It is now possible and advisable to subclass SchemaNode
in order to
create a bundle of default node behavior. The subclass can define the
following methods and attributes: preparer
, validator
, default
,
missing
, name
, title
, description
, widget
, and
after_bind
.
For example, the older, more imperative style that looked like this still
works, of course::
from colander import SchemaNode
ranged_int = colander.SchemaNode(
validator=colander.Range(0, 10),
default = 10,
title='Ranged Int'
)
But you can alternately now do something like this::
from colander import SchemaNode
class RangedIntSchemaNode(SchemaNode):
validator = colander.Range(0, 10)
default = 10
title = 'Ranged Int'
ranged_int = RangedInt()
Values that are expected to be callables can now alternately be methods of
the schemanode subclass instead of plain attributes::
from colander import SchemaNode
class RangedIntSchemaNode(SchemaNode):
default = 10
title = 'Ranged Int'
def validator(self, node, cstruct):
if not 0 < cstruct < 10:
raise colander.Invalid(node, 'Must be between 0 and 10')
ranged_int = RangedInt()
Note that when implementing a method value such as validator
that
expects to receive a node
argument, node
must be provided in the
call signature, even though node
will almost always be the same as
self
. This is because Colander simply treats the method as another
kind of callable, be it a method, or a function, or an instance that has a
__call__
method. It doesn't care that it happens to be a method of
self
, and it needs to support callables that are not methods, so it
sends node
in regardless.
You can't currently use method definitions as colander.deferred
callables. For example this will not work::
from colander import SchemaNode
class RangedIntSchemaNode(SchemaNode):
default = 10
title = 'Ranged Int'
@colander.deferred
def validator(self, node, kw):
request = kw['request']
def avalidator(node, cstruct):
if not 0 < cstruct < 10:
if request.user != 'admin':
raise colander.Invalid(node, 'Must be between 0 and 10')
return avalidator
ranged_int = RangedInt()
bound_ranged_int = ranged_int.bind(request=request)
This will result in::
TypeError: avalidator() takes exactly 3 arguments (2 given)
However, if you treat the thing being decorated as a function instead of a
method (remove the self
argument from the argument list), it will
indeed work)::
from colander import SchemaNode
class RangedIntSchemaNode(SchemaNode):
default = 10
title = 'Ranged Int'
@colander.deferred
def validator(node, kw):
request = kw['request']
def avalidator(node, cstruct):
if not 0 < cstruct < 10:
if request.user != 'admin':
raise colander.Invalid(node, 'Must be between 0 and 10')
return avalidator
ranged_int = RangedInt()
bound_ranged_int = ranged_int.bind(request=request)
In previous releases of Colander, the only way to defer the computation of
values was via the colander.deferred
decorator. In this release,
however, you can instead use the bindings
attribute of self
to
obtain access to the bind parameters within values that are plain old
methods::
from colander import SchemaNode
class RangedIntSchemaNode(SchemaNode):
default = 10
title = 'Ranged Int'
def validator(self, node, cstruct):
request = self.bindings['request']
if not 0 < cstruct < 10:
if request.user != 'admin':
raise colander.Invalid(node, 'Must be between 0 and 10')
ranged_int = RangedInt()
bound_range_int = ranged_int.bind(request=request)
If the things you're trying to defer aren't callables like validator
,
but they're instead just plain attributes like missing
or default
,
instead of using a colander.deferred
, you can use after_bind
to set
attributes of the schemanode that rely on binding variables::
from colander import SchemaNode
class UserIdSchemaNode(SchemaNode):
title = 'User Id'
def after_bind(self, node, kw):
self.default = kw['request'].user.id
You can override the default values of a schemanode subclass in its
constructor::
from colander import SchemaNode
class RangedIntSchemaNode(SchemaNode):
default = 10
title = 'Ranged Int'
validator = colander.Range(0, 10)
ranged_int = RangedInt(validator=colander.Range(0, 20))
In the above example, the validation will be done on 0-20, not 0-10.
If a schema node name conflicts with a schema value attribute name on the
same class, you can work around it by giving the schema node a bogus name
in the class definition but providing a correct name
argument to the
schema node constructor::
from colander import SchemaNode, Schema
class SomeSchema(Schema):
title = 'Some Schema'
thisnamewillbeignored = colander.SchemaNode(
colander.String(),
name='title'
)
Note that such a workaround is only required if the conflicting names are
attached to the exact same class definition. Colander scrapes off schema
node definitions at each class' construction time, so it's not an issue for
inherited values. For example::
from colander import SchemaNode, Schema
class SomeSchema(Schema):
title = colander.SchemaNode(colander.String())
class AnotherSchema(SomeSchema):
title = 'Some Schema'
schema = AnotherSchema()
In the above example, even though the title = 'Some Schema'
appears to
override the superclass' title
SchemaNode, a title
SchemaNode will
indeed be present in the child list of the schema
instance
(schema['title']
will return the title
SchemaNode) and the schema's
title
attribute will be Some Schema
(schema.title
will return
Some Schema
).
Normal inheritance rules apply to class attributes and methods defined in
a schemanode subclass. If your schemanode subclass inherits from another
schemanode class, your schemanode subclass' methods and class attributes
will override the superclass' methods and class attributes.
Ordering of child schema nodes when inheritance is used works like this:
the "deepest" SchemaNode class in the MRO of the inheritance chain is
consulted first for nodes, then the next deepest, then the next, and so on.
So the deepest class' nodes come first in the relative ordering of schema
nodes, then the next deepest, and so on. For example::
class One(colander.Schema):
a = colander.SchemaNode(
colander.String(),
id='a1',
)
b = colander.SchemaNode(
colander.String(),
id='b1',
)
d = colander.SchemaNode(
colander.String(),
id='d1',
)
class Two(One):
a = colander.SchemaNode(
colander.String(),
id='a2',
)
c = colander.SchemaNode(
colander.String(),
id='c2',
)
e = colander.SchemaNode(
colander.String(),
id='e2',
)
class Three(Two):
b = colander.SchemaNode(
colander.String(),
id='b3',
)
d = colander.SchemaNode(
colander.String(),
id='d3',
)
f = colander.SchemaNode(
colander.String(),
id='f3',
)
three = Three()
The ordering of child nodes computed in the schema node three
will be
['a2', 'b3', 'd3', 'c2', 'e2', 'f3']
. The ordering starts a1
,
b1
, d1
because that's the ordering of nodes in One
, and
One
is the deepest SchemaNode in the inheritance hierarchy. Then it
processes the nodes attached to Two
, the next deepest, which causes
a1
to be replaced by a2
, and c2
and e2
to be appended to
the node list. Then finally it processes the nodes attached to Three
,
which causes b1
to be replaced by b3
, and d1
to be replaced by
d3
, then finally f
is appended.
Multiple inheritance works the same way::
class One(colander.Schema):
a = colander.SchemaNode(
colander.String(),
id='a1',
)
b = colander.SchemaNode(
colander.String(),
id='b1',
)
d = colander.SchemaNode(
colander.String(),
id='d1',
)
class Two(colander.Schema):
a = colander.SchemaNode(
colander.String(),
id='a2',
)
c = colander.SchemaNode(
colander.String(),
id='c2',
)
e = colander.SchemaNode(
colander.String(),
id='e2',
)
class Three(Two, One):
b = colander.SchemaNode(
colander.String(),
id='b3',
)
d = colander.SchemaNode(
colander.String(),
id='d3',
)
f = colander.SchemaNode(
colander.String(),
id='f3',
)
three = Three()
The resulting node ordering of three
is the same as the single
inheritance example: ['a2', 'b3', 'd3', 'c2', 'e2', 'f3']
due to the
MRO deepest-first ordering (One
, then Two
, then Three
).