deb-python-colander/docs/null.rst

The Null Value

colander.null is a sentinel value which may be passed to colander.SchemaNode.serialize during serialization or to colander.SchemaNode.deserialize during deserialization.

During serialization, the use of colander.null indicates that the appstruct value corresponding to the node it's passed to is missing and the value of the default attribute of the corresponding node should be used instead.

During deserialization, the use of colander.null indicates that the cstruct value corresponding to the node it's passed to is missing, and if possible, the value of the missing attribute of the corresponding node should be used instead.

Note that colander.null has no relationship to the built-in Python None value. colander.null is used instead of None because None is a potentially valid value for some serializations and deserializations, and using it as a sentinel would prevent None from being used in this way.

Serializing The Null Value

A node will attempt to serialize its default value during colander.SchemaNode.serialize if the value it is passed as an appstruct argument is the colander.null sentinel value.

The default value of a node is specified during schema creation as its default attribute / argument. For example, the hair_color node below has a default value of brown:

import colander

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

Because the hair_color node is passed a default value, if the above schema is used to serialize a mapping that does not have a hair_color key, the default will be serialized:

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

Even though we did not include the hair_color attribute in the appstruct we fed to serialize, the value of serialized above will be {'name':'Fred, 'age':'20', 'hair_color':'brown'}. This is because a default value of brown was provided during schema node construction for hair_color.

The same outcome would have been true had we fed the schema a mapping for serialization which had the colander.null sentinel as the hair_color value:

import colander

schema = Person()
serialized = schema.serialize({'name':'Fred', 'age':20,
                               'hair_color':colander.null})

When the above is run, the value of serialized will be {'name':'Fred, 'age':'20', 'hair_color':'brown'} just as it was in the example where hair_color was not present in the mapping.

As we can see, serializations may be done of partial data structures; the colander.null value is inserted into the serialization whenever a corresponding value in the data structure being serialized is missing.

Note

The injection of the colander.null value into a serialization when a default doesn't exist for the corresponding node is not a behavior shared during both serialization and deserialization. While a serialization can be performed against a partial data structure without corresponding node defaults, a deserialization cannot be done to partial data without corresponding node missing values. When a value is missing from a data structure being deserialized, and no missing value exists for the node corresponding to the missing item in the data structure, a colander.Invalid exception will be the result.

If, during serialization, a value for the node is missing from the cstruct and the node does not possess an explicit default value, the colander.null sentinel value is passed to the type's serialize method directly, instructing the type to serialize a type-specific null value.

Serialization of a null value is completely type-specific, meaning each type is free to serialize colander.null to a value that makes sense for that particular type. For example, the null serialization value of a colander.String type is the empty string.

For example:

import colander

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


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

In the above example, the hair_color value is missing and the schema does not name a default value for hair_color. However, when we attempt to serialize the data structure, an error is not raised. Instead, the value for serialized above will be {'name':'Fred, 'age':'20', 'hair_color':colander.null}.

Because we did not include the hair_color attribute in the data we fed to serialize, and there was no default value associated with hair_color to fall back to, the colander.null value is passed as the appstruct value to the serialize method of the underlying type (colander.String). The return value of that type's serialize method when colander.null is passed as the appstruct is placed into the serialization. colander.String happens to return colander.null when it is passed colander.null as its appstruct argument, so this is what winds up in the resulting cstruct.

The colander.null value will be passed to a type either directly or indirectly:

• directly: because colander.null is passed directly to the serialize method of a node.
• indirectly: because every schema node uses a colander.null value as its default attribute when no explicit default is provided.

When a particular type cannot serialize the null value to anything sensible, that type's serialize method must return the null object itself as a serialization. For example, when the colander.Boolean type is asked to serialize the colander.null value, its serialize method simply returns the colander.null value (because null is conceptually neither true nor false).

Therefore, when colander.null is used as input to serialization, or as the default value of a schema node, it is possible that the colander.null value will placed into the serialized data structure. The consumer of the serialization must anticipate this and deal with the special colander.null value in the output however it sees fit.

Serialization Combinations

Within this table, the Value column represents the value passed to the colander.SchemaNode.serialize method of a particular schema node, the Default column represents the default value of that schema node, and the Result column is a description of the result of invoking the colander.SchemaNode.serialize method of the schema node with the effective value.

Value	Default	Result
colander.null	value	value serialized
<missing>	value	value serialized
colander.null	colander.null	null serialized
<missing>	colander.null	null serialized
value	<missing>	value serialized
value_a	value_b	value_a serialized
value	colander.null	value serialized
colander.null	<missing>	null serialized
colander.null	value	null serialized

Note

<missing> in the above table represents the circumstance in which a key present in a colander.MappingSchema is not present in a mapping passed to its colander.SchemaNode.serialize method. In reality, <missing> means exactly the same thing as colanderr.null, because the colander.Mapping type does the equivalent of mapping.get(keyname, colander.null) to find a subvalue during serialization.

Deserializing The Null Value

The data structure passed to colander.SchemaNode.deserialize may contain one or more colander.null sentinel markers.

When a colander.null sentinel marker is passed to the colander.SchemaNode.deserialize method of a particular node in a schema, the node will take the following steps:

• The type object's deserialize method will be called with the null value to allow the type to convert the null value to a type-specific default. The resulting "appstruct" is used instead of the value passed directly to colander.SchemaNode.deserialize in subsequent operations. Most types, when they receive the null value will simply return it, however.
• If the appstruct value computed by the type's deserialize method is colander.null and the schema node has an explicit missing attribute (the node's constructor was supplied with an explicit missing argument), the missing value will be returned. Note that when this happens, the missing value is not validated by any schema node validator: it is simply returned.
• If the appstruct value computed by the type's deserialize method is colander.null and the schema node does not have an explicitly provided missing attribute (the node's constructor was not supplied with an explicit missing value), a colander.Invalid exception will be raised with a message indicating that the field is required.

Note

There are differences between serialization and deserialization involving the colander.null value. During serialization, if an colander.null value is encountered, and no valid default attribute exists on the node related to the value the null value for that node is returned. Deserialization, however, doesn't use the default attribute of the node to find a default deserialization value in the same circumstance; instead it uses the missing attribute instead. Also, if, during deserialization, an colander.null value is encountered as the value passed to the deserialize method, and no explicit missing value exists for the node, a colander.Invalid exception is raised (colander.null is not returned, as it is during serialization).

Here's an example of a deserialization which uses a missing value in the schema as a deserialization default value:

import colander

class Person(colander.MappingSchema):
    name = colander.SchemaNode(colander.String())
    age = colander.SchemaNode(colander.Int(), missing=None)

schema = Person()
deserialized = schema.deserialize({'name':'Fred', 'age':colander.null})

The value for deserialized above will be {'name':'Fred, 'age':None}.

Because the age schema node is provided a missing value of None, if that schema is used to deserialize a mapping that has an an age key of colander.null, the missing value of None is serialized into the appstruct output for age.

Note

Note that None can be used for the missing schema node value as required, as in the above example. It's no different than any other value used as missing. The empty string can also be used as the missing value if that is helpful.

The colander.null value is also the default, so it needn't be specified in the cstruct. Therefore, the deserialized value of the below is equivalent to the above's:

import colander

class Person(colander.MappingSchema):
    name = colander.SchemaNode(colander.String())
    age = colander.SchemaNode(colander.Int(), missing=None)

schema = Person()
deserialized = schema.deserialize({'name':'Fred'})

Deserialization Combinations

Within this table, the Value column represents the value passed to the colander.SchemaNode.deserialize method of a particular schema node, the Missing column represents the missing value of that schema node, and the Result column is a description of the result of invoking the colander.SchemaNode.deserialize method of the schema node with the effective value.

Value	Missing	Result
colander.null	<missing>	Invalid exception raised
<missing>	<missing>	Invalid exception raised
colander.null	value	value used
<missing>	value	value used
<missing>	colander.null	colander.null used
value	<missing>	value used
value	colander.null	value used
value_a	value_b	value_a used

Note

<missing> in the above table represents the circumstance in which a key present in a colander.MappingSchema is not present in a mapping passed to its colander.SchemaNode.deserialize method. In reality, <missing> means exactly the same thing as colander.null, because the colander.Mapping type does the equivalent of mapping.get(keyname, colander.null) to find a subvalue during deserialization.