Singleton categories
Bases: object
Returns whether x is an object in this category.
More specifically, returns True if and only if x has a category which is a subcategory of this one.
EXAMPLES:
sage: ZZ in Sets()
True
Bases: sage.categories.category.Category
A base class for implementing singleton category
A singleton category is a category whose class takes no parameters like Fields() or Rings(). See also the Singleton design pattern.
This is a subclass of Category, with a couple optimizations for singleton categories.
The main purpose is to make the idioms:
sage: QQ in Fields() True sage: ZZ in Fields() False
as fast as possible, and in particular competitive to calling a constant Python method, in order to foster its systematic use throughout the Sage library. Such tests are time critical, in particular when creating a lot of polynomial rings over small fields like in the elliptic curve code.
EXAMPLES:
sage: from sage.categories.category_singleton import Category_singleton
sage: class MyRings(Category):
....: def super_categories(self): return Rings().super_categories()
sage: class MyRingsSingleton(Category_singleton):
....: def super_categories(self): return Rings().super_categories()
We create three rings. One of them is contained in the usual category of rings, one in the category of “my rings” and the third in the category of “my rings singleton”:
sage: R = QQ['x,y']
sage: R1 = Parent(category = MyRings())
sage: R2 = Parent(category = MyRingsSingleton())
sage: R in MyRings()
False
sage: R1 in MyRings()
True
sage: R1 in MyRingsSingleton()
False
sage: R2 in MyRings()
False
sage: R2 in MyRingsSingleton()
True
One sees that containment tests for the singleton class is a lot faster than for a usual class:
sage: timeit("R in MyRings()", number=10000) # not tested
10000 loops, best of 3: 7.12 µs per loop
sage: timeit("R1 in MyRings()", number=10000) # not tested
10000 loops, best of 3: 6.98 µs per loop
sage: timeit("R in MyRingsSingleton()", number=10000) # not tested
10000 loops, best of 3: 3.08 µs per loop
sage: timeit("R2 in MyRingsSingleton()", number=10000) # not tested
10000 loops, best of 3: 2.99 µs per loop
So this is an improvement, but not yet competitive with a pure Cython method:
sage: timeit("R.is_ring()", number=10000) # not tested
10000 loops, best of 3: 383 ns per loop
However, it is competitive with a Python method. Actually it is faster, if one stores the category in a variable:
sage: _Rings = Rings()
sage: R3 = Parent(category = _Rings)
sage: R3.is_ring.__module__
'sage.categories.rings'
sage: timeit("R3.is_ring()", number=10000) # not tested
10000 loops, best of 3: 2.64 µs per loop
sage: timeit("R3 in Rings()", number=10000) # not tested
10000 loops, best of 3: 3.01 µs per loop
sage: timeit("R3 in _Rings", number=10000) # not tested
10000 loops, best of 3: 652 ns per loop
This might not be easy to further optimize, since the time is consumed in many different spots:
sage: timeit("MyRingsSingleton.__classcall__()", number=10000)# not tested
10000 loops, best of 3: 306 ns per loop
sage: X = MyRingsSingleton()
sage: timeit("R in X ", number=10000) # not tested
10000 loops, best of 3: 699 ns per loop
sage: c = MyRingsSingleton().__contains__
sage: timeit("c(R)", number = 10000) # not tested
10000 loops, best of 3: 661 ns per loop
Warning
A singleton concrete class should not have a subclass
(necessarily concrete). Otherwise, creating an instance
of
and an instance
of
would break the singleton
principle:
would have two instances
and
.
With the current implementation only direct subclasses of Category_singleton are supported:
sage: class MyRingsSingleton(Category_singleton):
....: def super_categories(self): return Rings().super_categories()
sage: class Disaster(MyRingsSingleton): pass
sage: Disaster()
Traceback (most recent call last):
...
AssertionError: <class '__main__.Disaster'> is not a direct subclass of <class 'sage.categories.category_singleton.Category_singleton'>
However, it is acceptable for a direct subclass of
Category_singleton to create its unique instance as
an instance of a subclass of itself (in which case, its the
subclass of
which is concrete, not
itself). This is
used for example to plug in extra category code via a dynamic
subclass:
sage: from sage.categories.category_singleton import Category_singleton
sage: class R(Category_singleton):
....: def super_categories(self): return [Sets()]
sage: R()
Category of r
sage: R().__class__
<class '__main__.R_with_category'>
sage: R().__class__.mro()
[<class '__main__.R_with_category'>,
<class '__main__.R'>,
<class 'sage.categories.category_singleton.Category_singleton'>,
<class 'sage.categories.category.Category'>,
<class 'sage.structure.unique_representation.UniqueRepresentation'>,
<class 'sage.structure.unique_representation.CachedRepresentation'>,
<type 'sage.misc.fast_methods.WithEqualityById'>,
<type 'sage.structure.sage_object.SageObject'>,
<class '__main__.R.subcategory_class'>,
<class 'sage.categories.sets_cat.Sets.subcategory_class'>,
<class 'sage.categories.sets_with_partial_maps.SetsWithPartialMaps.subcategory_class'>,
<class 'sage.categories.objects.Objects.subcategory_class'>,
<type 'object'>]
sage: R() is R()
True
sage: R() is R().__class__()
True
In that case, R is an abstract class and has a single concrete subclass, so this does not break the Singleton design pattern.
See also
TESTS:
sage: import __main__
sage: __main__.MyRings = MyRings
sage: __main__.MyRingsSingleton = MyRingsSingleton
sage: TestSuite(MyRingsSingleton()).run(skip=["_test_category"])
Note
The _test_category test is failing because MyRingsSingleton() is not a subcategory of the join of its super categories:
sage: C = MyRingsSingleton()
sage: C.super_categories()
[Category of rngs, Category of semirings]
sage: Rngs() & Semirings()
Category of rings
sage: C.is_subcategory(Rings())
False
Oh well; it’s not really relevant for those tests.