Properties for dataclass fields
Project description
field-properties
Properties for dataclass fields
Example
from dataclasses import dataclass
from field_properties import field_property, unwrap_property
@dataclass(frozen=True)
class Foo:
bar: int = field_property(default=0) # Same parameter than dataclasses.field
@field_property(bar) # Equivalent to @field_property(bar).getter
def get_bar(self) -> int:
# unwrap_property(self).bar is equivalent to self._bar
# but it's type-checked and linter-friendly
return unwrap_property(self).bar
# When not declared, getter, setter and deleter are generated like the following:
# @field_property(bar).setter
# def set_bar(self, value: int):
# unwrap_property(self).bar = value
assert repr(Foo()) == repr(Foo(0)) == "Foo(bar=0)"
How does it works?
When a dataclass field has a default value, this value is assigned as a class attribute. field_property
use this mechanism and create a field with a property as default value.
If a default value/factory is registered with field_property
, the property setter will be called with it in __init__
.
Default getter/setter/deleter
field_property
generates default getter/setter/deleter as simple wrappers around an instance attribute whose name is the field name prefixed with an underscore _
. unwrap_property
allows accessing this attribute in a type-checked/linter-friendly way.
By the way, if all the getter/setter/deleter are declared (and thus not generated), the protected attribute will not be created.
Overriding
Field properties can be overridden, but the dataclass field must be overridden too — this is because a new field must be created, as property is declared as its default value (see previous section).
from dataclasses import dataclass
from field_properties import field_property, unwrap_property
@dataclass
class Foo:
bar: int = field_property(default=0)
@field_property(bar)
def get_bar(self):
return unwrap_property(self).bar + 1
class Foo2(Foo):
bar: int = field_property(default=0) # field property must be overridden
# field_property(inherit=True) is a shortcut to override a field
# and reusing all it's arguments
@field_property(bar)
def get_bar(self):
return unwrap_property(self).bar + 2
assert Foo() == 1
assert Foo2() == 2
In fact, because field is redeclared, it's also possible to override normal fields with a field property
from dataclasses import dataclass
from field_properties import field_property
@dataclass
class Foo:
bar: int = 0
class Foo2(Foo):
bar: int = field_property(default=1)
assert Foo2() == 1
Raw property
field_property
comes with a default implementation for its getter/setter/deleter. This can be turned off with raw=False
parameter. Here is an example of a read_only field:
from dataclasses import dataclass
from field_properties import field_property
@dataclass
class Foo:
bar: int = field_property(init=False, raw=True)
@field_property(bar)
def get_bar(self) -> int:
return 0
assert Foo().bar == 0
assert str(Foo()) == "Foo(bar=0)"
try:
Foo().bar = 1
except AttributeError:
assert True
else:
assert False
PEP 614
Decorator syntax @field_property(bar).setter
is only valid in Python 3.9. Previous version can use the following hack:
from dataclasses import dataclass
from field_properties import field_property
@dataclass
class Foo:
bar: int = field_property()
def set_bar(self, value: int):
...
field_property(bar).setter(set_bar)
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