[Python-3000] Requiring () around lambda's argument list

[Guido van Rossum]

I'm running into something that may make me require parentheses around
lambda's argument list. Basically, I'm settled on the following syntax
for adding optional annotations (formerly known as type declarations
:-) to parameters definitions:

  def foo(arg1name: typeexpr = defaultexpr, arg2: typeexpr =
defaultexpr, ...): ...

But the same syntax production that's used for formal parameters
inside parentheses is also used *without* parentheses in the syntax
for lambda. Unfortunately the above extension makes it ambiguous; when
the parser sees

  lambda arg1name : ...

it can't tell whether the colon is the colon that ends the formal
parameter list or whether it's a colon that introduces an optional
parameter annotation.

I really don't want to restart the syntax debate for parameter
annotations, so I feel forced to require parentheses around lambda's
parameter list.

Fortunately this is at least one of those things that we can fix with
a purely syntactical source code transformation...

--
--Guido van Rossum (home page: http://www.python.org/~guido/)