[Python-Dev] bug in PEP 318 (original) (raw)

Phillip J. Eby pje at telecommunity.com
Tue May 30 23:17:53 CEST 2006

At 08:56 PM 5/30/2006 +0200, Alexander Bernauer wrote:

Hi

I found two bugs in example 4 of the PEP 318 [1]. People on #python pointed me to this list. So here is my report. Additionally I appended an afaics correct implementation for this task. [1] http://www.python.org/dev/peps/pep-0318/ Bug 1) The decorator "accepts" gets the function which is returned by the decorator "returns". This is the function "newf" which is defined differently from the function "func". Because the first has an argument count of zero, the assertion on line 3 is wrong.

The simplest fix for this would be to require that returns() be used before accepts().

Bug 2) The assertion on line 6 does not work correctly for tuples. If the second argument of "isinstance" is a tuple, the function returns true, if the first argument is an instance of either type of the tuple.

This is intentional, to allow saying that the given argument may be (for example) an int or a float. What it doesn't support is nested-tuple arguments, but that's a reasonable omission given the examples' nature as examples.

def check(*args, **kwds): checktype(args, self.types) self.func(*args, **kwds)

This needs a 'return', since it otherwise loses the function's return value.

To be honest, I didn't understand what the purpose of setting "funcname" is, so I left it out. If its neccessary please feel free to correct me.

It's needed for Python documentation tools such as help(), pydoc, and so on display something more correct for the decorated function's documentation, although also copying the doc attribute would really also be required for that.

In contrast to tuples lists and dictionaries are not inspected. The reason is that I don't know how to express: "the function accepts a list of 3 or more integers" or alike. Perhaps somebody has an idea for this.

I think perhaps you've mistaken the PEP examples for an attempt to implement some kind of typechecking feature. They are merely examples to show an idea of what is possible with decorators, nothing more. They are not even intended for anybody to actually use!

I wonder, how it can be, that those imho obvious mistakes go into a PEP and stay undetected there for almost 3 years.

That's because nobody uses them; a PEP example is not intended or required to be a robust production implementation of the idea it sketches. They are proofs-of-concept, not source code for a utility. If they were intended to be used, they would be in a reference library or in the standard library, or otherwise offered in executable form.

More information about the Python-Dev mailing list