bpo-32427: Expose dataclasses.MISSING object. (#5045)
diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
index 7a725df..eaaed63 100644
--- a/Lib/dataclasses.py
+++ b/Lib/dataclasses.py
@@ -8,6 +8,7 @@
'field',
'FrozenInstanceError',
'InitVar',
+ 'MISSING',
# Helper functions.
'fields',
@@ -29,11 +30,11 @@
return '<factory>'
_HAS_DEFAULT_FACTORY = _HAS_DEFAULT_FACTORY_CLASS()
-# A sentinel object to detect if a parameter is supplied or not.
-class _MISSING_FACTORY:
- def __repr__(self):
- return '<missing>'
-_MISSING = _MISSING_FACTORY()
+# A sentinel object to detect if a parameter is supplied or not. Use
+# a class to give it a better repr.
+class _MISSING_TYPE:
+ pass
+MISSING = _MISSING_TYPE()
# Since most per-field metadata will be unused, create an empty
# read-only proxy that can be shared among all fields.
@@ -114,7 +115,7 @@
# This function is used instead of exposing Field creation directly,
# so that a type checker can be told (via overloads) that this is a
# function whose type depends on its parameters.
-def field(*, default=_MISSING, default_factory=_MISSING, init=True, repr=True,
+def field(*, default=MISSING, default_factory=MISSING, init=True, repr=True,
hash=None, compare=True, metadata=None):
"""Return an object to identify dataclass fields.
@@ -130,7 +131,7 @@
It is an error to specify both default and default_factory.
"""
- if default is not _MISSING and default_factory is not _MISSING:
+ if default is not MISSING and default_factory is not MISSING:
raise ValueError('cannot specify both default and default_factory')
return Field(default, default_factory, init, repr, hash, compare,
metadata)
@@ -149,12 +150,12 @@
def _create_fn(name, args, body, globals=None, locals=None,
- return_type=_MISSING):
+ return_type=MISSING):
# Note that we mutate locals when exec() is called. Caller beware!
if locals is None:
locals = {}
return_annotation = ''
- if return_type is not _MISSING:
+ if return_type is not MISSING:
locals['_return_type'] = return_type
return_annotation = '->_return_type'
args = ','.join(args)
@@ -182,7 +183,7 @@
# initialize this field.
default_name = f'_dflt_{f.name}'
- if f.default_factory is not _MISSING:
+ if f.default_factory is not MISSING:
if f.init:
# This field has a default factory. If a parameter is
# given, use it. If not, call the factory.
@@ -210,10 +211,10 @@
else:
# No default factory.
if f.init:
- if f.default is _MISSING:
+ if f.default is MISSING:
# There's no default, just do an assignment.
value = f.name
- elif f.default is not _MISSING:
+ elif f.default is not MISSING:
globals[default_name] = f.default
value = f.name
else:
@@ -236,14 +237,14 @@
# For example, the equivalent of 'x:int=3' (except instead of 'int',
# reference a variable set to int, and instead of '3', reference a
# variable set to 3).
- if f.default is _MISSING and f.default_factory is _MISSING:
+ if f.default is MISSING and f.default_factory is MISSING:
# There's no default, and no default_factory, just
# output the variable name and type.
default = ''
- elif f.default is not _MISSING:
+ elif f.default is not MISSING:
# There's a default, this will be the name that's used to look it up.
default = f'=_dflt_{f.name}'
- elif f.default_factory is not _MISSING:
+ elif f.default_factory is not MISSING:
# There's a factory function. Set a marker.
default = '=_HAS_DEFAULT_FACTORY'
return f'{f.name}:_type_{f.name}{default}'
@@ -261,13 +262,13 @@
for f in fields:
# Only consider fields in the __init__ call.
if f.init:
- if not (f.default is _MISSING and f.default_factory is _MISSING):
+ if not (f.default is MISSING and f.default_factory is MISSING):
seen_default = True
elif seen_default:
raise TypeError(f'non-default argument {f.name!r} '
'follows default argument')
- globals = {'_MISSING': _MISSING,
+ globals = {'MISSING': MISSING,
'_HAS_DEFAULT_FACTORY': _HAS_DEFAULT_FACTORY}
body_lines = []
@@ -368,7 +369,7 @@
# If the default value isn't derived from field, then it's
# only a normal default value. Convert it to a Field().
- default = getattr(cls, a_name, _MISSING)
+ default = getattr(cls, a_name, MISSING)
if isinstance(default, Field):
f = default
else:
@@ -404,7 +405,7 @@
# Special restrictions for ClassVar and InitVar.
if f._field_type in (_FIELD_CLASSVAR, _FIELD_INITVAR):
- if f.default_factory is not _MISSING:
+ if f.default_factory is not MISSING:
raise TypeError(f'field {f.name} cannot have a '
'default factory')
# Should I check for other field settings? default_factory
@@ -474,7 +475,7 @@
# with the real default. This is so that normal class
# introspection sees a real default value, not a Field.
if isinstance(getattr(cls, f.name, None), Field):
- if f.default is _MISSING:
+ if f.default is MISSING:
# If there's no default, delete the class attribute.
# This happens if we specify field(repr=False), for
# example (that is, we specified a field object, but