[Python-Dev] decorators (not transformations) on functions vs classes (original) (raw)

Jewett, Jim J jim.jewett at eds.com
Fri Mar 26 13:09:40 EST 2004

I would also guess that where people are using function attributes, there's a good chance that they're using decorators as well, since they probably need to do something with the attributes.

Depends entirely on the usage.

An attribute affects how the object looks when viewed as data. For a function, that is all it does; it does not affect semantics. For a class, it can also affect how the class acts.

If the object is just for storage/persistence, it is probably a class rather than an function. (Example: Values class of optparse)

class foo:
    pass
foo.x = 27

has some meaning. Any subclass or instance of foo (including those already created) will now have an attribute of x, with a default value of 27. Methods can use this value. Other code may (idiomatically) use the value to control its own settings.

def bar():
    pass
bar.x = 27

is not so useful.

There is no intermediate "self" scope between function locals and global, so bar itself can't see x. Functions do not inherit, so there won't be any instances or subfunctions that could be affected. Using a function's attributes to control another function's execution path is surprising.

For a function, the attribute is only meaningful when viewing the object as data rather than code. Many programmers take this opportunity to replace the function entirely with a class; others use naming conventions. If these solutions aren't careful enough then you may not want to trust an attribute; you should run the function on known input and test the output. (And then maybe cache the answer on the function, hoping that no one else will change it, even if they would not have followed the naming convention.)

-jJ

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