Fields

5.0.0 verified Fri May 01 auth: no python

A Python library for eliminating class boilerplate by providing an attrs/dataclass-like container class with built-in validation and transformation. Current version is 5.0.0, released on 2025-12-05. Development appears slow; pre-5.0.0 releases were irregular.

pip install fields

Common errors

error TypeError: 'Field' object is not callable ↓

cause Trying to call a Field instance as a function (e.g., after defining it as a class attribute).

fix

Fields are descriptors; access via instance attribute, not class attribute. Use instance.field_name.

error ValueError: Field 'x' is required ↓

cause Required field not provided during instantiation.

fix

Pass all required fields as keyword arguments to the constructor.

Warnings

breaking Classmethod and staticmethod descriptors conflict with field inspection; avoid using them inside @fields classes. ↓

fix Define classmethods/staticmethods outside the @fields class or use a different decorator pattern.

gotcha Field name checking uses both the slot name and the variable name; renaming a field variable but not updating the Field's 'name' argument can cause silent failures. ↓

fix Use keyword argument 'name' only when necessary; prefer relying on the variable name.

deprecated The 'validate' parameter in Field is deprecated in 5.0.0; use the 'check' parameter instead. ↓

fix Replace Field(validate=func) with Field(check=func).

Imports

Field
wrong
```
from fields.field import Field
```
correct
```
from fields import Field
```
Field is exported at package level; no submodule needed.
fields
wrong
```
import fields
```
correct
```
from fields import fields
```
Direct import gives you the decorator; for container use 'from fields import fields'.

Quickstart

Define a container class with automatic __init__, __repr__, and validation via type hints and Field descriptors.

from fields import fields, Field

@fields
class Point:
    x: int = Field(required=True)
    y: int = 0

p = Point(x=3)
print(p.x, p.y)  # 3 0