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bpo-32427: Expose MISSING object. #5045

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Merged
ericvsmith merged 1 commit into from
Dec 29, 2017
Merged

bpo-32427: Expose MISSING object. #5045

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41 changes: 21 additions & 20 deletions Lib/dataclasses.py
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,7 @@
'field',
'FrozenInstanceError',
'InitVar',
'MISSING',

# Helper functions.
'fields',
Expand All @@ -29,11 +30,11 @@ def __repr__(self):
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.
Expand Down Expand Up @@ -114,7 +115,7 @@ def __repr__(self):
# 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.

Expand All @@ -130,7 +131,7 @@ def field(*, default=_MISSING, default_factory=_MISSING, init=True, repr=True,
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)
Expand All @@ -149,12 +150,12 @@ def _tuple_str(obj_name, fields):


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)
Expand Down Expand Up @@ -182,7 +183,7 @@ def _field_init(f, frozen, globals, self_name):
# 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.
Expand Down Expand Up @@ -210,10 +211,10 @@ def _field_init(f, frozen, globals, self_name):
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:
Expand All @@ -236,14 +237,14 @@ def _init_param(f):
# 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}'
Expand All @@ -261,13 +262,13 @@ def _init_fn(fields, frozen, has_post_init, self_name):
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 = []
Expand Down Expand Up @@ -368,7 +369,7 @@ def _get_field(cls, a_name, a_type):

# 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:
Expand Down Expand Up @@ -404,7 +405,7 @@ def _get_field(cls, a_name, a_type):

# 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
Expand Down Expand Up @@ -474,7 +475,7 @@ def _process_class(cls, repr, eq, order, hash, init, frozen):
# 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
Expand Down
52 changes: 49 additions & 3 deletions Lib/test/test_dataclasses.py
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Original file line number Diff line number Diff line change
@@ -1,6 +1,6 @@
from dataclasses import (
dataclass, field, FrozenInstanceError, fields, asdict, astuple,
make_dataclass, replace, InitVar, Field
make_dataclass, replace, InitVar, Field, MISSING
)

import pickle
Expand Down Expand Up @@ -917,12 +917,12 @@ def validate_class(cls):
param = next(params)
self.assertEqual(param.name, 'k')
self.assertIs (param.annotation, F)
# Don't test for the default, since it's set to _MISSING
# Don't test for the default, since it's set to MISSING
self.assertEqual(param.kind, inspect.Parameter.POSITIONAL_OR_KEYWORD)
param = next(params)
self.assertEqual(param.name, 'l')
self.assertIs (param.annotation, float)
# Don't test for the default, since it's set to _MISSING
# Don't test for the default, since it's set to MISSING
self.assertEqual(param.kind, inspect.Parameter.POSITIONAL_OR_KEYWORD)
self.assertRaises(StopIteration, next, params)

Expand All @@ -948,6 +948,52 @@ class C:

validate_class(C)

def test_missing_default(self):
# Test that MISSING works the same as a default not being
# specified.
@dataclass
class C:
x: int=field(default=MISSING)
with self.assertRaisesRegex(TypeError,
r'__init__\(\) missing 1 required '
'positional argument'):
C()
self.assertNotIn('x', C.__dict__)

@dataclass
class D:
x: int
with self.assertRaisesRegex(TypeError,
r'__init__\(\) missing 1 required '
'positional argument'):
D()
self.assertNotIn('x', D.__dict__)

def test_missing_default_factory(self):
# Test that MISSING works the same as a default factory not
# being specified (which is really the same as a default not
# being specified, too).
@dataclass
class C:
x: int=field(default_factory=MISSING)
with self.assertRaisesRegex(TypeError,
r'__init__\(\) missing 1 required '
'positional argument'):
C()
self.assertNotIn('x', C.__dict__)

@dataclass
class D:
x: int=field(default=MISSING, default_factory=MISSING)
with self.assertRaisesRegex(TypeError,
r'__init__\(\) missing 1 required '
'positional argument'):
D()
self.assertNotIn('x', D.__dict__)

def test_missing_repr(self):
self.assertIn('MISSING_TYPE object', repr(MISSING))

def test_dont_include_other_annotations(self):
@dataclass
class C:
Expand Down