pyschema

This project provides a way to validate recursive data structures created using dictionaries, tuples, lists etc. Proper way of adding validations should be to add python classes in the mix, but lot of times we end up with simple data structures as they are easy to start. This project provides a way to create schema definition for such objects.

Basic Usage

>>> from pyschema import Schema
>>> s = Schema({
...     'type': 'number',
...     'minimum_value':0,
...     'maximum_value':120,
...     'display_name':'Age'
... })
>>> s.validate(10)
[]
>>> s.validate(200)
['Value 200 is greater than 120 at root(Age)']

Basic Schema

Schema definition is done recursively using attributes given here and allow any depth to of the metadata to be specified.

Each level is described by the following properties in a python dictionary:

Name	Description	Required	Lambda Support	Default
display_name	Name to be used in the display	True	False	n/a
description	Helpful information about this element	False	False	Blank string
type	Data type of the value. Should be one of map, list, string, number, boolean or any(4)	False	True	n/a
value_schema	Schema to be used for any of the child values by default.	Depends(1)	True	n/a
verbatim	Any information to be saved along with schema. (3)	False	False	None
allow_none	Allow None value. No validations are made if the value is None	False	False	False
custom_validation	Python function that peforms custom validation (in addition to other). In the realized schema, it will just say True.	False	False	None
Map Specific
known_children	A map of named children that are allowed for a map. Each named children can have their own schema the value. If it is blank dictionary, or None, value_schema is used.	False	True	Blank Map
allow_unknown_children	Should the UI allow adding children whose names are not known. If this is set to true, value schema attribute above must be defined.	False	False	False
mandatory_children	Names for which values must be provided. Useful for a map only.	False	True	No Restrictions
allow_list	Allows list of maps in addition to map. If List is given each item is validated.	False	False	False
Number Specific
minimum_value	Used for int type leaf nodes only.	False	True	No Limit
maximum_value	Used for int type leaf nodes only.	False	True	No Limit
String Specific
allowed_pattern	Used for string type values only. Regular expression to validate given input strings.	False	False	No restrictions
allowed_values	Used for string type values only. List of values allowed to be set.	False	True	No restrictions
Boolean Specific
true_value	Used for boolean type leaf nodes only. String to be displayed if the underlying value is True. For example: Yes, Enabled, True etc.	False	False	True
false_value	Used for boolean type leaf nodes only. String to be displayed if the underlying value is False. For example: No, Disabled, False etc.	False	False	False
List Specific
minimum_size	Minimum number of entries in list	False	False	0
maximum_size	Maximum number of entries in list	False	False	Unlimited
unique	Require all list entries be unique (5)	False	False	No restriction

1. For each complex type of object, schema is required either directly or through inheritance. Schema defined here is used for all children where an explicit schema is not defined.
2. Data is only validated and never modified. Validation errors are listed as a simple list of strings.
3. Verbatim data is for other consumers for example UI to drive the widgets or to give hints. It has no meaning for the backend.
4. Any type has no validations done except for any custom validations. Use with care. You are creating the schema to avoid any in the first place :-)
5. If a list contains complex values unique check can become complicated. Current implementation simple adds values to a set for comparison.

Lambda Support

Lambda support allows the schema to be generated by python code instead of defining it as a static python data. Advantage of such approach is to give out list of names or children that must be valid. While we can make everything dynamic, we choose to support a sensible subset.

A process called schema realization must be executed to run the python code and realize the true schema. First call to validate or an explicit call performs the realization of schema. However note that, validate realizes only the part of the schema that is actually used, where as realize call truly realizes the entire schema dynamically.

See the samples/basics.py code for an example of lambda support. Running this code produces the following output:

Realized schema
---------------
{'display_name': 'Root Configuration',
 'known_children': {'listed_names': {'display_name': 'Name List',
                                     'type': 'list',
                                     'value_schema': {'allowed_values': ['a',
                                                                         'b',
                                                                         'c'],
                                                      'display_name': 'Names',
                                                      'type': 'string'}},
                    'notify_via_mail': {'display_name': 'Mail Preference',
                                        'false_value': 'False',
                                        'true_value': 'True',
                                        'type': 'boolean'},
                    'number_of_names': {'display_name': 'Name Count Limit',
                                        'minimum_value': 2,
                                        'type': 'number'}},
 'mandatory_children': ['listed_names'],
 'type': 'map'}



Sample Validation Errors: 1
---------------
["Expecting value to be (<type 'list'>, <type 'set'>, <type 'tuple'>) but got <type 'int'> for root.listed_names(Name List)"]



Sample Validation Errors: 2
---------------
["Expecting value to be (<type 'str'>, <type 'unicode'>) but got <type 'int'> for root.listed_names.n(Names)",
 'invalid-name is not a allowed value for root.listed_names.n(Names). Expect it to be one of: a,b,c']



Sample Validation Errors: 3 (No errors)
---------------
[]