Q: Python 2.0 preliminary features?

[Jeremy Hylton]

>>>>> "MS" == skaller  <skaller at maxtal.com.au> writes:

  MS> Michael Hudson wrote:
  >>  skaller <skaller at maxtal.com.au> writes:

  >> Hold it there a second; you are proposing that knowing that x has
  >> a certain type in the expression
  >> 
  >> x = x + "hello"
  >> 
  >> has an impact on inference?

  MS> 	Yes. Rather, the type of x is _deduced_ from the fact that
  MS> operator + must be string concatenation in this context, since
  MS> the second argument is a string, and thus x must be a string.

I assume you're handling some of the special cases, and I'd be
interested to hear how.

It is easy enough to write a class that will do something when added
to a string.  Arbitrary example:

class Foo:
    def __init__(self):
            self.strings = []
    def __add__(self, other):
            new = Foo() 
            new.strings = self.strings[:]
            new.strings.append(other)
            return new

x = Foo()
x = x + "hello"

I assume you can do some analysis to discover that x *might* be a
class that implements __add__.  How specific is your analysis?  If you
see a class like Foo, are all bets off about the meaning of x +
"hello"?

The second variation is a library that accepts some object as an
argument and adds "hello" to it.  Do you do whole-program analysis to
determine whether any client of the library might pass something other
than a string?  What are the implications of that for writing a
generic library?

Jeremy