On Sat, Mar 28, 2015 at 9:53 AM, Matthew Rocklin <mrocklin@gmail.com> wrote:
Responding to comments off list:

I'm not referring to C-style preprocessor macros, I'm referring to macros historically found in functional languages and commonly found in many user-targeted languages built in the last few years.

Do you have examples and references? IIRC there's something named macros in Scheme but Scheme, unlike Python, completely unifies code and data, and there is a standard in-memory representation for code.
The goal is to create things that look like functions but have access to the expression that was passed in.  

Some examples where this is useful:

    plot(year, miles / gallon)  # Plot with labels determined by input-expressions, e.g. miles/gallon

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

    run_concurrently(f(x), f(y), f(z))  # Run f three times in three threads controlled by run_concurrently

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.  

None of the examples need the syntax tree though. The first wants the string, the last probably just want a way to turn an argument into a lambda.
There are lots of arguments against this, mostly focused around potential misuse.  I'm looking for history of such arguments and for a general "Yes, this is theoretically possible" or "Not a chance in hell" from the community.  Both are fine.

I don't think this is a mainline need in Python, so it's probably both. :-)
--Guido van Rossum (python.org/~guido)