Cobble: Create Python Data Objects

0.1.4 · active · verified Thu Apr 09

Cobble is a Python library designed for the easy creation of data objects. It automatically implements common methods such as `__eq__` and `__repr__`, simplifying the boilerplate typically associated with simple data-holding classes. The current version is 0.1.4, released on June 1, 2024, and it is actively maintained with a stable release cadence.

Warnings

gotcha Mutable default arguments in `cobble.field(default=...)` can lead to unexpected shared state across instances. This is a common Python footgun, also applicable to `dataclasses` and `cobble`.
Fix: Use `default_factory` for mutable defaults, e.g., `my_list = cobble.field(default_factory=list)`.
gotcha Overriding `__init__` in a `cobble.data` class will prevent Cobble from automatically generating its own `__init__` method, potentially leading to uninitialized fields or `AttributeError` if not handled carefully.
Fix: Prefer using `__post_init__` for custom initialization logic or ensure your custom `__init__` explicitly initializes all fields defined by `cobble.field()`.
gotcha Accessing fields directly on a `cobble.data` class before instantiation (e.g., `MyClass.field_name`) will result in an `AttributeError` if the field is intended for instances.
Fix: Always instantiate the class (e.g., `instance = MyClass(...)`) before attempting to access instance-specific fields. Use `typing.ClassVar` if an attribute truly needs to be a class-level variable and should not be part of the `__init__` generated by `cobble`.

Install

pip install cobble Install latest version

Imports

data
```
import cobble
@cobble.data
```
Used as a decorator to define data objects.
field
```
import cobble
cobble.field()
```
Used within a cobble.data class to define fields, similar to dataclasses.field.
visitor
```
import cobble
cobble.visitor(BaseClass)
```
Used to create visitor patterns for hierarchies of cobble data objects.

Quickstart

This example demonstrates how to define a simple data object `Song` using the `@cobble.data` decorator and `cobble.field()`. It also shows how to create a more complex structure like an expression tree with `Literal` and `Add` and then process it using a visitor pattern with `cobble.visitor`.

import cobble

@cobble.data
class Song(object):
    name = cobble.field()
    artist = cobble.field()
    album = cobble.field(default=None)

song = Song("MFEO", artist="Jack's Mannequin", album="Everything in Transit")
print(song)

@cobble.data
class Literal:
    value = cobble.field()

@cobble.data
class Add:
    left = cobble.field()
    right = cobble.field()

class Evaluator(cobble.visitor(object)):
    def visit_literal(self, literal):
        return literal.value
    def visit_add(self, add):
        return self.visit(add.left) + self.visit(add.right)

expression = Add(Literal(2), Literal(4))
result = Evaluator().visit(expression)
print(f"Evaluation of {expression} is: {result}")

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