Suggestion for a Python Extension

Luis Cortes lcortes at flash.net
Thu Mar 23 03:55:05 CET 2000


Hello,

    I would like to suggest an extension to python which I think would make it a
little nicer.  I know there might be other ways to do this, but I think this is
pretty intuative.  I have included the code for those people that might like to try
it out.  I basically got the idea from Prolog.  In Prolog you might have something
like this:

fact( 1, 3 ).
fact( 2, 3 ).
fact( 3, 4 ).

you can query the relation with a simple statement like so:

? fact( _X, 3 )

which returns:

fact( 1, 3 ).
fact( 2, 3 ).

I thought it might be nice to do the same with Python.  Except that facts are really
lists in Python.  For example:

fact = [ (1,3), (2,3), (3,4)]

ans = fact( SomeAnonymousVariable, 3 )

so that ans has:

ans = [ (1,3), (2,3) ]

I have create a couple of classes in Python to allow me to simulate what I want, but
it would be nice if it where built-in instead, using my classes the last example
would be:

fact = plist( [ (1,3), (2,3), (3,4)] )

ans = fact.query( Anonymous(), 3 )

so that ans has:

ans = [ (1,3), (2,3) ]

other examples might be:

ans = fact.query( 1, 3 )

so that ans has:

ans = [ (1,3) ]

OR:

ans = fact.query( 1, Anonymous() )

so that ans has:

ans = [ (1,3), (2,3) ]

OR

ans = fact.query( Anonymous(), Anonymous() )

so that ans has:

ans = [ (1,3), (2,3), (3,4) ]



What do you'all think?


best regards,
Luis.


P.S.  Please also reply to sender with your comments, suggests, and corrections.
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# -------------------------------------------------------
# copyleft: Author Luis Cortes, 2000
# -------------------------------------------------------

# -------------------------------------------------------
# Empty class to select on so that I know what is an anonymous
# variable and what is not.  This might be replaced by None, but 
# I am not sure of the semantics of None enough to know what
# will happen in all cases.  This makes it obvious.
# -------------------------------------------------------

class Anonymous:
	""" Empty class """


# ------------------------------------------------------
# plist - stands for Prolog List.  We are allowed to use the Anonymous class
# so that we can Query the list.  Currently I have a few assumptions:
#
# 1.  you must have exactly the correct number of arguments to each tuple as
#      the number of arguments for query.  ( this might be relaxed )
# 2.  It is a separate function, it might be nice if it was part of the list data-structure.
#      i.e. 
#	a = [ (1,2), (2,3), (5,3) ]
#	b = a( Anonymous(), 3 )
#
#	therefore b = [ (2,3), (5,3) ]
#
# 3.  It might be nice if we could inherit from the list data-structure.
#    i.e.
#	class plist( type( [])):
#    		def __init__( ....)
#    		....
#    		
#    	
# 4.  I don't know if python currently supports something like this already, I think
#	mxtools might have several functions that do similar things, but one nice
#	property about doing it this way is that if it is built into the language, then
#	we have an easy way to query a list or tuple without having to remember
#	the name of another function (or functions).
#
# ------------------------------------------------------
class plist:
	_oMyList=[]

	def __init__( self, oList ):
		assert( type( oList ) == type ([]) )
		self._oMyList = oList


	def query( self, *parms ):
		NewTuple = []
		
		# for all tuples in our list do ...
		for grp in self._oMyList:
			bNoMatch = 0
			
			# Check All Parameters to see if 1st assumption is true
			for i in range(len(grp)):
				if  not ( isinstance( parms[i], Anonymous ) or parms[i] == grp[i]):
					bNoMatch = 1
					break
			
			if (not bNoMatch):
				NewTuple.append( grp )
		return NewTuple

def main():
	test =  plist( [ (1,2), (0,4), (1,3), (4,4),(1,5)] )
	print test.query( 4, 4 )
	print test.query( Anonymous(), 4 )
	print test.query( 1, Anonymous())
	print test.query( Anonymous(), Anonymous())
	
	print "done"


main()


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