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Ultimate transformation library that supports validation, contexts and aiohttp.
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Build status @ Circle CI Gitter Chat Latest release BSD license

Ultimate transformation library that supports validation, contexts and aiohttp.

Trafaret is rigid and powerful lib to work with foreign data, configs etc. It provides simple way to check anything, and convert it accordingly to your needs.

It has shortcut syntax and ability to express anything that you can code:

>>> from trafaret.constructor import construct
>>> validator = construct({'a': int, 'b': [str]})
>>> validator({'a': 5, 'b': ['lorem', 'ipsum']})
{'a': 5, 'b': ['lorem', 'ipsum']}

>>> validator({'a': 5, 'b': ['gorky', 9]})
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/Users/mkrivushin/w/trafaret/trafaret/", line 204, in __call__
    return self.check(val)
  File "/Users/mkrivushin/w/trafaret/trafaret/", line 144, in check
    return self._convert(self.check_and_return(value))
  File "/Users/mkrivushin/w/trafaret/trafaret/", line 1105, in check_and_return
    raise DataError(error=errors, trafaret=self)
trafaret.DataError: {'b': DataError({1: DataError(value is not a string)})}

Read The Docs hosted documentation or look to the docs/intro.rst for start.

Trafaret can even generate Trafarets instances to build transformators from json, like in json schema implementation Trafaret Schema


  • converters and convert=False are deleted in favor of And and &
  • String parameter regex deleted in favor of Regexp and RegexpRaw usage
  • new OnError to customize error message
  • context=something argument for __call__ and check Trafaret methods. Supported by Or, And, Forward etc.
  • new customizable method transform like change_and_return but takes context= arg
  • new trafaret_instance.async_check method that works with await


For simple example what can be done:

import datetime
import trafaret as t

date = t.Dict(year=t.Int, month=t.Int, day=t.Int) >> (lambda d: datetime.datetime(**d))
assert date.check({'year': 2012, 'month': 1, 'day': 12}) == datetime.datetime(2012, 1, 12)

Work with regex:

>>> c = t.RegexpRaw(r'^name=(\w+)$') >> (lambda m:
>>> c.check('name=Jeff')

Rename dict keys:

>>> c = t.Dict({(t.Key('uNJ') >> 'user_name'): t.String})
>>> c.check({'uNJ': 'Adam'})
{'user_name': 'Adam'}

Arrow date checking:

import arrow

def check_datetime(str):
        return arrow.get(str).naive
    except arrow.parser.ParserError:
        return t.DataError('value is not in proper date/time format')

Yes, you can write trafarets that simple.

Related projects

Trafaret Config

Trafaret Validator

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