Class-level read-only properties in Python

python

Is there some way to make a class-level read-only property in Python? For instance, if I have a class Foo, I want to say:

x = Foo.CLASS_PROPERTY

but prevent anyone from saying:

Foo.CLASS_PROPERTY = y

EDIT: I like the simplicity of Alex Martelli's solution, but not the syntax that it requires. Both his and ~unutbu's answers inspired the following solution, which is closer to the spirit of what I was looking for:

class const_value (object):
    def __init__(self, value):
        self.__value = value

    def make_property(self):
        return property(lambda cls: self.__value)

class ROType(type):
    def __new__(mcl,classname,bases,classdict):
        class UniqeROType (mcl):
            pass

        for attr, value in classdict.items():
            if isinstance(value, const_value):
                setattr(UniqeROType, attr, value.make_property())
                classdict[attr] = value.make_property()

        return type.__new__(UniqeROType,classname,bases,classdict)

class Foo(object):
    __metaclass__=ROType
    BAR = const_value(1)
    BAZ = 2

class Bit(object):
    __metaclass__=ROType
    BOO = const_value(3)
    BAN = 4

Now, I get:

Foo.BAR
# 1
Foo.BAZ
# 2
Foo.BAR=2
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
# AttributeError: can't set attribute
Foo.BAZ=3
#

I prefer this solution because:

The members get declared inline instead of after the fact, as with type(X).foo = ...
The members' values are set in the actual class's code as opposed to in the metaclass's code.

It's still not ideal because:

I have to set the __metaclass__ in order for const_value objects to be interpreted correctly.
The const_values don't "behave" like the plain values. For example, I couldn't use it as a default value for a parameter to a method in the class.

The existing solutions are a bit complex -- what about just ensuring that each class in a certain group has a unique metaclass, then setting a normal read-only property on the custom metaclass. Namely:

>>> class Meta(type):
...   def __new__(mcl, *a, **k):
...     uniquemcl = type('Uniq', (mcl,), {})
...     return type.__new__(uniquemcl, *a, **k)
... 
>>> class X: __metaclass__ = Meta
... 
>>> class Y: __metaclass__ = Meta
... 
>>> type(X).foo = property(lambda *_: 23)
>>> type(Y).foo = property(lambda *_: 45)
>>> X.foo
23
>>> Y.foo
45
>>>

this is really much simpler, because it's based on nothing more than the fact that when you get an instance's attribute descriptors are looked up on the class (so of course when you get a class's attribute descriptors are looked on the metaclass), and making class/metaclass unique isn't terribly hard.

Oh, and of course:

>>> X.foo = 67
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: can't set attribute

just to confirm it IS indeed read-only!

The ActiveState solution that Pynt references makes instances of ROClass have read-only attributes. Your question seems to ask if the class itself can have read-only attributes.

Here is one way, based on Raymond Hettinger's comment:

#!/usr/bin/env python
def readonly(value):
    return property(lambda self: value)

class ROType(type):
    CLASS_PROPERTY = readonly(1)

class Foo(object):
    __metaclass__=ROType

print(Foo.CLASS_PROPERTY)
# 1

Foo.CLASS_PROPERTY=2
# AttributeError: can't set attribute

The idea is this: Consider first Raymond Hettinger's solution:

class Bar(object):
    CLASS_PROPERTY = property(lambda self: 1)
bar=Bar()
bar.CLASS_PROPERTY=2

It shows a relatively simple way to give bar a read-only property.

Notice that you have to add the CLASS_PROPERTY = property(lambda self: 1) line to the definition of the class of bar, not to bar itself.

So, if you want the class Foo to have a read-only property, then the parent class of Foo has to have CLASS_PROPERTY = property(lambda self: 1) defined.

The parent class of a class is a metaclass. Hence we define ROType as the metaclass:

class ROType(type):
    CLASS_PROPERTY = readonly(1)

Then we make Foo's parent class be ROType:

class Foo(object):
    __metaclass__=ROType

Class-level read-only properties in Python

Related

Recent Posts