[Python-Dev] Python Library Addition: First-class Procedure Signatures (original) (raw)

Isaac Morland ijmorlan at cs.uwaterloo.ca
Wed Nov 14 19:30:21 CET 2007

For another project (see my previous email on named tuples), I needed to represent procedure signatures, and use them to expand arguments into the dictionary of values that exists when execution of a procedure starts. To my surprise, this capability didn't seem to be provided by the Python library, even though it clearly is present within the Python system somewhere.

So I wrote a Signature class. Instances of the class represent all the information present between the parentheses of a procedure definition. Properties are provided to get the information out, and an expand_args method can be called to expand arguments into a dictionary. This expand_args method implements (if I've done it right) the argument conversion part of section 5.3.4 of the Python Reference Manual (http://docs.python.org/ref/calls.html).

I've put the code below, but I wonder if the real solution is just to create an interface to already-existing capability? It occurs to me that the implementation is likely to be in the interpreter itself and not written in Python.

One possible improvement (and I'm not sure it's better, so I'm just putting it out there): perhaps expand_args should be renamed to call. Then essentially a Signature object would be a procedure whose body is just "return locals ()".

class Signature (object): def init (self, argnames, excessargs=None, excesskeys=None, defaults=None): self.__argnames = tuple (argnames) self.__excessargs = excessargs self.__excesskeys = excesskeys if defaults is None: defaults = {} self.__defaults = dict (defaults)

 @property
 def argnames (self):
     return self.__argnames

 @property
 def excessargs (self):
     return self.__excessargs

 @property
 def excesskeys (self):
     return self.__excesskeys

 def defaults (self):
     return dict (self.__defaults)

 def expand_args (self, *args, **keys):
     # Start with defaults
     result = self.defaults ()

     # Assign positional arguments
     for i in range (min (len (args), len (self.argnames))):
         result[self.argnames[i]] = args[i]

     # Assign keyword arguments
     for arg in self.argnames:
         if arg in keys:
             if arg in result:
                 raise TypeError
             result[arg] = keys[arg]
             del keys[arg]

     # Check for missing arguments
     for i in range (len (args), len (self.argnames)):
         if not self.argnames[i] in result:
             raise TypeError

     # Excess positional arguments (*args parameter)
     if self.excessargs is None:
         if len (args) > len (self.argnames):
             raise TypeError
     else:
         result[self.excessargs] = args[len (self.argnames):]

     # Excess keyword arguments (**keys parameter)
     if self.excesskeys is None:
         if keys:
             raise TypeError
     else:
         result[self.excesskeys] = keys

     return result

Isaac Morland CSCF Web Guru DC 2554C, x36650 WWW Software Specialist

More information about the Python-Dev mailing list