yamlize

yamlize is a package for serialization of Python objects to and from YAML. yamlize:

• Retains round trip data
  • comments
  • spacing
  • alias/anchor names
  • YAML merge tags,
  • and other markup options
• Checks types
• Sets defaults.
• Allows for arbirtrary data validation.
• Does not require !!python/object: type annotations within the YAML.

Table of Contents

• Package documentation
  • Yamlizable.load
  • Yamlizable.dump
  • Objects
  • Attributes
    • Using key to rename YAML inputs
    • Using type to for type data validation
    • Using default to specify default Python object attribute values
    • Data validation with Attribute validators
  • Maps
  • Keyed Lists
  • Sequences
• Alias and Anchor Treatment
• Merge tags
• Round trip information
• Customization
  • Yamizable.from_yaml
    • Data validation with Yamlizable.from_yaml
    • Subclassing
    • Subclassing 2 -- when you can't subclass
• Why not just serialze with PyYAML?

A couple important notes:

• yamlize and Yamlizable.load do not call __init__. Instead they use __new__ to create an instance and setattr(obj, name, value) to set attributes read from YAML. If you would like to customize some sort of initialization you can create your own __new__ method, or override Yamlizable.from_yaml
• The actual storage name of an attribute is different than the name of the attribute. Specifically, the storage name is '_yamlized_' + name.
• yamlize can be used for data validation, search "data validation" within the page for various topics of interest.
• Within the examples, you may notice <BLANKLINE> simply indicating that there is a newline at the end of a string. This is an artifact of using doctest.

Package documentation

Yamlizable.load :

A class method that exists on all Yamlizable subclasses to de-serialize YAML into an instance of that subclass.

Yamlizable.dump :

A class method that exists on all Yamlizable subclasses to serialize an instance of that subclass to YAML.

Attributes : a YAML scalar, kind of

yamlize doesn't really have support for scalars, but it can do type checking on scalar types and data validation. An Attribute is used to define an instance attribute of something created by yamlize.

Objects : a YAML map converted to a Python object

yamlize class for serialization to and from a YAML map. Each key of the YAML map is an attribute of the class instance.

Maps : a YAML map

Basically the same as an OrderedDict, except that it can also have Attributes.

Keyed Lists : a YAML map with special keys and values

Similar to Maps, the KeyedList takes the value of a Python object's attribute as a key. This is best explained with examples...

Sequences : a YAML sequence of objects

This corresponds to a sequence of objects, and can be used to simply validate types (by using something like yamlize.StrList), or to convert a list of other Python yamlizable objects.

Yamlizable.load

Before seeing much about yamlize, this may be out of context, but it is important to know. All subclasses implement a load class method. The class method is then used to create class instances from YAML (again, yamlize does not call __init__, only __new__).

arguments :

stream : str or file

The load method can accept either a YAML string, or a file-like object.

Loader : ruamel.yaml.Loader, optional

A YAML loader; it has only been tested with the ruamel.yaml.RoundTripLoader.

return type : instance of subclass

This returns an instance of the subclass used. So, for example, Thing.load('...') returns an instance of a Thing.

Yamlizable.dump

All subclasses implement a dump class method. The class method is used to write YAML from Python object instances.

arguments :

data : instance of subclass

This is the object to be written, it should be of the same class as the type being used; e.g. Thing.dump(data=thing_instance)

stream : file-like object, optional

If provided, dump writes to the stream, otherwise it returns a string.

return type : None if stream was provided, otherwise string

If stream was provided, the output is written to the stream, otherwise returns an instance of a string.

Objects

>>> from yamlize import Object, Attribute
>>>
>>> class Pet(Object):
...
...     name = Attribute()  # declare a yamlize.Attribute
...
...     age = Attribute()
>>>
>>> lucy = Pet.load(u'''
... name: Lucy  # yay it is some YAML!
... age: 8
... ''')
>>>
>>> lucy.name, lucy.age
('Lucy', 8)

Using ruamel.yaml, the formatting can be retained allowing for hand-generated YAML files to retain important information and legibility.

>>> print(Pet.dump(lucy))
name: Lucy  # yay it is some YAML!
age: 8
<BLANKLINE>

Attributes

Taking a step back from the introduction to yamlize Objects, we should really look at Attributes. An Attribute is a way to map between YAML keys/values to a Python object's attributes.

The Attribute constructor has the following arguments:

name : str, optional

Name of the Python object's attribute. By default this will be the name provided in the declaration (i.e. in the Pet example above, we could have written age = Attribute(name='age'), but that is a bit redundant.

key : str, optional (See renaming keys)

Key in a YAML file. For example, if you had an attribute with an underscore (_) in it, and would instead like to use spaces in the YAML file. Or if your Python object's attributes are camelCase, or PascalCase, but you'd like the YAML to be sane.

type : type, optional (See attribute types)

This can be used to force an object to be cast to a specific type, or to ensure that the YAML input is valid.

default : optional (See attribute defaults)

Provides a default value if the attribute is not defined within the YAML.

validator: callable, optional (See attribute validators)

Callable used to confirm a value is valid the signature is validator(value) -> False to indicate an invalid value, or a custom exception can be raised. Note: False is False and nothing else is, so don't return 0, [], {}, etc. when you meant False.

Using key to rename YAML inputs

The Attributes key argument can be used to "map" from a YAML input name to the Python object's attribute name.

>>> from yamlize import Object, Attribute
>>>
>>> class ThingWithAttribute(Object):
...
...     python_name = Attribute(key='YAML key')
>>>
>>> twa = ThingWithAttribute.load('YAML key: this is the value from YAML')
>>> twa.python_name
'this is the value from YAML'

Note

yamlize doesn't prevent you from doing silly things like using names that shouldn't be valid python attributes, or keys that shouldn't be valid YAML.

getattr(obj, 'why did I do this?')

Using type to for type data validation

The Attributes type argument can be used to perform type data validation on the input YAML. (Sorry for using "type data validation" instead of "data type validation", but this way one can search "data validation" within the documentation and find all relevant topics.)

>>> from yamlize import Object, Attribute
>>>
>>> class StronglyTypedThing(Object):
...
...     my_int = Attribute(type=int)
...     my_float = Attribute(type=float)
...     my_str = Attribute(type=str)
...
>>>
>>> stt = StronglyTypedThing.load(u'''
... my_int: 42
... my_float: 9.9
... my_str: this is a string.   still
... ''')

The above worked just fine because all the types correspond. Giving incorrect types will result in YamlizingErrors indicating the line of input that is erroneous.

>>> StronglyTypedThing.load(u'''
... my_int: 12.1
... my_float: 9.9
... my_str: this is a string.   still
... ''') # doctest: +IGNORE_EXCEPTION_DETAIL
Traceback (most recent call last):
    ...
YamlizingError: Coerced `<class 'ruamel.yaml.scalarfloat.ScalarFloat'>` to `<type 'int'>`, but the new value `12` is not equal to old `12.1`.
start:   in "<unicode string>", line 2, column 9:
    my_int: 12.1
            ^ (line: 2)
end:   in "<unicode string>", line 2, column 13:
    my_int: 12.1
                ^ (line: 2)

Note that we tried to coerce one type to another, so it is possible to trick the logic.

>>> stt2 = StronglyTypedThing.load(u'''
... my_int: 81.0      # this will be cast to an integer
... my_float: 92.1
... my_str: another boring message
... ''')
>>> stt2.my_int
81

Not all types can be tricked, and pull requests are welcome to fix unintended side effects.

>>> StronglyTypedThing.load(u'''
... my_int: 1001
... my_float: 1e99
... my_str: 1.234
... ''') # doctest: +IGNORE_EXCEPTION_DETAIL
Traceback (most recent call last):
    ...
YamlizingError: Coerced `<class 'ruamel.yaml.scalarfloat.ScalarFloat'>` to `<type 'str'>`, but the new value `1.234` is not equal to old `1.234`.
start:   in "<unicode string>", line 4, column 9:
    my_str: 1.234
            ^ (line: 4)
end:   in "<unicode string>", line 4, column 14:
    my_str: 1.234
                 ^ (line: 4)

The type data validation also works for attribute assignment.

>>> from yamlize import Object, Attribute
>>>
>>> class StronglyTypedThing(Object):
...
...     my_int = Attribute(type=int)
...     my_float = Attribute(type=float)
...     my_str = Attribute(type=str)
...
>>>
>>> stt = StronglyTypedThing()
>>> stt.my_int = 12
>>> stt.my_float = 1.01
>>> stt.my_str = 'abc'
>>> # now... lets try a badly typed operand
>>> stt.my_int = 12.34  # doctest: +IGNORE_EXCEPTION_DETAIL
Traceback (most recent call last):
    ...
yamlize.yamlizing_error.YamlizingError: Coerced `<class 'float'>` to `<class 'int'>`, but the new value `12` is not equal to old `12.34`.

Using default to specify default Python object attribute values

The Attributes default argument can be used to simplify YAML input when an attribute can have a default value.

>>> from yamlize import Object, Attribute
>>>
>>> class Point(Object):
...     x = Attribute()
...     y = Attribute()
...     z = Attribute(default=0.0)
>>>
>>> p0 = Point.load(u'''
... x: 1.0
... y: 2.2
... ''')
>>> p0.x, p0.y, p0.z
(1.0, 2.2, 0.0)

The default obviously, only applies to the specific attribute, so the following results in an error.

>>> Point.load(u'''
... x: 1000.0001    # missing non-default z value
... z: 2000.0002
... ''') # doctest: +IGNORE_EXCEPTION_DETAIL
Traceback (most recent call last):
    ...
YamlizingError: Missing <class '__main__.Point'> attributes without default: ['y']
start:   in "<unicode string>", line 2, column 1:
    x: 1000.0001    # missing non-de ...
    ^ (line: 2)
end:   in "<unicode string>", line 4, column 1:
<BLANKLINE>
    ^ (line: 4)

Warning

The default argument should work more similar to collections.defaultdict accepting a callable object. This will likely be changed in future versions. The issue with this is that we need to known when a value should and should not be written out.

Data validation with Attribute validators

Attribute data validation is available through validators. Your validator method will be called whenever assigning a value to the attribute. You should get very accurate line numbers for the failing YAML node.

>>> from yamlize import Object, AttributeCollection
>>>
>>> class PositivePoint(Object):
...
...     x = Attribute(type=float)
...
...     # raise a custom exception
...     @x.validator
...     def x(self, x):
...         if x < 0.0:
...             raise ValueError('Cannot set PositivePoint.x to {}'.format(x))
...
...     # or, return False when the value is not valid
...     y = Attribute(type=float, validator=lambda self, y: y >= 0)
>>>
>>> PositivePoint.load(u'{ x: -0.0000001, y: 1.0}')  # doctest: +IGNORE_EXCEPTION_DETAIL
Traceback (most recent call last):
    ...
YamlizingError: Failed to assign attribute `x` to `-1e-07`, got: Cannot set PositivePoint.x to -1e-07
start:   in "<unicode string>", line 1, column 6:
    { x: -0.0000001, y: 1.0}
         ^ (line: 1)
end:   in "<unicode string>", line 1, column 16:
    { x: -0.0000001, y: 1.0}
                   ^ (line: 1)

As noted, the validator is called every time the Attribute is assigned, so the attribute can never be invalid.

>>> pp = PositivePoint()
>>> pp.x = 101.1
>>> pp.y = -101.1  # doctest: +IGNORE_EXCEPTION_DETAIL
Traceback (most recent call last):
    ...
ValueError: Cannot set `PositivePoint.y` to invalid value `-101.1`

When I say it can never be invalid, the value will not be assigned...

>>> pp.y  # doctest: +IGNORE_EXCEPTION_DETAIL
Traceback (most recent call last):
    ...
YamlizingError: Attribute `y` was not defined on `<__main__.PositivePoint object at 0x10da75a08>`

As noted this is rather cumbersome, so you may wish to use Yamlizable.from_yaml for data validation instead.

Maps

yamlize.Map is a subclass of the yamlize.Object that can be used to define a Python class that has both attributes and keys/values. Attribute names are exclusive, and cannot also be provided as a key name.

>>> from yamlize import Map, Typed, Attribute
>>>
>>> class Student(Map):
...     key_type = Typed(str)
...     value_type = Typed(float)
...     first = Attribute()
...     last = Attribute()
>>>
>>> f = Student.load(u'''
... first: Failing
... last: Student
... homework 1: 15.0  # turned in late
... homework 2: 45.0  # turned in late, again
... homework 3: 60.0  # turned in late, again again
... homework 4: 95.0
... exam 1: 65.0
... ''')
>>> f.first
'Failing'
>>> f['homework 1']
15.0

Note

Now for the neat stuff.

You can use yamlize types as arguments to other classes.

>>> from yamlize import Map, Typed
...
>>> class GradeBook(Map):
...     key_type = Typed(str)
...     value_type = Student  # no need to use Typed, as Student is already Yamlizable
>>>
>>> gb = GradeBook()
>>> gb['Failing Student'] = f
>>> print(GradeBook.dump(gb))
Failing Student:
  first: Failing
  last: Student
  homework 1: 15.0 # turned in late
  homework 2: 45.0 # turned in late, again
  homework 3: 60.0 # turned in late, again again
  homework 4: 95.0
  exam 1: 65.0
<BLANKLINE>

Keyed Lists

yamlize.KeyedList is a subclass of the yamlize.Object that can be used to define a Python class that has both attributes and keys/values. Attribute names are exclusive, and cannot also be provided as a key name.

The difference between a yamlize.Map and a yamlize.KeyedList is that the KeyedList key points to an attribute on the value. This operates under the assumption that the value type is another Yamlizable type. The purpose of pointing to an attribute on the value is to reduce duplication of data. In the previous example of the GradeBook we specified "Failing Student" twice.

>>> from yamlize import KeyedList, Attribute
>>>
>>> class GradeBook(KeyedList):
...     key_attr = Student.first  # attribute of the value that is the key
...     item_type = Student
>>>
>>> grade_book = GradeBook()
>>> grade_book.add(f)  # f is failing student from above
>>> print(GradeBook.dump(grade_book))
Failing:
  last: Student
  homework 1: 15.0 # turned in late
  homework 2: 45.0 # turned in late, again
  homework 3: 60.0 # turned in late, again again
  homework 4: 95.0
  exam 1: 65.0
<BLANKLINE>

Sequences

A yamlize.Sequence should be used effectively as a Python strong-typed list. Unlike the other yamlize decorators / classes, a Sequence cannot have attributes. The lack of attributes is a functionality of YAML itself; a YAML sequence cannot have attributes.

>>> from yamlize import Object, Sequence
>>>
>>> class Person(Object):
...     first = Attribute(type=str)
...     last = Attribute(type=str)
>>>
>>> class People(Sequence):
...     item_type = Person
>>>
>>> peeps = People.load(u'''
... - {first: g, last: m}
... - {first: First, last: Last}
... - first: First2
...   last: Last2
... ''')
>>> peeps[0].first, peeps[2].last
('g', 'Last2')

Alias and Anchor Treatment

A yamlize correctly handles YAML anchors (&), aliases (*), and merge tags (<<).

>>> from yamlize import Object, Sequence
>>>
>>> class Person(Object):
...     first = Attribute(type=str)
...     last = Attribute(type=str)
>>>
>>> class People(Sequence):
...     item_type = Person
>>>
>>> peeps = People.load(u'''
... - &g {first: g, last: m}
... - {first: First, last: Last}
... - {first: First, last: Last}
... - *g
... ''')

Since an anchor and alias were used to define g twice, there is one object reference for g.

>>> g0 = peeps[0]
>>> g3 = peeps[3]
>>> g0 == g3, id(g0) == id(g3)
(True, True)

Conversely, despite having the same definition for {first: First, last: Last} twice, they are different objects.

>>> peeps[1] == peeps[2], id(peeps[1]) == id(peeps[2])
(False, False)

When dumping back to YAML, anchor and alias names are retained:

>>> print(People.dump(peeps))
- &g {first: g, last: m}
- {first: First, last: Last}
- {first: First, last: Last}
- *g
<BLANKLINE>

Merge tags

One neat aspect of YAML is the ability to use merge tags <<: to reduce user input. yamlize will retain these.

>>> from yamlize import Object, KeyedList
>>>
>>> class Thing(Object):
...     name = Attribute(type=str)
...     int_attr = Attribute(type=int)
...     str_attr = Attribute(type=str)
...     float_attr = Attribute(type=float)
>>>
>>> class Things(KeyedList):
...     key_attr = Thing.name
...     item_type = Thing
>>>
>>> things = Things.load(u'''
... thing1: &thing1
...   int_attr: 1
...   str_attr: '1'
...   float_attr: 99.2
... thing2: &thing2
...   <<: *thing1
...   str_attr: an actual string
... thing3:
...   <<: *thing1
...   <<: *thing2
...   float_attr: 42.42
... ''')

The last merged value is the one that is applied, so:

>>> thing1, thing2, thing3 = list(things.values())
>>> thing1.int_attr == thing2.int_attr
True
>>> thing2.str_attr == thing3.str_attr
True

And of course, merge tags are retained when dumping back to YAML.

>>> print(Things.dump(things))
thing1: &thing1
  int_attr: 1
  str_attr: '1'
  float_attr: 99.2
thing2: &thing2
  <<: *thing1
  str_attr: an actual string
thing3:
  <<: *thing1
  <<: *thing2
  float_attr: 42.42
<BLANKLINE>

Round trip information

Note this will retain block or flow style and comments when dumping back to yaml.

>>> formatted_people = People.load(u'''
... - {first: f, last: l} # comment 1
... - first: First  # value-add comment 2
...   last: Last    #
... ''')
>>> print(People.dump(formatted_people))
- {first: f, last: l} # comment 1
- first: First  # value-add comment 2
  last: Last    #
<BLANKLINE>

Customization

We have already discussed the Yamlizable.load and Yamlizable.dump class methods. These two methods only get called to open / create the "root" of the document tree and begin the parsing. The actual bulk of the work is done using Yamlizable.from_yaml and Yamlizable.to_yaml_.

Yamizable.from_yaml

The from_yaml method is also a class method that is used to create a new instance for a Yamlizable object.

arguments :

loader : Loader (See ruamel.yaml)

A loader class, this should generally be used to parse the node, and register the created object.

node : Node (See ruamel.yaml)

A YAML node.

round_trip_data : yamlize.round_trip_data.RoundTripData

An object for retaining round trip data. This is passed from the parent object (which may or may not be Yamlizable. Yamlizable objects have their own RoundTripData instances, but non-Yamlizable objects do not (i.e. int, float, str). In order to retain non-Yamlizable round trip data, a RoundTripData instance can store additional data from other nodes.

return type : Yamlizable subclass instance

The return type should be an instance of the subclass.

This method can be used effectively in place of a custom resolver.

Data validation with Yamlizable.from_yaml

Alternative to using attribute validators, you can override the Yamlizable.from_yaml classmethod to supply custom data validation.

>>> from yamlize import Object, Attribute, YamlizingError
>>>
>>> class PositivePoint2(Object):
...     x = Attribute(type=float)
...     y = Attribute(type=float)
...
...     @classmethod
...     def from_yaml(cls, loader, node, round_trip_data=None):
...         # from_yaml.__func__ is the unbound class method
...         self = Object.from_yaml.__func__(PositivePoint2, loader, node, round_trip_data)
...
...         if self.x < 0.0 or self.y < 0.0:
...             raise YamlizingError('Point x and y values must be positive', node)
...
...         return self
>>>
>>> PositivePoint2.load(u'{ x: -0.0000001, y: 1.0}')  # doctest: +IGNORE_EXCEPTION_DETAIL
Traceback (most recent call last):
    ...
YamlizingError: Point x and y values must be positive
start:   in "<unicode string>", line 1, column 1:
    { x: -0.0000001, y: 1.0}
    ^ (line: 1)
end:   in "<unicode string>", line 1, column 25:
    { x: -0.0000001, y: 1.0}
                            ^ (line: 1)

Subclassing

You can also use Yamlizable.from_yaml for handling subclassing.

>>> from yamlize import Object, Sequence
>>>
>>> class Shape(Object):
...
...     shape = Attribute(type=str)
...
...     @classmethod
...     def from_yaml(cls, loader, node, round_trip_data):
...         # the node is a map, let's find the "shape" key
...         for key_node, val_node in node.value:
...             key = loader.construct_object(key_node)
...             if key == 'shape':
...                 subclass_name = loader.construct_object(val_node)
...                 break
...         else:
...             raise YamlizingError('Missing "shape" key', node)
...
...         subclass = {
...             'Circle' : Circle,
...             'Square' : Square,
...             'Rectangle' : Rectangle
...             }[subclass_name]
...
...         # from_yaml.__func__ is the unbound class method
...         return Object.from_yaml.__func__(subclass, loader, node, round_trip_data)
>>>
>>> class Circle(Shape):
...
...     radius = Attribute(type=float)
>>>
>>> class Square(Shape):
...
...     side = Attribute(type=float)
>>>
>>> class Rectangle(Shape):
...
...     length = Attribute(type=float)
...     width = Attribute(type=float)
>>>
>>> class Shapes(Sequence):
...
...     item_type = Shape
>>>
>>> shapes = Shapes.load(u'''
... - {shape: Circle, radius: 1.0}
... - {shape: Square, side: 2.0}
... - {shape: Rectangle, length: 3.0, width: 4.0}
... ''')
>>>
>>> print(Shapes.dump(shapes))
- {shape: Circle, radius: 1.0}
- {shape: Square, side: 2.0}
- {shape: Rectangle, length: 3.0, width: 4.0}
<BLANKLINE>

Subclassing 2 -- when you can't subclass

Under most conditions yamlize needs attributes and items to be subclasses of yamlize.Yamlizable. This can cause problems when you have some data that otherwise just doesn't need to be subclassed. yamlize allows you to specify specific conversion methods to and from YAML in these instances.

>>> from yamlize import Object
>>> import aenum
>>>
>>> class Sex(aenum.Enum):
...     FEMALE = aenum.auto()
...     MALE = aenum.auto()
>>>
>>> TypedSex = Typed(Sex,
...         from_yaml=lambda loader, node, rtd: Sex[loader.construct_object(node)],
...         to_yaml=lambda dumper, data, rtd: dumper.represent_data(str(data).replace('Sex.', '')))
>>>

With the above, you can now use TypedSex to ensure proper typing of the enumeration. For example:

>>> class Person(Object):
...     name = Attribute(type=str)      # as an FYI, under the hood 'str' becomes Typed(str)
...     sex = Attribute(type=TypedSex)
>>>
>>> me = Person.load('{name: me, sex: MALE}')
>>> me.name
'me'
>>> str(me.sex)
'Sex.MALE'

Why not just serialze with PyYAML?

PyYAML serialization requires (without custom implicit tag resolvers) that your YAML indicate the Python object being represented. It may also not be possible to have a specific map represent specific types, and I don't think the root of a document can represent a single object. It may not be possible for multiple implicit resolvers to distinguish between a variety of Python objects. Also, using yamlize the YAML definition and class definition are one and the same; whereas with custom resolvers for different object types you would need to also clarify the YAML tree of where a certain type of object may exist. For example:

>>> class A(object):
...     def __init__(self, attr):
...         self.attr = attr
>>> a = A('attribute value')
>>> import yaml
>>> print(yaml.dump(a))
!!python/object:__main__.A
attr: attribute value
<BLANKLINE>

Unlike JSON and XML, one of the beauties of YAML is that it is mostly human readable and writable. Using PyYAML out of the box requires that you either muddle the YAML with Python types, or define custom resolvers/representers in addition to the types you already need. The other deficiency of PyYAML out of the box is that it does not support round trip data (spacing, block v. flow style, comments) retention, but ruamel.yaml does! ruamel.yaml similarly requires that specific resolvers/representers be created.

yamlize makes the assumption that whatever you are loading / dumping is representative of the type of the object expected. With this assumption, yamlize can create complex Python objects without requiring specialized YAML customizations.