Fast methods via Cython

Fast methods via Cython

This module provides extension classes with useful methods of cython speed, that python classes can inherit.

Note

In its original version, this module provides a cython base class WithEqualityById implementing unique instance behaviour, and a cython base class FastHashable_class, which has a quite fast hash whose value can be freely chosen at initialisation time.

AUTHOR:

  • Simon King (2013-02)
class sage.misc.fast_methods.FastHashable_class

Bases: object

A class that has a fast hash method, returning a pre-assigned value.

NOTE:

This is for internal use only. The class has a cdef attribute _hash, that needs to be assigned (for example, by calling the init method, or by a direct assignement using cython). This is slower than using provide_hash_by_id(), but has the advantage that the hash can be prescribed, by assigning a cdef attribute _hash.

TESTS:

sage: from sage.misc.fast_methods import FastHashable_class
sage: H = FastHashable_class(123)
sage: hash(H)
123
class sage.misc.fast_methods.WithEqualityById

Bases: object

Provide hash and equality test based on identity.

Note

This class provides the unique representation behaviour of UniqueRepresentation, together with CachedRepresentation.

EXAMPLES:

Any instance of UniqueRepresentation inherits from WithEqualityById.

sage: class MyParent(Parent):
...     def __init__(self, x):
...         self.x = x
...     def __cmp__(self,other):
...         return cmp(self.x^2,other.x^2)
...     def __hash__(self):
...         return hash(self.x)
sage: class MyUniqueParent(UniqueRepresentation, MyParent): pass
sage: issubclass(MyUniqueParent, sage.misc.fast_methods.WithEqualityById)
True

Inheriting from WithEqualityById provides unique representation behaviour. In particular, the comparison inherited from MyParent is overloaded:

sage: a = MyUniqueParent(1)
sage: b = MyUniqueParent(2)
sage: c = MyUniqueParent(1)
sage: a is c
True
sage: d = MyUniqueParent(-1)
sage: a == d
False

Note, however, that Python distinguishes between “comparison by cmp” and “comparison by binary relations”:

sage: cmp(a,d)
0

The comparison inherited from MyParent will be used in those cases in which identity does not give sufficient information to find the relation:

sage: a < b
True
sage: b > d
True

The hash inherited from MyParent is replaced by a hash that coincides with object‘s hash:

sage: hash(a) == hash(a.x)
False
sage: hash(a) == object.__hash__(a)
True

Warning

It is possible to inherit from UniqueRepresentation and then overload equality test in a way that destroys the unique representation property. We strongly recommend against it! You should use CachedRepresentation instead.

sage: class MyNonUniqueParent(MyUniqueParent):
...     def __eq__(self, other):
...         return self.x^2 == other.x^2
sage: a = MyNonUniqueParent(1)
sage: d = MyNonUniqueParent(-1)
sage: a is MyNonUniqueParent(1)
True
sage: a == d
True
sage: a is d
False

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