Basics of using colander include defining a colander schema, deserializing a data structure using a schema, serializing a data structure using a schema, and dealing with :exc:`colander.Invalid` exceptions.
Defining A Colander Schema
Imagine you want to deserialize and validate a serialization of data you've obtained by reading a YAML document. An example of such a data serialization might look something like this:
Let's further imagine you'd like to make sure, on demand, that a particular serialization of this type read from this YAML document or another YAML document is "valid".
Notice that all the innermost values in the serialization are strings, even though some of them (such as age and the position of each friend) are more naturally integer-like. Let's define a schema which will attempt to convert a serialization to a data structure that has different types.
For ease of reading, we've actually defined five schemas above, but
we coalesce them all into a single
Person schema. As the result
of our definitions, a
name, which must be a string.
age, which must be deserializable to an integer; after deserialization happens, a validator ensures that the integer is between 0 and 200 inclusive.
- A sequence of
friendstructures. Each friend structure is a two-element tuple. The first element represents an integer rank; it must be between 0 and 9999 inclusive. The second element represents a string name.
- A sequence of
phonestructures. Each phone structure is a mapping. Each phone mapping has two keys:
locationmust be one of
home. The number must be a string.
Schema Node Objects
A schema is composed of one or more schema node objects, each typically of the class :class:`colander.SchemaNode`, usually in a nested arrangement. Each schema node object has a required type, an optional preparer for adjusting data after deserialization, an optional validator for deserialized prepared data, an optional default, an optional missing, an optional title, an optional description, and a slightly less optional name. It also accepts arbitrary keyword arguments, which are attached directly as attributes to the node instance.
The preparer of a schema node is called after deserialization but before validation; it prepares a deserialized value for validation. Examples would be to prepend schemes that may be missing on url values or to filter html provided by a rich text editor. A preparer is not called during serialization, only during deserialization.
The validator of a schema node is called after deserialization and
preparation ; it makes sure the value matches a constraint. An example of
such a validator is provided in the schema above:
validator=colander.Range(0, 200). A validator is not called after
schema node serialization, only after node deserialization.
The default of a schema node indicates the value to be serialized if a value for the schema node is not found in the input data during serialization. It should be the deserialized representation. If a schema node does not have a default, it is considered "serialization required".
The missing of a schema node indicates the value if a value for the schema node is not found in the input data during deserialization. It should be the deserialized representation. If a schema node does not have a default, it is considered "deserialization required". This value is never validated; it is considered pre-validated.
The name of a schema node appears in error reports.
The title of a schema node is metadata about a schema node that can be used by higher-level systems. By default, it is a capitalization of the name.
The description of a schema node is metadata about a schema node that can be used by higher-level systems. By default, it is empty.
Any other keyword arguments to a schema node constructor will be
attached to the node unmolested (e.g. when
foo=1 is passed, the
resulting schema node will have an attribute named
foo with the
You may see some higher-level systems (such as Deform) pass a
widget argument to a SchemaNode constructor. Such systems make use of
the fact that a SchemaNode can be passed arbitrary keyword arguments for
widget and other keyword arguments not enumerated
here but which are passed during schema node construction by someone
constructing a schema for a particular purpose are not used internally by
Colander; they are instead only meaningful to higher-level systems which
consume Colander schemas. Abitrary keyword arguments are allowed to a
schema node constructor in Colander 0.9+. Prior version disallow them.
The name of a schema node that is introduced as a class-level attribute of a :class:`colander.MappingSchema`, :class:`colander.TupleSchema` or a :class:`colander.SequenceSchema` is its class attribute name. For example:
The name of the schema node defined via
colander.SchemaNode(..) within the schema above is
The title of the same schema node is
In the examples above, if you've been paying attention, you'll have noticed that we're defining classes which subclass from :class:`colander.MappingSchema`, :class:`colander.TupleSchema` and :class:`colander.SequenceSchema`.
It's turtles all the way down: the result of creating an instance of any of :class:`colander.MappingSchema`, :class:`colander.TupleSchema` or :class:`colander.SequenceSchema` object is also a :class:`colander.SchemaNode` object.
Earlier we defined a schema:
Let's now use this schema to try to deserialize some concrete data structures.
Each of thse concrete data structures is called a :term:`cstruct`. "cstruct" is an abbreviation of "colander structure": you can think of a cstruct as a serialized representation of some application data. A "cstruct" is usually generated by the :meth:`colander.SchemaNode.serialize` method, and is converted back into an application structure (aka :term:`appstruct`) via :meth:`colander.SchemaNode.deserialize`.
Deserializing A Valid Serialization
schema.deserialize(cstruct) is called, because all the data in
the schema is valid, and the structure represented by
conforms to the schema,
deserialized will be the following:
Note that all the friend rankings have been converted to integers, likewise for the age.
Deserializing An Invalid Serialization
cstruct structure has some problems. The
age is a
negative number. The rank for
t which is not a valid
location of the first phone is
bar, which is not
a valid location (it is not one of "work" or "home"). What happens
when a cstruct cannot be deserialized due to a data type error or a
deserialize method will raise an exception, and the
clause above will be invoked, causing an error message to be printed.
It will print something like:
The above error is telling us that:
- The top-level age variable failed validation.
- Bob's rank (the Friend tuple name
bob's zeroth element) is not a valid number.
- The zeroth phone number has a bad location: it should be one of "home" or "work".
We can optionally catch the exception raised and obtain the raw error dictionary:
This will print something like:
The exceptions raised by Colander during deserialization are instances of the :exc:`colander.Invalid` exception class. We saw previously that instances of this exception class have a :meth:`colander.Invalid.asdict` method which returns a dictionary of error messages. This dictionary is composed by Colander by walking the exception tree. The exception tree is composed entirely of :exc:`colander.Invalid` exceptions.
While the :meth:`colander.Invalid.asdict` method is useful for simple error reporting, a more complex application, such as a form library that uses Colander as an underlying schema system, may need to do error reporting in a different way. In particular, such a system may need to present the errors next to a field in a form. It may need to translate error messages to another language. To do these things effectively, it will almost certainly need to walk and introspect the exception graph manually.
The :exc:`colander.Invalid` exceptions raised by Colander validation are very rich. They contain detailed information about the circumstances of an error. If you write a system based on Colander that needs to display and format Colander exceptions specially, you will need to get comfy with the Invalid exception API.
When a validation-related error occurs during deserialization, each
node in the schema that had an error (and any of its parents) will be
represented by a corresponding :class:`colander.Invalid` exception.
To support this behavior, each :exc:`colander.Invalid` exception has a
children attribute which is a list. Each element in this list (if
any) will also be an :exc:`colander.Invalid` exception, recursively,
representing the error circumstances for a particular schema
Each exception in the graph has a
msg attribute, which will either
be the value
unicode object, or a
translation string instance representing a freeform error value set
by a particular type during an unsuccessful deserialization.
Exceptions that exist purely for structure will have a
attribute with the value
None. Each exception instance will also
have an attribute named
node, representing the schema node to
which the exception is related.
Translation strings are objects which behave like Unicode objects but have extra metadata associated with them for use in translation systems. See http://docs.repoze.org/projects/translationstring/dev/ for documentation about translation strings. All error messages used by Colander internally are translation strings, which means they can be translated to other languages. In particular, they are suitable for use as gettext message ids.
See the :class:`colander.Invalid` API documentation for more information.
Preparing deserialized data for validation
In certain circumstances, it is necessary to modify the deserialized value before validating it.
For example, a :class:`~colander.String` node may be required to
contain content, but that content may come from a rich text
editor. Such an editor may return
<b></b> which may appear to be
valid but doesn't contain content, or
only after some processing.
Serializing a data structure is obviously the inverse operation from deserializing a data structure. The :meth:`colander.SchemaNode.serialize` method of a schema performs serialization of application data (aka an :term:`appstruct`). If you pass the :meth:`colander.SchemaNode.serialize` method data that can be understood by the schema types in the schema you're calling it against, you will be returned a data structure of serialized values.
For example, given the following schema:
We can serialize a matching data structure:
The value for
serialized above will be
'name':'Bob'}. Note that the
age integer has become a string.
Serialization and deserialization are not completely symmetric, however. Although schema-driven data conversion happens during serialization, and default values are injected as necessary, :mod:`colander` types are defined in such a way that structural validation and validation of values does not happen as it does during deserialization. For example, the :attr:`colander.null` value is substituted into the cstruct for every missing subvalue in an appstruct, and none of the validators associated with the schema or any of is nodes is invoked.
This usually means you may "partially" serialize an appstruct where
some of the values are missing. If we try to serialize partial data
serialize method of the schema:
The value for
serialized above will be
'name':colander.null}. Note the
age integer has become a
string, and the missing
name attribute has been replaced with
:attr:`colander.null`. Above, even though we did not include the
name attribute in the appstruct we fed to
serialize, an error
is not raised. For more information about :attr:`colander.null`
substitution during serialization, see :ref:`serializing_null`.
The corollary: it is the responsibility of the developer to ensure he serializes "the right" data; :mod:`colander` will not raise an error when asked to serialize something that is partially nonsense.
Defining A Schema Imperatively
The above schema we defined was defined declaratively via a set of
class statements. It's often useful to create schemas more
dynamically. For this reason, Colander offers an "imperative" mode of
schema configuration. Here's our previous declarative schema:
We can imperatively construct a completely equivalent schema like so:
Defining a schema imperatively is a lot uglier than defining a schema declaratively, but it's often more useful when you need to define a schema dynamically. Perhaps in the body of a function or method you may need to disinclude a particular schema field based on a business condition; when you define a schema imperatively, you have more opportunity to control the schema composition.
Serializing and deserializing using a schema created imperatively is done exactly the same way as you would serialize or deserialize using a schema created declaratively:
You may be using a module scope schema definition with the expectation that calling a :class:`colander.SchemaNode` constructor will clone all of its subnodes. This is not the case.
For example, in a Python module, you might have code that looks like this:
from colander import MappingSchema from colander import Int class MySchema1(MappingSchema): a = SchemaNode(Int()) class MySchema2(MappingSchema): b = MySchema1() def afunction(): s = MySchema2() s['a'].add(SchemaNode(Int(), name='c'))
Because you're mutating
a (by appending a child node to it via the
:meth:`colander.SchemaNode.add` method) you are probably expecting
that you are working with a copy of
a. This is incorrect:
you're mutating the module-scope copy of the
a instance defined
MySchema1 class. This is almost certainly not what you
mean to do. The symptom of making such a mistake might be that
c nodes are added as children of
a over the course of
the Python process lifetime.
To get around this, use the :meth:`colander.SchemaNode.clone` method to create a deep copy of an instance of a schema otherwise defined at module scope before mutating any of its subnodes:
def afunction(): s = MySchema2().clone() s['a'].add(SchemaNode(Int(), name='c'))
:meth:`colander.SchemaNode.clone` clones all the nodes in the schema, so you can work with a "deep copy" of the schema without disturbing the "template" schema nodes defined at a higher scope.