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A function argument validation for humans

Project description

Function argument validation for humans for Python 3!


  • Straight port from the amazing library ow from Sindre Sorhus
  • Expressive chainable API
  • Pythonic approach via decorator
  • Lots of built-in validations
  • Supports custom validations
  • Written in Python3 with Type hinting


Since this is a straight up port from the JavaScript library not all features are available. Partly since this is a port and I haven't caught up and also since Python doesn't support all usecases as JavaScript does.


$ pip3 install pyow


from pyow import pyow

def unicorn(input):
	pyow(input, pyow.string.min_length(5))

	# ...

>>> ArgumentError: 'Expected argument to be of type `str` but received type `int`

>>> ArgumentError: Expected string to have a minimum length of `3`, got `yo`

or via decorator

from pyow.decorator import validate

def unicorn(input):
    return 1


pyow(value, predicate)

Test if value matches the provided predicate. Throws an ArgumentError if the test fails.

pyow.is_valid(value, predicate)

Returns True if the value matches the predicate, otherwise returns False.


Create a reusable validator.

check_password = pyow.create(pyow.string.min_length(6))

>>> ArgumentError: ('Expected string to have a minimum length of `6`, got `foo`')

pyow.any(predicate: List[Predicate])

Returns a predicate that verifies if the value matches at least one of the given predicates.

pyow('foo', pyow.any(pyow.string.max_length(3), pyow.number))


All the below types return a predicate. Every predicate has some extra operators that you can use to test the value even more fine-grained.


Built-in types


The following predicates are available on every type.


Inverts the following predicates.

pyow(1, pyow.number.nix.infinite)
pyow(1, pyow.number.isnot.infinite)

pyow('', pyow.string.isnot.empty);
>>> ArgumentError: [NOT] Expected string to be empty, got ``


Use a custom validation function. Return True if the value matches the validation, return False if it doesn't.

pyow(1, pyow.number.is_(lambda x: x < 10))

pyow(1, pyow.number.is_(lambda x: x > 10))
>>> ArgumentError: Expected `1` to pass custom validation function

Instead of returning False, you can also return a custom error message which results in a failure.

def greater_than(max_number: int, x: int):
	return x > max_number or f'Expected `{x}` to be greater than `{max_number}`'

pyow(5, pyow.number.is_(lambda x: greater_than(10, x)))
>>> ArgumentError: Expected `5` to be greater than `10`





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