE"), self.assertRaises(TypeError):
ImplementsLessThanEqualTo(3) <= ImplementsGreaterThanEqualTo(3)
with self.subTest("GT > GE"), self.assertRaises(TypeError):
ImplementsGreaterThan(4) > ImplementsGreaterThanEqualTo(4)
with self.subTest("GT > LT"), self.assertRaises(TypeError):
ImplementsGreaterThan(5) > ImplementsLessThan(5)
with self.subTest("GE >= GT"), self.assertRaises(TypeError):
ImplementsGreaterThanEqualTo(6) >= ImplementsGreaterThan(6)
with self.subTest("GE >= LE"), self.assertRaises(TypeError):
ImplementsGreaterThanEqualTo(7) >= ImplementsLessThanEqualTo(7)
with self.subTest("GE when equal"):
a = ComparatorNotImplemented(8)
b = ComparatorNotImplemented(8)
self.assertEqual(a, b)
with self.assertRaises(TypeError):
a >= b
with self.subTest("LE when equal"):
a = ComparatorNotImplemented(9)
b = ComparatorNotImplemented(9)
self.assertEqual(a, b)
with self.assertRaises(TypeError):
a <= b
class TestLRU(unittest.TestCase):

1
Misc/ACKS
View File

@ -862,6 +862,7 @@ Chad Miller
Damien Miller
Jason V. Miller
Jay T. Miller
Katie Miller
Roman Milner
Julien Miotte
Andrii V. Mishkovskyi

4
Misc/NEWS
View File

@ -13,6 +13,10 @@ Core and Builtins
Library
-------
- Issue #10042: functools.total_ordering now correctly handles
NotImplemented being returned by the underlying comparison function (Patch
by Katie Miller)
- Issue #19092: contextlib.ExitStack now correctly reraises exceptions
from the __exit__ callbacks of inner context managers (Patch by Hrvoje
Nikšić)