Add a way to 'automatically' convert values to types not native to YAML

[akaIDIOT]

Either at construction time or at value retrieval time, some values may need conversion. Examples include date(time)s and binary blobs. Converters are easy to do (strptime / b64decode?), the way to attach / register them and where to apply them requires some thinking.

[akaIDIOT]

This also goes for values read from environment variables, these can only be strings. Can we determine a 'safe' way to auto-convert values like 'true' to a bool and '12345' to an int?

[akaIDIOT]

TOML (config language, used in proposed Pipfile thingy) seems to be using pattern matching, if a string matches an RFC 3339 date, it'll be a datetime instance. We could adopt a similar approach, or use YAML typing, which puts a strain on the user to explicitly type it as such.

[akaIDIOT]

Interestingly, pyyaml seems to do this partially already:

# date / datetime (seems to ignore tz hints, sadly)
yaml.load('test: 2001-01-02')
{'test': datetime.date(2001, 1, 2)}
yaml.load('test: 2001-01-02 12:34:00')
{'test': datetime.datetime(2001, 1, 2, 12, 34)}
yaml.load('test: !!timestamp 2001-01-02 12:34:00 +1')

# binary data is a tad more involved
yaml.load('test: YmluYXJ5IGRhdGE=')
{'test': 'YmluYXJ5IGRhdGE='}
yaml.load('test: !!binary YmluYXJ5IGRhdGE=')
{'test': b'binary data'}

http://yaml.org/type/ lists a number of builtin tags that might be quite useful for this.

yaml/pyyaml#113 proposes support for timezone-aware timestamps.

[akaIDIOT]

Issue above has been merged in the meantime 🎉