[Python-ideas] Anonymous blocks (again):

[Stephen J. Turnbull]

Martin Morrison writes:

 > That is instead of a decorator-like syntax, make the "in" keyword
 > reusable to introduce a new block, whose "argument" is a statement
 > that can forward reference some names, which are then defined
 > within the block. This allows multiple temporary names to be
 > defined (each in a separate statement within the block).

This idea and its presumed defects are described (using the "given"
syntax of PEP 3150) in the section "Using a nested suite" in PEP 403.

 > Some further thought is required on whether only def (and maybe
 > class) statements should be allowed within the "in" block. Although
 > I guess there's technically nothing wrong with:
 > 
 > in x = y + z:
 >     y = 12
 >     z = 30
 > 
 > Other than it's a very verbose way of doing something simple. ;-)

Violates TOOWTDI according to PEP 403.

David Mertz and Juancarlo Añez riff on the theme:

 >[Why not spell it something like]:
 >
 > in x = do_something(in_arg, success_hdlr, error_hdlr, const) let:
 >     def success_hdlr(result):
 >         ... # Do something with result
 >     def error_hdlr(error):
 >         ... # Do something with error
 >     const = 42

(Note the "let" at the end of the "in" clause.)

Python doesn't use redundant keywords for a single construct.
"let" is redundant with the following "def"s.  On top of that, "let"
being a new keyword will kill this syntax, I think.