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An experimental Python DSL for validation
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An experimental Python DSL for validation


Here is an example of the use of Violet:

from violet import is_type, cast, passes
validator = is_type(
            "name": is_type(str) & passes(len),
            "age": cast(int) & passes(lambda x: 18 <= x <= 99),
            "sex": cast(str.lower) & passes(lambda s: s in ["male", "female"])

data = [{'name': 'Sue', 'age': '28', 'sex': 'FEMALE'},
        {'name': 'Sam', 'age': '42', 'sex': 'Male'},
        {'name': 'Sacha', 'age': '20', 'sex': 'Male'}]

assert [{'name': 'Sue', 'age': 28, 'sex': 'female'},
        {'name': 'Sam', 'age': 42, 'sex': 'male'},
        {'name': 'Sacha', 'age': 20, 'sex': 'male'}] == validator(data)


The idea behind violet is to create a composition of directives using the & and | operators as well as container types.

There are currently three directives, is_type, cast and passes.


This directive raises a TypeError if the type check fails.

Possible arguments for is_type are type classes such as int, str, float, etc:

validator = is_type(int)
assert 10 == validator(10)

validator = is_type(str)
assert "foo" == validator("foo")

Constant values:

validator = is_type(10)
assert 10 == validator(10)

Other directives and composites of those directives:

validator = is_type(is_type(int) | is_type(str))
assert 10 == validator(10)
assert "foo" == validator("foo")

validator = is_type(cast(int) & passes(lambda x: x > 10))
assert 11 == validator("11")

Containers of types or directives:

# list
validator = is_type([int])
assert [10, 11, 12] == validator([10, 11, 12])

# set
validator = is_type({int})
assert {10, 11, 12} == validator({10, 11, 12})

# dict
validator = is_type({"name": str, "age": int})
assert {"name": "Eric", "age": 32} == validator({"name": "Eric", "age": 32})

validator = is_type({"name": str, "age": is_type(cast(int))})
assert {"name": "Eric", "age": 32} == validator({"name": "Eric", "age": "32"})

# nested containers
validator = is_type({"name": str, "friends": {str}})

assert {"name": "Eric", "friends": {"Glenn", "Mark"}}
  == validator({"name": "Eric", "friends": {"Glenn", "Mark"}})


Simply converts the value as another:

validator = cast(int)
assert 10 == validator("10")


Raises an AssertionError if the predicate returns False, passes the value through if the predicate returns True:

validator = passes(lambda x: x > 10)
assert 11 == validator(11)

Not implemented

Currently there is no way to support optional keys in dictionary types. If a key is not found in the value being validated, a KeyError is currently raised. Support for optional keys is a planned feature.

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