Base class for parent objects
CLASS HIERARCHY:
SageObject
CategoryObject
Parent
A simple example of registering coercions:
sage: class A_class(Parent):
....: def __init__(self, name):
....: Parent.__init__(self, name=name)
....: self._populate_coercion_lists_()
....: self.rename(name)
....: #
....: def category(self):
....: return Sets()
....: #
....: def _element_constructor_(self, i):
....: assert(isinstance(i, (int, Integer)))
....: return ElementWrapper(self, i)
....:
sage: A = A_class("A")
sage: B = A_class("B")
sage: C = A_class("C")
sage: def f(a):
....: return B(a.value+1)
....:
sage: class MyMorphism(Morphism):
....: def __init__(self, domain, codomain):
....: Morphism.__init__(self, Hom(domain, codomain))
....: #
....: def _call_(self, x):
....: return self.codomain()(x.value)
....:
sage: f = MyMorphism(A,B)
sage: f
Generic morphism:
From: A
To: B
sage: B.register_coercion(f)
sage: C.register_coercion(MyMorphism(B,C))
sage: A(A(1)) == A(1)
True
sage: B(A(1)) == B(1)
True
sage: C(A(1)) == C(1)
True
sage: A(B(1))
Traceback (most recent call last):
...
AssertionError
When implementing an element of a ring, one would typically provide the element class with _rmul_ and/or _lmul_ methods for the action of a base ring, and with _mul_ for the ring multiplication. However, prior to trac ticket #14249, it would have been necessary to additionally define a method _an_element_() for the parent. But now, the following example works:
sage: from sage.structure.element import RingElement
sage: class MyElement(RingElement):
....: def __init__(self, parent, x, y):
....: RingElement.__init__(self, parent)
....: def _mul_(self, other):
....: return self
....: def _rmul_(self, other):
....: return self
....: def _lmul_(self, other):
....: return self
sage: class MyParent(Parent):
....: Element = MyElement
Now, we define
sage: P = MyParent(base=ZZ, category=Rings())
sage: a = P(1,2)
sage: a*a is a
True
sage: a*2 is a
True
sage: 2*a is a
True
TESTS:
This came up in some subtle bug once:
sage: gp(2) + gap(3)
5
Bases: sage.structure.category_object.CategoryObject
Base class for all parents.
Parents are the Sage/mathematical analogues of container objects in computer science.
INPUT:
If facade is specified, then Sets().Facades() is added to the categories of the parent. Furthermore, if facade is not True, the internal attribute _facade_for is set accordingly for use by Sets.Facades.ParentMethods.facade_for().
Internal invariants:
Todo
Eventually, category should be Sets by default.
This is the generic call method for all parents.
When called, it will find a map based on the Parent (or type) of x. If a coercion exists, it will always be chosen. This map will then be called (with the arguments and keywords if any).
By default this will dispatch as quickly as possible to _element_constructor_() though faster pathways are possible if so desired.
TESTS:
We check that the invariant:
self._element_init_pass_parent == guess_pass_parent(self, self._element_constructor)
is preserved (see trac ticket #5979):
sage: class MyParent(Parent):
....: def _element_constructor_(self, x):
....: print self, x
....: return sage.structure.element.Element(parent = self)
....: def _repr_(self):
....: return "my_parent"
....:
sage: my_parent = MyParent()
sage: x = my_parent("bla")
my_parent bla
sage: x.parent() # indirect doctest
my_parent
sage: x = my_parent() # shouldn't this one raise an error?
my_parent 0
sage: x = my_parent(3) # todo: not implemented why does this one fail???
my_parent 3
This function allows one to specify coercions, actions, conversions and embeddings involving this parent.
IT SHOULD ONLY BE CALLED DURING THE __INIT__ method, often at the end.
INPUT:
coerce_list – a list of coercion Morphisms to self and parents with canonical coercions to self
action_list – a list of actions on and by self
parents with conversions to self
embedding – a single Morphism from self
convert_method_name – a name to look for that other elements can implement to create elements of self (e.g. _integer_)
element_constructor – A callable object used by the __call__ method to construct new elements. Typically the element class or a bound method (defaults to self._element_constructor_).
init_no_parent – if True omit passing self in as the first argument of element_constructor for conversion. This is useful if parents are unique, or element_constructor is a bound method (this latter case can be detected automatically).
This is a multiplication method that more or less directly calls another attribute _mul_ (single underscore). This is because __mul__ can not be implemented via inheritance from the parent methods of the category, but _mul_ can be inherited. This is, e.g., used when creating twosided ideals of matrix algebras. See trac ticket #7797.
EXAMPLE:
sage: MS = MatrixSpace(QQ,2,2)
This matrix space is in fact an algebra, and in particular it is a ring, from the point of view of categories:
sage: MS.category()
Category of algebras over Rational Field
sage: MS in Rings()
True
However, its class does not inherit from the base class Ring:
sage: isinstance(MS,Ring)
False
Its _mul_ method is inherited from the category, and can be used to create a left or right ideal:
sage: MS._mul_.__module__
'sage.categories.rings'
sage: MS*MS.1 # indirect doctest
Left Ideal
(
[0 1]
[0 0]
)
of Full MatrixSpace of 2 by 2 dense matrices over Rational Field
sage: MS*[MS.1,2]
Left Ideal
(
[0 1]
[0 0],
[2 0]
[0 2]
)
of Full MatrixSpace of 2 by 2 dense matrices over Rational Field
sage: MS.1*MS
Right Ideal
(
[0 1]
[0 0]
)
of Full MatrixSpace of 2 by 2 dense matrices over Rational Field
sage: [MS.1,2]*MS
Right Ideal
(
[0 1]
[0 0],
[2 0]
[0 2]
)
of Full MatrixSpace of 2 by 2 dense matrices over Rational Field
True if there is an element of self that is equal to x under ==, or if x is already an element of self. Also, True in other cases involving the Symbolic Ring, which is handled specially.
For many structures we test this by using __call__() and then testing equality between x and the result.
The Symbolic Ring is treated differently because it is ultra-permissive about letting other rings coerce in, but ultra-strict about doing comparisons.
EXAMPLES:
sage: 2 in Integers(7)
True
sage: 2 in ZZ
True
sage: Integers(7)(3) in ZZ
True
sage: 3/1 in ZZ
True
sage: 5 in QQ
True
sage: I in RR
False
sage: SR(2) in ZZ
True
sage: RIF(1, 2) in RIF
True
sage: pi in RIF # there is no element of RIF equal to pi
False
sage: sqrt(2) in CC
True
sage: pi in RR
True
sage: pi in CC
True
sage: pi in RDF
True
sage: pi in CDF
True
TESTS:
Check that trac ticket #13824 is fixed:
sage: 4/3 in GF(3)
False
sage: 15/50 in GF(25, 'a')
False
sage: 7/4 in Integers(4)
False
sage: 15/36 in Integers(6)
False
Override this method to specify coercions beyond those specified in coerce_list.
If no such coercion exists, return None or False. Otherwise, it may return either an actual Map to use for the coercion, a callable (in which case it will be wrapped in a Map), or True (in which case a generic map will be provided).
Override this method to provide additional conversions beyond those given in convert_list.
This function is called after coercions are attempted. If there is a coercion morphism in the opposite direction, one should consider adding a section method to that.
This MUST return a Map from S to self, or None. If None is returned then a generic map will be provided.
Override this method to provide an action of self on S or S on self beyond what was specified in action_list.
This must return an action which accepts an element of self and an element of S (in the order specified by self_on_left).
Returns an element of self. Want it in sufficient generality that poorly-written functions won’t work when they’re not supposed to. This is cached so doesn’t have to be super fast.
EXAMPLES:
sage: QQ._an_element_()
1/2
sage: ZZ['x,y,z']._an_element_()
x
TESTS:
Since Parent comes before the parent classes provided by categories in the hierarchy of classes, we make sure that this default implementation of _an_element_() does not override some provided by the categories. Eventually, this default implementation should be moved into the categories to avoid this workaround:
sage: S = FiniteEnumeratedSet([1,2,3])
sage: S.category()
Category of facade finite enumerated sets
sage: super(Parent, S)._an_element_
Cached version of <function _an_element_from_iterator at ...>
sage: S._an_element_()
1
sage: S = FiniteEnumeratedSet([])
sage: S._an_element_()
Traceback (most recent call last):
...
EmptySetError
Metadata about the _repr_() output.
INPUT:
Valid key arguments are:
OUTPUT:
Boolean.
EXAMPLES:
sage: ZZ._repr_option('ascii_art')
False
sage: MatrixSpace(ZZ, 2)._repr_option('element_ascii_art')
True
Initialize the category framework
Most parents initialize their category upon construction, and this is the recommended behavior. For example, this happens when the constructor calls Parent.__init__() directly or indirectly. However, some parents defer this for performance reasons. For example, sage.matrix.matrix_space.MatrixSpace does not.
EXAMPLES:
sage: P = Parent()
sage: P.category()
Category of sets
sage: class MyParent(Parent):
....: def __init__(self):
....: self._init_category_(Groups())
sage: MyParent().category()
Category of groups
Return the homspace Hom(self, codomain, category).
INPUT:
OUTPUT:
The homspace of all homomorphisms from self to codomain in the category category.
See also
EXAMPLES:
sage: R.<x,y> = PolynomialRing(QQ, 2)
sage: R.Hom(QQ)
Set of Homomorphisms from Multivariate Polynomial Ring in x, y over Rational Field to Rational Field
Homspaces are defined for very general Sage objects, even elements of familiar rings:
sage: n = 5; Hom(n,7)
Set of Morphisms from 5 to 7 in Category of elements of Integer Ring
sage: z=(2/3); Hom(z,8/1)
Set of Morphisms from 2/3 to 8 in Category of elements of Rational Field
This example illustrates the optional third argument:
sage: QQ.Hom(ZZ, Sets())
Set of Morphisms from Rational Field to Integer Ring in Category of sets
A parent may specify how to construct certain homsets by implementing a method _Hom_`(codomain, category). See :func:`~sage.categories.homset.Hom() for details.
Returns a (preferably typical) element of this parent.
This is used both for illustration and testing purposes. If the set self is empty, an_element() raises the exception EmptySetError.
This calls _an_element_() (which see), and caches the result. Parent are thus encouraged to override _an_element_().
EXAMPLES:
sage: CDF.an_element()
1.0*I
sage: ZZ[['t']].an_element()
t
In case the set is empty, an EmptySetError is raised:
sage: Set([]).an_element()
Traceback (most recent call last):
...
EmptySetError
EXAMPLES:
sage: P = Parent()
sage: P.category()
Category of sets
sage: class MyParent(Parent):
....: def __init__(self): pass
sage: MyParent().category()
Category of sets
Return x as an element of self, if and only if there is a canonical coercion from the parent of x to self.
EXAMPLES:
sage: QQ.coerce(ZZ(2))
2
sage: ZZ.coerce(QQ(2))
Traceback (most recent call last):
...
TypeError: no canonical coercion from Rational Field to Integer Ring
We make an exception for zero:
sage: V = GF(7)^7
sage: V.coerce(0)
(0, 0, 0, 0, 0, 0, 0)
Return the embedding of self into some other parent, if such a parent exists.
This does not mean that there are no coercion maps from self into other fields, this is simply a specific morphism specified out of self and usually denotes a special relationship (e.g. sub-objects, choice of completion, etc.)
EXAMPLES:
sage: K.<a>=NumberField(x^3+x^2+1,embedding=1)
sage: K.coerce_embedding()
Generic morphism:
From: Number Field in a with defining polynomial x^3 + x^2 + 1
To: Real Lazy Field
Defn: a -> -1.465571231876768?
sage: K.<a>=NumberField(x^3+x^2+1,embedding=CC.gen())
sage: K.coerce_embedding()
Generic morphism:
From: Number Field in a with defining polynomial x^3 + x^2 + 1
To: Complex Lazy Field
Defn: a -> 0.2327856159383841? + 0.7925519925154479?*I
Return a Map object to coerce from S to self if one exists, or None if no such coercion exists.
EXAMPLES:
By trac ticket #12313, a special kind of weak key dictionary is used to store coercion and conversion maps, namely MonoDict. In that way, a memory leak was fixed that would occur in the following test:
sage: import gc
sage: _ = gc.collect()
sage: K = GF(1<<55,'t')
sage: for i in range(50):
....: a = K.random_element()
....: E = EllipticCurve(j=a)
....: b = K.has_coerce_map_from(E)
sage: _ = gc.collect()
sage: len([x for x in gc.get_objects() if isinstance(x,type(E))])
1
TESTS:
The following was fixed in trac ticket #12969:
sage: R = QQ['q,t'].fraction_field()
sage: Sym = sage.combinat.sf.sf.SymmetricFunctions(R)
sage: H = Sym.macdonald().H()
sage: P = Sym.macdonald().P()
sage: m = Sym.monomial()
sage: Ht = Sym.macdonald().Ht()
sage: phi = m.coerce_map_from(P)
Returns a pair (functor, parent) such that functor(parent) return self. If this ring does not have a functorial construction, return None.
EXAMPLES:
sage: QQ.construction()
(FractionField, Integer Ring)
sage: f, R = QQ['x'].construction()
sage: f
Poly[x]
sage: R
Rational Field
sage: f(R)
Univariate Polynomial Ring in x over Rational Field
This function returns a Map from \(S\) to \(self\), which may or may not succeed on all inputs. If a coercion map from S to self exists, then the it will be returned. If a coercion from \(self\) to \(S\) exists, then it will attempt to return a section of that map.
Under the new coercion model, this is the fastest way to convert elements of \(S\) to elements of \(self\) (short of manually constructing the elements) and is used by __call__().
EXAMPLES:
sage: m = ZZ.convert_map_from(QQ)
sage: m
Generic map:
From: Rational Field
To: Integer Ring
sage: m(-35/7)
-5
sage: parent(m(-35/7))
Integer Ring
The (default) class for the elements of this parent
FIXME’s and design issues:
Returns an action of self on S or S on self.
To provide additional actions, override _get_action_().
TESTS:
sage: M = QQ['y']^3
sage: M.get_action(ZZ['x']['y'])
Right scalar multiplication by Univariate Polynomial Ring in y over Univariate Polynomial Ring in x over Integer Ring on Ambient free module of rank 3 over the principal ideal domain Univariate Polynomial Ring in y over Rational Field
sage: M.get_action(ZZ['x']) # should be None
Return True if there is a natural map from S to self. Otherwise, return False.
EXAMPLES:
sage: RDF.has_coerce_map_from(QQ)
True
sage: RDF.has_coerce_map_from(QQ['x'])
False
sage: RDF['x'].has_coerce_map_from(QQ['x'])
True
sage: RDF['x,y'].has_coerce_map_from(QQ['x'])
True
Return the unique homomorphism from self to codomain that sends self.gens() to the entries of im_gens. Raises a TypeError if there is no such homomorphism.
INPUT:
OUTPUT:
A homomorphism self –> codomain
Note
As a shortcut, one can also give an object X instead of im_gens, in which case return the (if it exists) natural map to X.
EXAMPLES:
Polynomial Ring: We first illustrate construction of a few homomorphisms involving a polynomial ring:
sage: R.<x> = PolynomialRing(ZZ)
sage: f = R.hom([5], QQ)
sage: f(x^2 - 19)
6
sage: R.<x> = PolynomialRing(QQ)
sage: f = R.hom([5], GF(7))
Traceback (most recent call last):
...
TypeError: images do not define a valid homomorphism
sage: R.<x> = PolynomialRing(GF(7))
sage: f = R.hom([3], GF(49,'a'))
sage: f
Ring morphism:
From: Univariate Polynomial Ring in x over Finite Field of size 7
To: Finite Field in a of size 7^2
Defn: x |--> 3
sage: f(x+6)
2
sage: f(x^2+1)
3
Natural morphism:
sage: f = ZZ.hom(GF(5))
sage: f(7)
2
sage: f
Ring Coercion morphism:
From: Integer Ring
To: Finite Field of size 5
There might not be a natural morphism, in which case a TypeError is raised:
sage: QQ.hom(ZZ)
Traceback (most recent call last):
...
TypeError: Natural coercion morphism from Rational Field to Integer Ring not defined.
The old way to signal atomic string reps.
True if the elements have atomic string representations, in the sense that if they print at s, then -s means the negative of s. For example, integers are atomic but polynomials are not.
EXAMPLES:
sage: Parent().is_atomic_repr()
doctest:...: DeprecationWarning: Use _repr_option to return metadata about string rep
See http://trac.sagemath.org/14040 for details.
False
Test whether the ring is exact.
Note
This defaults to true, so even if it does return True you have no guarantee (unless the ring has properly overloaded this).
OUTPUT:
Return True if elements of this ring are represented exactly, i.e., there is no precision loss when doing arithmetic.
EXAMPLES:
sage: QQ.is_exact()
True
sage: ZZ.is_exact()
True
sage: Qp(7).is_exact()
False
sage: Zp(7, type='capped-abs').is_exact()
False
Update the coercion model to use action to act on self.
action should be of type sage.categories.action.Action.
EXAMPLES:
sage: import sage.categories.action
sage: import operator
sage: class SymmetricGroupAction(sage.categories.action.Action):
....: "Act on a multivariate polynomial ring by permuting the generators."
....: def __init__(self, G, M, is_left=True):
....: sage.categories.action.Action.__init__(self, G, M, is_left, operator.mul)
....:
....: def _call_(self, g, a):
....: if not self.is_left():
....: g, a = a, g
....: D = {}
....: for k, v in a.dict().items():
....: nk = [0]*len(k)
....: for i in range(len(k)):
....: nk[g(i+1)-1] = k[i]
....: D[tuple(nk)] = v
....: return a.parent()(D)
sage: R.<x, y, z> = QQ['x, y, z']
sage: G = SymmetricGroup(3)
sage: act = SymmetricGroupAction(G, R)
sage: t = x + 2*y + 3*z
sage: act(G((1, 2)), t)
2*x + y + 3*z
sage: act(G((2, 3)), t)
x + 3*y + 2*z
sage: act(G((1, 2, 3)), t)
3*x + y + 2*z
This should fail, since we haven’t registered the left action:
sage: G((1,2)) * t
Traceback (most recent call last):
...
TypeError: ...
Now let’s make it work:
sage: R._unset_coercions_used()
sage: R.register_action(act)
sage: G((1, 2)) * t
2*x + y + 3*z
Update the coercion model to use \(mor : P \to \text{self}\) to coerce from a parent P into self.
For safety, an error is raised if another coercion has already been registered or discovered between P and self.
EXAMPLES:
sage: K.<a> = ZZ['a']
sage: L.<b> = ZZ['b']
sage: L_into_K = L.hom([-a]) # non-trivial automorphism
sage: K.register_coercion(L_into_K)
sage: K(0) + b
-a
sage: a + b
0
sage: K(b) # check that convert calls coerce first; normally this is just a
-a
sage: L(0) + a in K # this goes through the coercion mechanism of K
True
sage: L(a) in L # this still goes through the convert mechanism of L
True
sage: K.register_coercion(L_into_K)
Traceback (most recent call last):
...
AssertionError: coercion from Univariate Polynomial Ring in b over Integer Ring to Univariate Polynomial Ring in a over Integer Ring already registered or discovered
Update the coercion model to use \(\text{mor} : P \to \text{self}\) to convert from P into self.
EXAMPLES:
sage: K.<a> = ZZ['a']
sage: M.<c> = ZZ['c']
sage: M_into_K = M.hom([a]) # trivial automorphism
sage: K._unset_coercions_used()
sage: K.register_conversion(M_into_K)
sage: K(c)
a
sage: K(0) + c
Traceback (most recent call last):
...
TypeError: ...
Add embedding to coercion model.
This method updates the coercion model to use \(\text{embedding} : \text{self} \to P\) to embed self into the parent P.
There can only be one embedding registered; it can only be registered once; and it must be registered before using this parent in the coercion model.
EXAMPLES:
sage: S3 = AlternatingGroup(3)
sage: G = SL(3, QQ)
sage: p = S3[2]; p.matrix()
[0 0 1]
[1 0 0]
[0 1 0]
By default, one can’t mix matrices and permutations:
sage: G(p)
Traceback (most recent call last):
...
TypeError: entries must be coercible to a list or integer
sage: G(1) * p
Traceback (most recent call last):
...
TypeError: ...
sage: phi = S3.hom(lambda p: G(p.matrix()), codomain = G)
sage: phi(p)
[0 0 1]
[1 0 0]
[0 1 0]
sage: S3._unset_coercions_used()
sage: S3.register_embedding(phi)
By trac ticket #14711, coerce maps should be copied when using outside of the coercion system:
sage: phi = copy(S3.coerce_embedding()); phi
Generic morphism:
From: Alternating group of order 3!/2 as a permutation group
To: Special Linear Group of degree 3 over Rational Field
sage: phi(p)
[0 0 1]
[1 0 0]
[0 1 0]
This does not work since matrix groups are still old-style parents (see trac ticket #14014):
sage: G(p) # todo: not implemented
sage: G(1) * p # todo: not implemented
Some more advanced examples:
sage: x = QQ['x'].0
sage: t = abs(ZZ.random_element(10^6))
sage: K = NumberField(x^2 + 2*3*7*11, "a"+str(t))
sage: a = K.gen()
sage: K_into_MS = K.hom([a.matrix()])
sage: K._unset_coercions_used()
sage: K.register_embedding(K_into_MS)
sage: L = NumberField(x^2 + 2*3*7*11*19*31, "b"+str(abs(ZZ.random_element(10^6))))
sage: b = L.gen()
sage: L_into_MS = L.hom([b.matrix()])
sage: L._unset_coercions_used()
sage: L.register_embedding(L_into_MS)
sage: K.coerce_embedding()(a)
[ 0 1]
[-462 0]
sage: L.coerce_embedding()(b)
[ 0 1]
[-272118 0]
sage: a.matrix() * b
[-272118 0]
[ 0 -462]
sage: a * b.matrix()
[-272118 0]
[ 0 -462]
Return the (unique) Parent that represents the set of Python objects of a specified type.
EXAMPLES:
sage: from sage.structure.parent import Set_PythonType
sage: Set_PythonType(list)
Set of Python objects of type 'list'
sage: Set_PythonType(list) is Set_PythonType(list)
True
sage: S = Set_PythonType(tuple)
sage: S([1,2,3])
(1, 2, 3)
S is a parent which models the set of all lists:
sage: S.category()
Category of sets
EXAMPLES:
sage: R = sage.structure.parent.Set_PythonType(int)
sage: S = sage.structure.parent.Set_PythonType(float)
sage: Hom(R, S)
Set of Morphisms from Set of Python objects of type 'int' to Set of Python objects of type 'float' in Category of sets
Bases: sage.structure.parent.Set_generic
EXAMPLES:
sage: S = sage.structure.parent.Set_PythonType(float)
sage: S.category()
Category of sets
EXAMPLES:
sage: S = sage.structure.parent.Set_PythonType(bool)
sage: S.cardinality()
2
sage: S = sage.structure.parent.Set_PythonType(int)
sage: S.cardinality()
4294967296 # 32-bit
18446744073709551616 # 64-bit
sage: S = sage.structure.parent.Set_PythonType(float)
sage: S.cardinality()
18437736874454810627
sage: S = sage.structure.parent.Set_PythonType(long)
sage: S.cardinality()
+Infinity
EXAMPLES:
sage: S = sage.structure.parent.Set_PythonType(tuple)
sage: S.object()
<type 'tuple'>
Bases: sage.structure.parent.Parent
Abstract base class for sets.
TESTS:
sage: Set(QQ).category()
Category of sets
Return True if x is a parent object, i.e., derives from sage.structure.parent.Parent and False otherwise.
EXAMPLES:
sage: from sage.structure.parent import is_Parent
sage: is_Parent(2/3)
False
sage: is_Parent(ZZ)
True
sage: is_Parent(Primes())
True
TESTS:
sage: sage.structure.parent.normalize_names(5, 'x')
('x0', 'x1', 'x2', 'x3', 'x4')
sage: sage.structure.parent.normalize_names(2, ['x','y'])
('x', 'y')