[Python-ideas] History on proposals for Macros?

[Steven D'Aprano]

On Sat, Mar 28, 2015 at 09:53:48AM -0700, Matthew Rocklin wrote:
[...]
> The goal is to create things that look like functions but have access to
> the expression that was passed in.

>     assertRaises(ZeroDivisionError, 1/0)  # Evaluate the rhs 1/0 within
> assertRaises function, not before

> Generally one constructs something that looks like a function but, rather
> than receiving a pre-evaluated input, receives a syntax tree along with the
> associated context.  This allows that function-like-thing to manipulate the
> expression and to control the context in which the evaluation occurs.

How will the Python compiler determine that assertRaises should receive 
the syntax tree rather than the evaluated 1/0 expression (which of 
course will raise)? The information that assertRaises is a "macro" is 
not available at compile time.

I really like the idea of delaying the evaluation of certain expressions 
until a time of the caller's choosing, but I don't see how to make that 
work automatically. Of course we can do it manually by wrapping the 
expression in a function, or by writing it as a string and compiling it 
with compile() for later eval'ing, but the sort of thing you show above 
strikes me as fundamentally incompatible with Python's execution model.

Happy to be proven wrong though.


-- 
Steve