# Basic usage

Serializing and deserializing with cbor2 is pretty straightforward:

    from cbor2 import dump, dumps, load, loads

    # Serialize an object as a bytestring
    data = dumps(['hello', 'world'])

    # Deserialize a bytestring
    obj = loads(data)

    # Efficiently deserialize from a file
    with open('input.cbor', 'rb') as fp:
        obj = load(fp)

    # Efficiently serialize an object to a file
    with open('output.cbor', 'wb') as fp:
        dump(obj, fp)

Some data types, however, require extra considerations, as detailed below.

## Date/time handling

The CBOR specification does not support naïve datetimes (that is, datetimes where `tzinfo` is missing). When the encoder encounters such a datetime, it needs to know which timezone it belongs to. To this end, you can specify a default timezone by passing a `tzinfo` instance to `dump()`/`dumps()` call as the `timezone` argument. Decoded datetimes are always timezone aware.

By default, datetimes are serialized in a manner that retains their timezone offsets. You can optimize the data stream size by passing `datetime_as_timestamp=False` to `dump()`/`dumps()`, but this causes the timezone offset information to be lost.

In versions prior to 4.2 the encoder would convert a `datetime.date` object into a `datetime.datetime` prior to writing. This can cause confusion on decoding so this has been disabled by default in the next version. The behaviour can be re-enabled as follows:

    from cbor2 import dumps
    from datetime import date, timezone

    # Serialize dates as datetimes
    encoded = dumps(date(2019, 10, 28), timezone=timezone.utc, date_as_datetime=True)

A default timezone offset must be provided also.

## Cyclic (recursive) data structures

If the encoder encounters a shareable object (ie. list or dict) that it has seen before, it will by default raise `CBOREncodeError` indicating that a cyclic reference has been detected and value sharing was not enabled. CBOR has, however, an extension specification that allows the encoder to reference a previously encoded value without processing it again. This makes it possible to serialize such cyclic references, but value sharing has to be enabled by passing `value_sharing=True` to `dump()`/`dumps()`.

Warning

Support for value sharing is rare in other CBOR implementations, so think carefully whether you want to enable it. It also causes some line overhead, as all potentially shareable values must be tagged as such.

## String references

When `string_referencing=True` is passed to `dump()`/`dumps()`, if the encoder would encode a string that it has previously encoded and where a reference would be shorter than the encoded string, it instead encodes a reference to the nth sufficiently long string already encoded.

Warning

Support for string referencing is rare in other CBOR implementations, so think carefully whether you want to enable it.

## Tag support

In addition to all standard CBOR tags, this library supports many extended tags:

Tag | Semantics | Python type(s)
--- | --- | ---
0 | Standard date/time string | datetime.date / datetime.datetime
1 | Epoch-based date/time | datetime.date / datetime.datetime
2 | Positive bignum | int / long
3 | Negative bignum | int / long
4 | Decimal fraction | decimal.Decimal
5 | Bigfloat | decimal.Decimal
25 | String reference | str / bytes
28 | Mark shared value | N/A
29 | Reference shared value | N/A
30 | Rational number | fractions.Fraction
35 | Regular expression | re.Pattern (result of `re.compile(...)`)
36 | MIME message | email.message.Message
37 | Binary UUID | uuid.UUID
256 | String reference namespace | N/A
258 | Set of unique items | set
260 | Network address | `ipaddress.IPv4Address` (or IPv6)
261 | Network prefix | `ipaddress.IPv4Network` (or IPv6)
43000 | Single complex number | complex
55799 | Self-Described CBOR | object

Arbitary tags can be represented with the `CBORTag` class.

If you want to write a file that is detected as CBOR by the Unix `file` utility, wrap your data in a `CBORTag` object like so:

    from cbor2 import dump, CBORTag

    with open('output.cbor', 'wb') as fp:
        dump(CBORTag(55799, obj), fp)

This will be ignored on decode and the original data content will be returned.

## Use Cases

Here are some things that the cbor2 library could be (and in some cases, is being) used for:

- Experimenting with network protocols based on CBOR encoding

- Designing new data storage formats

- Submitting binary documents to ElasticSearch without base64 encoding overhead

- Storing and validating file metadata in a secure backup system

- RPC which supports Decimals with low overhead