Hi,
In another thread on this list I was reminded of types.SimpleNamespace. This is
nice, but I wanted to create a bag class with constants that are read-only. My
main question is about example #3 below (example #2 just illustrates my thought
process). Is this a use case to a metaclass? Or can I do it some other way
(maybe a class decorator?). I would like to create a metaclass that converts
any non-special attributes (=not starting with '_') into properties, if needed.
That way I can specify my bag class in a very clean way: I only specify the
metaclass, and I list the attributes as normal attrbutes, because the metaclass
will convert them into properties.
Why does following the line (in #3) not convert the normal attribute into a
property?
setattr(cls, attr, property(lambda self: obj)) # incorrect!
# 1-------------------------------------------------
# nice, but I want the constants to be read-only
from types import SimpleNamespace
const = SimpleNamespace(YES=1, NO=0, DUNNO=9)
const.YES = 0
print(const)
# 2-------------------------------------------------
# works, but I wonder if there's a builtin way
class Const(object):
"""Adding attributes is ok, modifying them is not"""
YES = property(lambda self: 1)
NO = property(lambda self: 0)
DUNNO = property(lambda self: 42)
#THROWS_ERROR = 666
def __new__(cls):
for attr in dir(cls):
if attr.startswith('_'):
continue
elif not isinstance(getattr(cls, attr), property):
raise ValueError("Only properties allowed")
return super().__new__(cls)
def __repr__(self):
kv = ["%s=%s" % (attr, getattr(self, attr)) for \
attr in sorted(dir(Const)) if not attr.startswith('_')]
return "ReadOnlyNamespace(" + ", ".join(kv) + ")"
c = Const()
print(repr(c))
#c.YES = 42 # raises AttributeError (desired behavior)
print(c.YES)
# 3-------------------------------------------------
class Meta(type):
def __new__(cls, name, bases, attrs):
for attr, obj in attrs.items():
if attr.startswith('_'):
continue
elif not isinstance(obj, property):
import pdb;pdb.set_trace()
#setattr(cls, attr, property(lambda self: obj)) # incorrect!
raise ValueError("Only properties allowed")
return super().__new__(cls, name, bases, attrs)
class MyReadOnlyConst(metaclass=Meta):
__metaclass__ = Meta
YES = property(lambda self: 1)
NO = property(lambda self: 0)
DUNNO = property(lambda self: 42)
THROWS_ERROR = 666
c2 = MyReadOnlyConst()
print(c2.THROWS_ERROR)
#c2.THROWS_ERROR = 777
#print(c2.THROWS_ERROR)
Thank you in advance and sorry about the large amount of code!
Albert-Jan
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