What you can structure and how

The philosophy of cattrs structuring is simple: give it an instance of Python built-in types and collections, and a type describing the data you want out. cattrs will convert the input data into the type you want, or throw an exception.

All loading conversions are composable, where applicable. This is demonstrated further in the examples.

Primitive values

typing.Any

Use typing.Any to avoid applying any conversions to the object you’re loading; it will simply be passed through.

>>> cattr.structure(1, Any)
1
>>> d = {1: 1}
>>> cattr.structure(d, Any) is d
True

int, float, str, bytes

Use any of these primitive types to convert the object to the type.

>>> cattr.structure(1, str)
'1'
>>> cattr.structure("1", float)
1.0

In case the conversion isn’t possible, the expected exceptions will be propagated out. The particular exceptions are the same as if you’d tried to do the conversion yourself, directly.

>>> cattr.structure("not-an-int", int)
Traceback (most recent call last):
...
ValueError: invalid literal for int() with base 10: 'not-an-int'

Enums

Enums will be structured by their values. This works even for complex values, like tuples.

>>> @unique
... class CatBreed(Enum):
...    SIAMESE = "siamese"
...    MAINE_COON = "maine_coon"
...    SACRED_BIRMAN = "birman"
...
>>> cattr.structure("siamese", CatBreed)
<CatBreed.SIAMESE: 'siamese'>

Again, in case of errors, the expected exceptions will fly out.

>>> cattr.structure("alsatian", CatBreed)
Traceback (most recent call last):
...
ValueError: 'alsatian' is not a valid CatBreed

Collections and other generics

Optionals

Optional primitives and collections are supported out of the box.

>>> cattr.structure(None, int)
Traceback (most recent call last):
...
TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType'
>>> cattr.structure(None, Optional[int])
>>> # None was returned.

Bare Optional s (non-parameterized, just Optional, as opposed to Optional[str]) aren’t supported, use Optional[Any] instead.

This generic type is composable with all other converters.

>>> cattr.structure(1, Optional[float])
1.0

Lists

Lists can be produced from any iterable object. Types converting to lists are:

  • Sequence[T]

  • MutableSequence[T]

  • List[T]

In all cases, a new list will be returned, so this operation can be used to copy an iterable into a list. A bare type, for example Sequence instead of Sequence[int], is equivalent to Sequence[Any].

>>> cattr.structure((1, 2, 3), MutableSequence[int])
[1, 2, 3]

These generic types are composable with all other converters.

>>> cattr.structure((1, None, 3), List[Optional[str]])
['1', None, '3']

Sets and frozensets

Sets and frozensets can be produced from any iterable object. Types converting to sets are:

  • Set[T]

  • MutableSet[T]

Types converting to frozensets are:

  • FrozenSet[T]

In all cases, a new set or frozenset will be returned, so this operation can be used to copy an iterable into a set. A bare type, for example MutableSet instead of MutableSet[int], is equivalent to MutableSet[Any].

>>> cattr.structure([1, 2, 3, 4], Set)
{1, 2, 3, 4}

These generic types are composable with all other converters.

>>> cattr.structure([[1, 2], [3, 4]], Set[FrozenSet[str]])
{frozenset({'1', '2'}), frozenset({'4', '3'})}

Dictionaries

Dicts can be produced from other mapping objects. To be more precise, the object being converted must expose an items() method producing an iterable key-value tuples, and be able to be passed to the dict constructor as an argument. Types converting to dictionaries are:

  • Dict[K, V]

  • MutableMapping[K, V]

  • Mapping[K, V]

In all cases, a new dict will be returned, so this operation can be used to copy a mapping into a dict. Any type parameters set to typing.Any will be passed through unconverted. If both type parameters are absent, they will be treated as Any too.

>>> from collections import OrderedDict
>>> cattr.structure(OrderedDict([(1, 2), (3, 4)]), Dict)
{1: 2, 3: 4}

These generic types are composable with all other converters. Note both keys and values can be converted.

>>> cattr.structure({1: None, 2: 2.0}, Dict[str, Optional[int]])
{'1': None, '2': 2}

Homogeneous and heterogeneous tuples

Homogeneous and heterogeneous tuples can be produced from iterable objects. Heterogeneous tuples require an iterable with the number of elements matching the number of type parameters exactly. Use:

  • Tuple[A, B, C, D]

Homogeneous tuples use:

  • Tuple[T, ...]

In all cases a tuple will be returned. Any type parameters set to typing.Any will be passed through unconverted.

>>> cattr.structure([1, 2, 3], Tuple[int, str, float])
(1, '2', 3.0)

The tuple conversion is composable with all other converters.

>>> cattr.structure([{1: 1}, {2: 2}], Tuple[Dict[str, float], ...])
({'1': 1.0}, {'2': 2.0})

Unions

Unions of NoneType and a single other type are supported (also known as Optional s). All other unions a require a disambiguation function.

Automatic Disambiguation

In the case of a union consisting exclusively of attrs classes, cattrs will attempt to generate a disambiguation function automatically; this will succeed only if each class has a unique field. Given the following classes:

>>> @attr.s
... class A:
...     a = attr.ib()
...     x = attr.ib()
...
>>> @attr.s
... class B:
...     a = attr.ib()
...     y = attr.ib()
...
>>> @attr.s
... class C:
...     a = attr.ib()
...     z = attr.ib()
...

cattrs can deduce only instances of A will contain x, only instances of B will contain y, etc. A disambiguation function using this information will then be generated and cached. This will happen automatically, the first time an appropriate union is structured.

Manual Disambiguation

To support arbitrary unions, register a custom structuring hook for the union (see Registering custom structuring hooks).

attrs classes

Simple attrs classes

attrs classes using primitives, collections of primitives and their own converters work out of the box. Given a mapping d and class A, cattrs will simply instantiate A with d unpacked.

>>> @attr.s
... class A:
...     a = attr.ib()
...     b = attr.ib(convert=int)
...
>>> cattr.structure({'a': 1, 'b': '2'}, A)
A(a=1, b=2)

attrs classes deconstructed into tuples can be structured using cattr.structure_attrs_fromtuple (fromtuple as in the opposite of attr.astuple and converter.unstructure_attrs_astuple).

>>> @attr.s
... class A:
...     a = attr.ib()
...     b = attr.ib(convert=int)
...
>>> cattr.structure_attrs_fromtuple(['string', '2'], A)
A(a='string', b=2)

Loading from tuples can be made the default by creating a new Converter with unstruct_strat=cattr.UnstructureStrategy.AS_TUPLE.

>>> converter = cattr.Converter(unstruct_strat=cattr.UnstructureStrategy.AS_TUPLE)
>>> @attr.s
... class A:
...     a = attr.ib()
...     b = attr.ib(convert=int)
...
>>> converter.structure(['string', '2'], A)
A(a='string', b=2)

Structuring from tuples can also be made the default for specific classes only; see registering custom structure hooks below.

Complex attrs classes

Complex attrs classes are classes with type information available for some or all attributes. These classes support almost arbitrary nesting.

Type information is supported by attrs directly, and can be set using type annotations when using Python 3.6+, or by passing the appropriate type to attr.ib.

>>> @attr.s
... class A:
...     a: int = attr.ib()
...
>>> attr.fields(A).a
Attribute(name='a', default=NOTHING, validator=None, repr=True, cmp=True, hash=None, init=True, metadata=mappingproxy({}), type=<class 'int'>, converter=None, kw_only=False)

Type information, when provided, can be used for all attribute types, not only attributes holding attrs classes.

>>> @attr.s
... class A:
...     a: int = attr.ib(default=0)
...
>>> @attr.s
... class B:
...     b = attr.ib(type=A)  # Legacy syntax.
...
>>> cattr.structure({'b': {'a': '1'}}, B)
B(b=A(a=1))

Registering custom structuring hooks

cattrs doesn’t know how to structure non-attrs classes by default, so it has to be taught. This can be done by registering structuring hooks on a converter instance (including the global converter).

Here’s an example involving a simple, classic (i.e. non-attrs) Python class.

>>> class C(object):
...     def __init__(self, a):
...         self.a = a
...     def __repr__(self):
...         return f'C(a={self.a})'
>>> cattr.structure({'a': 1}, C)
Traceback (most recent call last):
...
ValueError: Unsupported type: <class '__main__.C'>. Register a structure hook for it.
>>>
>>> cattr.register_structure_hook(C, lambda d, t: C(**d))
>>> cattr.structure({'a': 1}, C)
C(a=1)

The structuring hooks are callables that take two arguments: the object to convert to the desired class and the type to convert to. The type may seem redundant but is useful when dealing with generic types.

When using cattr.register_structure_hook, the hook will be registered on the global converter. If you want to avoid changing the global converter, create an instance of cattr.Converter and register the hook on that.

In some situations, it is not possible to decide on the converter using typing mechanisms alone (such as with attrs classes). In these situations, cattrs provides a register_structure_func_hook instead, which accepts a function to determine whether that type can be handled instead.

The function-based hooks are evaluated after the class-based hooks. In the case where both a class-based hook and a function-based hook are present, the class-based hook will be used.

>>> class D(object):
...     custom = True
...     def __init__(self, a):
...         self.a = a
...     def __repr__(self):
...         return f'D(a={self.a})'
...     @classmethod
...     def deserialize(cls, data):
...         return cls(data["a"])
>>> cattr.register_structure_hook_func(lambda cls: getattr(cls, "custom", False), lambda d, t: t.deserialize(d))
>>> cattr.structure({'a': 2}, D)
D(a=2)