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Simple validation for function arguments using a decorator.

Project Description

This module is designed to solve the most basic of argument validations: types, clauses, and combinations of clauses. It is meant to remove some of the boiler plate code used to check the input types and checks such as between, or string lengths.

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  • Clause - A function that takes in the value as a parameter and returns True or False.
  • Operator - Allows you to “and” and “or” clauses together.

Basic example:

from validoot import validates, inst, typ, between

@validates(inst(basestring), typ(int), between(0, 100))
def do_something(name, id, age):

In the code above, a validoot.ValidationError will be thrown if the name is not a string or unicode, if the id is not an integer, or if the age is not between 0 and 100.

>>> do_something('Darth Vader', 0, 42)
>>> do_something('Boba Fett', 1, 123)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "validoot/", line 25, in __call__
    self.positional_validators[i], args[i], i))
validoot.exceptions.ValidationError: Validation <in range [0..100)> failed for value 123 ( arg[2] )


We can extend the first example by adding an additional check for the name: it must be between 5 and 40 characters. For this we use the validoot.And operator to combine the clauses.

from validoot import validates, inst, typ, between, len_between, And

@validates(And(inst(basestring), len_between(5, 40)), typ(int), between(0, 100))
def do_something(name, id, age):

An Or operator also exists. Both And and Or take in a variable number of clauses and can be nested further.

Operator shortcuts are provided for joining clauses in a different manner which reads differently (._and(...), ._or(...)). So our previous example can be changed to look like this:

from validoot import validates, inst, typ, between, len_between

@validates(inst(basestring)._and(len_between(5, 40)), typ(int), between(0, 100))
def do_something(name, id, age):

Operators can also be combined in more complicated ways:

inst(basestring)._and(len_between(5, 40))._or(typ(int))

Keyword arguments:

There is also support for keyword arguments:

from validoot import validates, inst, typ

@validates(inst(basestring), something=typ(float))
def do_something(name, something=1.0, anotherthing=2):

Here the something value must pass the validation checks as specified in the decorator. No checks exist for anotherthing so it has no restrictions.

Decorating Class/Static/Instance methods or Constructors:

Methods belonging to classes can be validated as well in exactly the same way as the examples above. Please make note of the order of the @validates decorator and other decorators such as @classmethod or @staticmethod.

class SomeClass(object):

    # classmethod MUST be the innermost decorator!
    def some_class_method(cls, an_integer):
        return an_integer

    # staticmethod can be outer or inner decorator
    def some_static_method(a_floater):
        return a_floater

    def some_instance_method(self, a_string):
        return a_string

In order to validate arguments passed through to a constructor, the validates decorator should be places on the class itself:

class SomeClass(object):

    def __init__(self, username):
        self.username = username

Additional Clauses:

There are some more complex clauses included with the package:

  • _ : The underscore only allows NoneType.
  • numeric : Only accepts int, float, or long types.
  • text : Only accepts instances of basestring (Python 2) or str (Python 3).
  • positive : Only positive numbers
  • negative : Only positive numbers
  • email_address : Simple regex email check (covers most basic examples)
  • ip_address : Only accept an IPv4 address
  • url : Simple regex url check (covers most basic examples)

These can be found in the validoot.builtins module.


What if I don’t want validation for one of the position arguments?

Simple. Just use None.

from validoot import validates, inst, between

@validates(inst(basestring), None, between(0, 100))
def do_something(name, id, age):

What validation clauses are built in?

  • typ(t) - value must be of exact type t
  • inst(t) - value must be of exact type t or of a subclass
  • between(lower, upper, lower_inc=True, upper_inc=False) - the value must between lower and upper. lower_inc and upper_inc indicate range inclusivity.
  • len_between(...) - identical to between but uses len(value)
  • regex(string) - value must match the regex string provided
  • list_of(v) - value must be a list of objects that pass the validation v
  • dict_of(v1, v2) - value must be a dictionary where each key passes validation v1 and each value passes validation v2

How do I create my own validation clauses?

The built in clauses provided by Validoot are all subclasses of the validoot.clauses.Clause object. Check out its source code to see how they work. Technically clauses can be any callable object so plain functions or lambdas also work.

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