multimethod (or rather overloading) in Python

[anton muhin]

Dear pythonistas!
I'd like to emulate overloading in Python (like C++).
Let's consider an example:
class A(object): pass
class B(object): pass
Of course, there are some standard overloading implementations for 
Python. For example:

def overload(cls):
  def wrapper(f):
gl = f.func_globals
next = gl.get(f.func_name, failedOverload_(f.func_name))
def _(obj, *args, **kwargs):
  if isinstance(obj, cls):
return f(obj, *args, **kwargs)
  else:
return next(obj, *args, **kwargs)
return _
@overload(A)
def foo(_): print '[EMAIL PROTECTED]'
@overload(B)
def foo(_): print '[EMAIL PROTECTED]'
However, it obviously doesn't work for classes: I cannot overload 
instance methods with such a decorator.

The best way I found is closures. Unfortunatley, in this case I need a hack:
  gl = f.func_globals
turns into:
  gl = sys._getframe(1).f_locals
and with this hack one can make the following trick:
def poorManOverloadedMethods(someInfo):
  @overload(A)
  def _(_): print '%s: [EMAIL PROTECTED]' % someInfo
  @overload(B)
  def _(_): print '%s: [EMAIL PROTECTED]' % someInfo
  return _
PMOM = poorManOverloadedMethods('test')
...
Of course, I can imagine some metaclasses magic that would allow to code:
class MyClass(WithOverloading):
  @overloadMethod(A)
  def someMetod(self, _): ...
But it would rather convoluted: the best idea I have so far is to mangle 
 methods name in the manner most of C++ compilers do.

Is there better way? Can I unify both @overload and @overloadMethod?
with the best regards,
anton.
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Re: multimethod (or rather overloading) in Python

[anton muhin]

anton muhin wrote:
Correction:
Of course, I can imagine some metaclasses magic that would allow to code:
class MyClass(WithOverloading):
  @overloadMethod(A)
  def someMetod(self, _): ...
But it would rather convoluted: the best idea I have so far is to mangle 
 methods name in the manner most of C++ compilers do.
Stupid me. Of course, name magling is impossible and unnecessary. Sorry.
Still the question remains.
with the best regards,
anton.
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Re: [perl-python] exercise: partition a list by equivalence

[anton muhin]

Xah Lee wrote:
here's another interesting algorithmic exercise, again from part of a
larger program in the previous series.
Here's the original Perl documentation:
=pod
merge($pairings) takes a list of pairs, each pair indicates the
sameness
of the two indexes. Returns a partitioned list of same indexes.
For example, if the input is
merge( [ [1,2], [2,4], [5,6] ] );
that means 1 and 2 are the same. 2 and 4 are the same. Therefore
1==2==4. The result returned is
[[4,2,1],[6,5]];
(ordering of the returned list and sublists are not specified.)
=cut
Almost a joke:
from numarray import *
def merge(*pairs):
  flattened = reduce(tuple.__add__, pairs, tuple())
  m, M = min(flattened), max(flattened)
  d = M - m + 1
  matrix = zeros((d, d), type = Bool)
  for x, y in pairs:
X, Y = x - m, y - m
matrix[X, X] = 1
matrix[X, Y] = 1
matrix[Y, X] = 1
matrix[Y, Y] = 1
  while True:
next = greater(dot(matrix, matrix), 0)
if alltrue(ravel(next == matrix)):
  break
matrix = next
  results = []
  for i in range(d):
eqls, = nonzero(matrix[i])
if eqls.size():
  if i == eqls[0]:
results.append(tuple(x + m for x in eqls))
  return results
Disclaimer: I'm not an expert in numarray and suppose the something can 
be dramatically imporved.
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Re: multimethod (or rather overloading) in Python

[anton muhin]

Nick Coghlan wrote:
anton muhin wrote:
anton muhin wrote:
Correction:
Of course, I can imagine some metaclasses magic that would allow to 
code:

class MyClass(WithOverloading):
  @overloadMethod(A)
  def someMetod(self, _): ...
But it would rather convoluted: the best idea I have so far is to 
mangle  methods name in the manner most of C++ compilers do.

PEAK has a fairly sophisticated implementation of method dispatch you 
may want to look at.

http://peak.telecommunity.com/Articles/WhatisPEAK.html
http://dirtsimple.org/2004/11/generic-functions-have-landed.html
http://peak.telecommunity.com/doc/src/dispatch/__init__.html
I'm compelled to point out that PEAK should still be considered a 'work 
in progress', but PJE's ideas should help you out :)

Cheers,
Nick.
Thank you very much, Nick!
anton.
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Re: Syntax for extracting multiple items from a dictionary

[anton muhin]

Stefan Behnel wrote:

shark schrieb:
row = {"fname" : "Frank", "lname" : "Jones", "city" : "Hoboken", 
"state" :
"Alaska"}
cols = ("city", "state")

Is there a best-practices way to ask for an object containing only the 
keys
named in cols out of row? In other words, to get this:
{"city" : "Hoboken", "state" : "Alaska"}

Untested:
dict( (key,value) for (key,value) in row.iteritems() if key in cols )
Works in Py2.4
Stefan
Or dict((key, row[key]) for key in cols).
regards,
anton.
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