Proper way to create class variable in Data Class
May 21
I've just begun playing around with Python's Data Classes, and I would like confirm that I am declaring Class Variables in the proper way.
Using regular python classes
class Employee:
raise_amount = .05
def __init__(self, fname, lname, pay):
self.fname = fname
self.lname = lname
self.pay = pay
Using python Data Class
@dataclass
class Employee:
fname: str
lname: str
pay: int
raise_amount = .05
The class variable I am referring to is raise_amount. Is this a properly declared class variable using Data Classes? Or is there a better way of doing so?
I have tested the data class implementation already and it provides the expected functionality, but I am mainly wondering if my implementation is following best practices.
python1 answer
Accepted answer · original discussion
May 21
To create a class variable, annotate the field as a typing.ClassVar or not at all.
from typing import ClassVar
from dataclasses import dataclass
@dataclass
class Foo:
ivar: float = 0.5
cvar: ClassVar[float] = 0.5
nvar = 0.5
foo = Foo()
Foo.ivar, Foo.cvar, Foo.nvar = 1, 1, 1
print(Foo().ivar, Foo().cvar, Foo().nvar) # 0.5 1 1
print(foo.ivar, foo.cvar, foo.nvar) # 0.5 1 1
print(Foo(), Foo(12)) # Foo(ivar=0.5) Foo(ivar=12)
There is a subtle difference in that the unannotated field is completely ignored by @dataclass, whereas the ClassVar field is stored but not converted to an attribute.
dataclasses— Data Classes
The member variables [...] are defined using PEP 526 type annotations.
Class variables
One of two places where
dataclass()actually inspects the type of a field is to determine if a field is a class variable as defined in PEP 526. It does this by checking if the type of the field istyping.ClassVar. If a field is aClassVar, it is excluded from consideration as a field and is ignored by the dataclass mechanisms. SuchClassVarpseudo-fields are not returned by the module-level fields() function.
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