Python Macros: Use Cases?

Question

If Python had a macro facility similar to Lisp/Scheme (something like MetaPython), how would you use it?

If you are a Lisp/Scheme programmer, what sorts of things do y

Answer 1

Currently, the only way features can be added to Python the language is through a PEP (Python Enhancement Proposal). This can be slow, and doesn't help you in the cases when you want to add a feature to the language that is only useful for your use case.

For example, there's a PEP to add a do-while loop. This will probably be added to Python, but the PEP was created in 2003. I'd like to write do-while loops today, and I could do that if Python had macros.

Similarly, there was a PEP to add labelled break and continue but this was rejected. If labelled break statements would make my code clearer, I could implement them with a macro.

PEPs aside, I would also like an unless macro. Rather than writing:

if not is_superman():
    dodge_bullet()

I could write:

unless is_superman():
    dodge_bullet()

I'd like a case macro (often called cond in Lisp). Rather than writing:

if x == FOO:
    do_stuff_with_foos()
elif x == BAR:
    do_stuff_with_bars()
elif x == BAZ:
    do_stuff_with_bazs()

I could write:

switch x:
   case FOO:
       do_stuff_with_foos()
   case BAR:
       do_stuff_with_bars()
   case BAZ:
       do_stuff_with_bazs()

These would be straightforward to implement as macros. More complex, useful macros would include:

• Ruby style string interpolation e.g. "hello there {user}" (probably best implemented as a reader macro)
• Pattern matching

Currently, these are only features in other languages. With macros, I could add them to Python. I could even write PEPs that included an example implementation. (Some PEPs already do this, but they are forced to modify the C source of the interpreter itself.)