Lib/dataclasses.py

cpython 3.14 @ ab2d84fe1023/Lib/dataclasses.py ↗

Lib/dataclasses.py implements the @dataclass decorator. It inspects class annotations, collects Field descriptors, then generates and execs __init__, __repr__, __eq__, and optionally __lt__/__hash__ source strings at class creation time. Nothing happens at instance creation beyond running the generated __init__.

Map

Lines	Symbol	Role
1-120	Field, field()	Descriptor for one dataclass field, carries default, repr, compare, hash flags
121-350	_process_class	Core: collect fields, build init/repr/eq code strings, exec them
351-500	dataclass	Decorator entry point; calls _process_class
501-700	fields, asdict, astuple	Introspection helpers
701-900	make_dataclass, replace	Dynamic class creation and functional copy
901-1310	Internal helpers	_field_init, _init_fn, _repr_fn, _eq_fn, _hash_fn code generators

Reading

Code generation via exec

dataclasses generates __init__ as a source string and execs it in a namespace that already contains the field defaults and type annotations. This avoids any runtime overhead per-instance since the result is a real function object.

# CPython: Lib/dataclasses.py:500 _create_fn
def _create_fn(name, args, body, *, globals=None, locals=None,
               return_type=MISSING):
    if locals is None:
        locals = {}
    if return_type is not MISSING:
        locals['_return_type'] = return_type
    args = ','.join(args)
    body = '\n'.join(f'  {b}' for b in body)
    txt = f' def {name}({args}):\n{body}'
    local_vars = ', '.join(locals.keys())
    txt = f"def __create_fn__({local_vars}):\n{txt}\n return {name}"
    ns = {}
    exec(txt, globals, ns)
    return ns['__create_fn__'](**locals)

Field and field()

field() returns a Field sentinel. _process_class recognizes it and substitutes the actual default or default_factory call into the generated __init__ body.

# CPython: Lib/dataclasses.py:285 Field.__init__
class Field:
    __slots__ = ('default', 'default_factory', 'repr', 'hash',
                 'init', 'compare', 'metadata', 'kw_only',
                 '_field_type', 'name', 'type')

__post_init__ hook

If the class defines __post_init__, the generated __init__ appends a call to it at the end, passing any InitVar pseudo-fields as positional arguments.

asdict recursion

asdict deep-copies the values using copy.deepcopy and recurses into nested dataclasses, lists, tuples, and dicts. The recursion preserves the original container types.

gopy notes

Not yet ported. The exec-based code generation requires a working compile/exec path. The planned port lives in module/dataclasses/. An interim approach could generate the methods directly in Go without source string compilation, but that would diverge from CPython's behaviour for classes that override __init_subclass__.