Preconfigured Converters#

The cattrs.preconf package contains factories for preconfigured converters, specifically adjusted for particular serialization libraries.

For example, to get a converter configured for BSON:

>>> from cattrs.preconf.bson import make_converter

>>> converter = make_converter() # Takes the same parameters as the `cattrs.Converter`

Converters obtained this way can be customized further, just like any other converter.

These converters support the following classes and type annotations, both for structuring and unstructuring:

  • str, bytes, int, float, pathlib.Path int enums, string enums

  • attrs classes and dataclasses

  • lists, homogenous tuples, heterogenous tuples, dictionaries, counters, sets, frozensets

  • optionals

  • sequences, mutable sequences, mappings, mutable mappings, sets, mutable sets

  • datetime.datetime

New in version 22.1.0: All preconf converters now have loads and dumps methods, which combine un/structuring and the de/serialization logic from their underlying libraries.

>>> from cattrs.preconf.json import make_converter

>>> converter = make_converter()

>>> @define
... class Test:
...     a: int

>>> converter.dumps(Test(1))
'{"a": 1}'

Particular libraries may have additional constraints documented below.

Third-party libraries can be specified as optional (extra) dependencies on cattrs during installation. Optional install targets should match the name of the cattrs.preconf modules.

# Using pip
pip install cattrs[ujson]

# Using poetry
poetry add --extras tomlkit cattrs

Standard Library json#

Found at cattrs.preconf.json.

Bytes are serialized as base 85 strings. Counters are serialized as dictionaries. Sets are serialized as lists, and deserialized back into sets. datetime s are serialized as ISO 8601 strings.

ujson#

Found at cattrs.preconf.ujson.

Bytes are serialized as base 85 strings. Sets are serialized as lists, and deserialized back into sets. datetime s are serialized as ISO 8601 strings.

ujson doesn’t support integers less than -9223372036854775808, and greater than 9223372036854775807, nor does it support float('inf').

orjson#

Found at cattrs.preconf.orjson.

Bytes are serialized as base 85 strings. Sets are serialized as lists, and deserialized back into sets. datetime s are serialized as ISO 8601 strings.

orjson doesn’t support integers less than -9223372036854775808, and greater than 9223372036854775807. orjson only supports mappings with string keys so mappings will have their keys stringified before serialization, and destringified during deserialization.

msgpack#

Found at cattrs.preconf.msgpack.

Sets are serialized as lists, and deserialized back into sets. datetime s are serialized as UNIX timestamp float values.

msgpack doesn’t support integers less than -9223372036854775808, and greater than 18446744073709551615.

When parsing msgpack data from bytes, the library needs to be passed strict_map_key=False to get the full range of compatibility.

cbor2#

Found at cattrs.preconf.cbor2.

cbor2 implements a fully featured CBOR encoder with several extensions for handling shared references, big integers, rational numbers and so on.

Sets are serialized and deserialized to sets. Tuples are serialized as lists.

datetime s are serialized as a text string by default (CBOR Tag 0). Use keyword argument datetime_as_timestamp=True to encode as UNIX timestamp integer/float (CBOR Tag 1) note: this replaces timezone information as UTC.

Use keyword argument canonical=True for efficient encoding to the smallest binary output.

Floats can be forced to smaller output by casting to lower-precision formats by casting to numpy floats (and back to Python floats). Example: float(np.float32(value)) or float(np.float16(value))

bson#

Found at cattrs.preconf.bson. Tested against the bson module bundled with the pymongo library, not the standalone PyPI bson package.

Sets are serialized as lists, and deserialized back into sets.

bson doesn’t support integers less than -9223372036854775808 or greater than 9223372036854775807 (64-bit signed). bson does not support null bytes in mapping keys. bson only supports mappings with string keys so mappings will have their keys stringified before serialization, and destringified during deserialization. The bson datetime representation doesn’t support microsecond accuracy.

When encoding and decoding, the library needs to be passed codec_options=bson.CodecOptions(tz_aware=True) to get the full range of compatibility.

pyyaml#

Found at cattrs.preconf.pyyaml.

Frozensets are serialized as lists, and deserialized back into frozensets.

tomlkit#

Found at cattrs.preconf.tomlkit.

Bytes are serialized as base 85 strings. Sets are serialized as lists, and deserialized back into sets. Tuples are serialized as lists, and deserialized back into tuples. tomlkit only supports mappings with string keys so mappings will have their keys stringified before serialization, and destringified during deserialization.