Type-safe immutable objects for Python
This is an work in progress.
emuobject is a Python library that provides type-safe (Emu)table objects. (Forgive the pun.)
from emuobject.Emu import Emu
class Person(Emu):
@classmethod
def schema(cls):
# cerberus schema
return {
"name": {"type": "string"},
"age": {"type": "integer", "min": 0},
"description": {"type": "string", "required": False},
}
def is_old_enough_to_vote(self) -> bool:
return self.age >= 18
p = Person({name="John", age=42})
print(p.get('name'))
print(p.get('description')) # error, because we have no descriptionThis is not on PyPI because it's my first Python module and I'm pretty darned sure I have some things wrong.
pip install git+https://github.com/Ovid/emuobjectWhen working on large projects, there are two key issues which can crop up with object-oriented code:
- The type you got was not the type you expected
- Something else changed the data!
emuobject attempts to handle both.
In "Pythonic" objects, we don't do data validation. For the above Person
class, we might write person.age = "forty-two" and it would work just fine
until we tried to do something with it later. This is a problem, because it
means that we can't trust the data we get back from our objects. Here's an
example:
if person.is_old_enough_to_vote():
do_something_with(person)
add_to_voter_queue(person)In the above, if do_something_with changes the age of the person to 17 or less, then
add_to_voter_queue has an object in an invalid state, despite our having a guard clause.
If you read one of the many of the "object-oriented programming is bad" articles on the web, you'll often find that many of their arguments go away in the face of immutable objects.
emuobject requires you to define a schema method which returns a
cerberus schema. When you create an
object, it will be validated against this schema, and if it fails, an
exception is thrown.
Because the object is immutable, this validation happens only once upon instantiation.
