Validating python class attributes

Question

I want to add validations for my class attributes. On exploring, I came across multiple libraries google appengine, dj have their own property classes to validate attributes. Since its a common problem, Is there any python library which just provides basic validators ?

Does this answer your question? [Correct approach to validate attributes of an instance of class](https://stackoverflow.com/questions/2825452/correct-approach-to-validate-attributes-of-an-instance-of-class) — Kermit, Mar 31 '22 at 12:42

score 1 · Answer 1 · answered Oct 14 '19 at 19:36

You can use pyfields, it makes it very easy to add validation to class fields, independently of how you create the constructor (although pyfields also provides ways to create the constructor easily).

For example:

from pyfields import field

class Position:
    x = field(validators=lambda x: x > 0)
    y = field(validators={'y should be between 0 and 100': lambda y: y > 0 and y < 100})

p = Position()
p.x = 1
p.y = 101  # error see below

yields

valid8.entry_points.ValidationError[ValueError]: 
  Error validating [Position.y=101]. InvalidValue: y should be between 0 and 100.
  Function [<lambda>] returned [False] for value 101.

It is fully compliant with mini-lambda and valid8's validation lib, so that it is easy to add many validators to your fields, while keeping code readability :

from pyfields import init_fields
from mini_lambda import x
from valid8.validation_lib import is_in

ALLOWED_COLORS = {'white', 'blue', 'red'}

class Wall:
    height: int = field(validators={'should be a positive number': x > 0,
                                    'should be a multiple of 100': x % 100 == 0},
                        doc="Height of the wall in mm.")
    color: str = field(validators=is_in(ALLOWED_COLORS),
                       default='white',
                       doc="Color of the wall.")

    @init_fields
    def __init__(self, msg):
        print(msg)


w = Wall("hello world!", height=100)
print(vars(w))

See documentation for details (I'm the author by the way ;) )

score 0 · Answer 2 · answered Jun 27 '18 at 09:18

validation in python classes can make from method class.you should see the object-oriented programming in python articles for more detail.

from datetime import datetime
class Person(object):
    def __init__(self, years):
        self.age =  years

    @property
    def age(self):
        return datetime.now().year - self.birthyear

    @age.setter
    def age(self, years):
        if years < 0:
            raise ValueError("Age cannot be negative")
        self.birthyear = datetime.now().year - years

# p = Person(-1)
# ValueError: Age cannot be negative

# p = Person(10)
# p.age = -1
# ValueError: Age cannot be negative

p = Person(19)
print(p.age)       # 19

p.age = p.age + 1
print(p.age)       # 20

p.birthyear = 1992
print(p.age)       # 23

or try this link: https://smarie.github.io/python-valid8/

Validating python class attributes

2 Answers2