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POClass-level read-only properties in Python
 

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  1. POClass-level read-only properties in Python
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    <p>Is there some way to make a class-level read-only property in <a href="http://en.wikipedia.org/wiki/Python%5F%28programming%5Flanguage%29" rel="nofollow noreferrer">Python</a>? For instance, if I have a class <code>Foo</code>, I want to say:</p> <pre><code>x = Foo.CLASS_PROPERTY </code></pre> <p>but prevent anyone from saying:</p> <pre><code>Foo.CLASS_PROPERTY = y </code></pre> <p><strong>EDIT:</strong> I like the simplicity of <a href="https://stackoverflow.com/questions/1735434/class-level-read-only-properties-in-python/1735726#1735726">Alex Martelli's solution</a>, but not the syntax that it requires. Both his and <a href="https://stackoverflow.com/questions/1735434/class-level-read-only-properties-in-python/1735615#1735615">~unutbu's answer</a>s inspired the following solution, which is <em>closer</em> to the spirit of what I was looking for:</p> <pre><code>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 </code></pre> <p>Now, I get:</p> <pre><code>Foo.BAR # 1 Foo.BAZ # 2 Foo.BAR=2 # Traceback (most recent call last): # File "&lt;stdin&gt;", line 1, in &lt;module&gt; # AttributeError: can't set attribute Foo.BAZ=3 # </code></pre> <p>I prefer this solution because:</p> <ul> <li>The members get declared inline instead of after the fact, as with <code>type(X).foo = ...</code></li> <li>The members' values are set in the actual class's code as opposed to in the metaclass's code.</li> </ul> <p>It's still not ideal because:</p> <ul> <li>I have to set the <code>__metaclass__</code> in order for <code>const_value</code> objects to be interpreted correctly.</li> <li>The <code>const_value</code>s 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.</li> </ul>
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    1. COWhy would anyone ever try to set Foo.CLASS_PROPERTY? It's typical style in Python to choose simplicity over planning for preventing a theoretical instance where someone does something stupid. There isn't anything you can do to keep people from doing stupid stuff with your code.
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      1. USMike Graham
    2. CO@Mike: You're right; common sense would tell people to not modify these properties. And yet, occasionally, they get modified. That's why I like for it to be spelled out (more explicitly than with all-caps). Same rationale as using private members. Explicit interfaces are good. Don't get me wrong: I like simplicity too. I'd much prefer if there were some built-in facility for this (like a @classproperty decorator or something). As it is, I'm not even sure I would use the above solution if it requires setting the `__metaclass__` each time. All-caps may win out, for now.
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    3. CONote to people from the future: Modern python versions can use `@property` in new-style classes: http://docs.python.org/library/functions.html#property
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