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Colander Basics

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 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:

{
 'name':'keith',
 'age':'20',
 'friends':[('1', 'jim'),('2', 'bob'), ('3', 'joe'), ('4', 'fred')],
 'phones':[{'location':'home', 'number':'555-1212'},
           {'location':'work', 'number':'555-8989'},],
}

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.

import colander

class Friend(colander.TupleSchema):
    rank = colander.SchemaNode(colander.Int(), 
                              validator=colander.Range(0, 9999))
    name = colander.SchemaNode(colander.String())

class Phone(colander.MappingSchema):
    location = colander.SchemaNode(colander.String(), 
                                  validator=colander.OneOf(['home', 'work']))
    number = colander.SchemaNode(colander.String())

class Friends(colander.SequenceSchema):
    friend = Friend()

class Phones(colander.SequenceSchema):
    phone = Phone()

class Person(colander.MappingSchema):
    name = colander.SchemaNode(colander.String())
    age = colander.SchemaNode(colander.Int(),
                             validator=colander.Range(0, 200))
    friends = Friends()
    phones = Phones()

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 Person represents:

  • A name, which must be a string.
  • An 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 friend structures. 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 phone structures. Each phone structure is a mapping. Each phone mapping has two keys: location and number. The location must be one of work or 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 colander.SchemaNode, usually in a nested arrangement. Each schema node object has a required type, an optional deserialization validator, an optional default, an optional missing, an optional title, an optional description, and a slightly less optional name.

The type of a schema node indicates its data type (such as colander.Int or colander.String).

The validator of a schema node is called after deserialization; it makes sure the deserialized 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 serialization, only after 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.

The name of a schema node that is introduced as a class-level attribute of a colander.MappingSchema, colander.TupleSchema or a colander.SequenceSchema is its class attribute name. For example:

import colander

class Phone(colander.MappingSchema):
    location = colander.SchemaNode(colander.String(), 
                                  validator=colander.OneOf(['home', 'work']))
    number = colander.SchemaNode(colander.String())

The name of the schema node defined via location = colander.SchemaNode(..) within the schema above is location. The title of the same schema node is Location.

Schema Objects

In the examples above, if you've been paying attention, you'll have noticed that we're defining classes which subclass from colander.MappingSchema, colander.TupleSchema and colander.SequenceSchema.

It's turtles all the way down: the result of creating an instance of any of colander.MappingSchema, colander.TupleSchema or colander.SequenceSchema object is also a colander.SchemaNode object.

Instantiating a colander.MappingSchema creates a schema node which has a type value of colander.Mapping.

Instantiating a colander.TupleSchema creates a schema node which has a type value of colander.Tuple.

Instantiating a colander.SequenceSchema creates a schema node which has a type value of colander.Sequence.

Deserialization

Earlier we defined a schema:

import colander

class Friend(colander.TupleSchema):
    rank = colander.SchemaNode(colander.Int(), 
                              validator=colander.Range(0, 9999))
    name = colander.SchemaNode(colander.String())

class Phone(colander.MappingSchema):
    location = colander.SchemaNode(colander.String(), 
                                  validator=colander.OneOf(['home', 'work']))
    number = colander.SchemaNode(colander.String())

class Friends(colander.SequenceSchema):
    friend = Friend()

class Phones(colander.SequenceSchema):
    phone = Phone()

class Person(colander.MappingSchema):
    name = colander.SchemaNode(colander.String())
    age = colander.SchemaNode(colander.Int(),
                             validator=colander.Range(0, 200))
    friends = Friends()
    phones = Phones()

Let's now use this schema to try to deserialize some concrete data structures.

Each of thse concrete data structures is called a 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 colander.SchemaNode.serialize method, and is converted back into an application structure (aka appstruct) via colander.SchemaNode.deserialize.

Deserializing A Valid Serialization

cstruct = {
       'name':'keith',
       'age':'20',
       'friends':[('1', 'jim'),('2', 'bob'), ('3', 'joe'), ('4', 'fred')],
       'phones':[{'location':'home', 'number':'555-1212'},
                 {'location':'work', 'number':'555-8989'},],
       }
schema = Person()
deserialized = schema.deserialize(cstruct)

When schema.deserialize(cstruct) is called, because all the data in the schema is valid, and the structure represented by cstruct conforms to the schema, deserialized will be the following:

{
'name':'keith',
'age':20,
'friends':[(1, 'jim'),(2, 'bob'), (3, 'joe'), (4, 'fred')],
'phones':[{'location':'home', 'number':'555-1212'},
          {'location':'work', 'number':'555-8989'},],
}

Note that all the friend rankings have been converted to integers, likewise for the age.

Deserializing An Invalid Serialization

Below, the cstruct structure has some problems. The age is a negative number. The rank for bob is t which is not a valid integer. The 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 validation error?

import colander

cstruct = {
       'name':'keith',
       'age':'-1',
       'friends':[('1', 'jim'),('t', 'bob'), ('3', 'joe'), ('4', 'fred')],
       'phones':[{'location':'bar', 'number':'555-1212'},
                 {'location':'work', 'number':'555-8989'},],
       }
schema = Person()
schema.deserialize(cstruct)

The deserialize method will raise an exception, and the except clause above will be invoked, causing an error messaage to be printed. It will print something like:

Invalid: {'age':'-1 is less than minimum value 0',
         'friends.1.0':'"t" is not a number',
         'phones.0.location:'"bar" is not one of "home", "work"'}

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:

import colander

cstruct = {
       'name':'keith',
       'age':'-1',
       'friends':[('1', 'jim'),('t', 'bob'), ('3', 'joe'), ('4', 'fred')],
       'phones':[{'location':'bar', 'number':'555-1212'},
                 {'location':'work', 'number':'555-8989'},],
       }
schema = Person()
try:
    schema.deserialize(cstruct)
except colander.Invalid, e:
    errors = e.asdict()
    print errors

This will print something like:

{'age':'-1 is less than minimum value 0',
 'friends.1.0':'"t" is not a number',
 'phones.0.location:'"bar" is not one of "home", "work"'}

colander.Invalid Exceptions

The exceptions raised by Colander during deserialization are instances of the colander.Invalid exception class. We saw previously that instances of this exception class have a 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 colander.Invalid exceptions.

While the 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 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 colander.Invalid exception. To support this behavior, each colander.Invalid exception has a children attribute which is a list. Each element in this list (if any) will also be an colander.Invalid exception, recursively, representing the error circumstances for a particular schema deserialization.

Each exception in the graph has a msg attribute, which will either be the value None, a str or 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 msg 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.

Note

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/translationstring/ 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 colander.Invalid API documentation for more information.

Serialization

Serializing a data structure is obviously the inverse operation from deserializing a data structure. The colander.SchemaNode.serialize method of a schema performs serialization of application data (aka an appstruct). If you pass the 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:

import colander

class Person(colander.MappingSchema):
    name = colander.SchemaNode(colander.String())
    age = colander.SchemaNode(colander.Int(),
                              validator=colander.Range(0, 200))

We can serialize a matching data structure:

appstruct = {'age':20, 'name':'Bob'}
schema = Person()
serialized = schema.serialize(appstruct)

The value for serialized above will be {'age':'20', '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, 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 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 using the serialize method of the schema:

appstruct = {'age':20}
schema = Person()
serialized = schema.serialize(appstruct)

The value for serialized above will be {'age':'20', 'name':colander.null}. Note the age integer has become a string, and the missing name attribute has been replaced with 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 colander.null substitution during serialization, see serializing_null.

The corollary: it is the responsibility of the developer to ensure he serializes "the right" data; 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:

import colander

class Friend(colander.TupleSchema):
    rank = colander.SchemaNode(colander.Int(), 
                              validator=colander.Range(0, 9999))
    name = colander.SchemaNode(colander.String())

class Phone(colander.MappingSchema):
    location = colander.SchemaNode(colander.String(), 
                                  validator=colander.OneOf(['home', 'work']))
    number = colander.SchemaNode(colander.String())

class Friends(colander.SequenceSchema):
    friend = Friend()

class Phones(colander.SequenceSchema):
    phone = Phone()

class Person(colander.MappingSchema):
    name = colander.SchemaNode(colander.String())
    age = colander.SchemaNode(colander.Int(),
                             validator=colander.Range(0, 200))
    friends = Friends()
    phones = Phones()

We can imperatively construct a completely equivalent schema like so:

import colander

friend = colander.SchemaNode(Tuple())
friend.add(colander.SchemaNode(colander.Int(),
                              validator=colander.Range(0, 9999),
           name='rank'))
friend.add(colander.SchemaNode(colander.String()), name='name')

phone = colander.SchemaNode(Mapping())
phone.add(colander.SchemaNode(colander.String(),
                             validator=colander.OneOf(['home', 'work']),
                             name='location'))
phone.add(colander.SchemaNode(colander.String(), name='number'))

schema = colander.SchemaNode(Mapping())
schema.add(colander.SchemaNode(colander.String(), name='name'))
schema.add(colander.SchemaNode(colander.Int(), name='age'), 
                              validator=colander.Range(0, 200))
schema.add(colander.SchemaNode(colander.Sequence(), friend, name='friends'))
schema.add(colander.SchemaNode(colander.Sequence(), phone, name='phones'))

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:

data = {
       'name':'keith',
       'age':'20',
       'friends':[('1', 'jim'),('2', 'bob'), ('3', 'joe'), ('4', 'fred')],
       'phones':[{'location':'home', 'number':'555-1212'},
                 {'location':'work', 'number':'555-8989'},],
       }
deserialized = schema.deserialize(data)

Gotchas

You may be using a module scope schema definition with the expectation that calling a 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 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 within the MySchema1 class. This is almost certainly not what you mean to do. The symptom of making such a mistake might be that multiple c nodes are added as children of a over the course of the Python process lifetime.

To get around this, use the 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'))

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.