Switch statement

Summary

Adds a switch statement to Python.

Limitation: a switch statement cannot contain another switch statement.

Source code

There have been 2 PEPs looking at the possibility of adding a switch statement to Python. First, in 2001, PEP 275 made this suggestion. This was later superceded by PEP 3103 written in 2006 by Guido van Rossum and, after a straw poll during his keynote at PyCon 2007, he rejected it.

In order to “play” with the syntax and try to evaluate the benefit, I decided to implement it. PEP 3103 looks at various possible syntax, and I chose the one which seemed the most in line with normal Python syntax, namely version 1 B. The implementation meant to replace code like:

switch EXPR:
    case EXPR_1:
        SUITE
    case EXPR_2:
        SUITE
    case in (EXPR_3, EXPR_4, ...):
        SUITE
    ...
    else:
        SUITE

by:

var_name = EXPR
if var_name == EXPR_1:
        SUITE
elif var_name == EXPR_2:
        SUITE
elif var_name in (EXPR_3, EXPR_4, ...):
        SUITE
else:
        SUITE
del var_name

where var_name is a unique variable name chosen randomly. Note that I didn’t bother to dedent the inner SUITE as it is never shown to the end user and would only complicate the code.

Result

The actual result is not particularly enlightning. PEP 3103 considers the fact that each SUITE has two levels of indentation to be a downside. Personally, after writing some testing code with it (not saved anywhere), I found the fact that, due to having all the case statements indented make the code stand out better than having a traditional if/elif/else suite; so, I consider the extra indentation to be a bonus. However, perhaps I might feel otherwise if I always limited myself to having line with no more than 79 characters which was the norm when the PEP was written.

switch.py

Implements something similar to version 1.B of PEP 3103

ideas.examples.switch.add_hook(predictable_names=False, **_kwargs)

Creates and adds the import hook in sys.meta_path

ideas.examples.switch.transform_source(source, callback_params=None, **_kwargs)

Replaces code like:

switch EXPR:
    case EXPR_1:
        SUITE
    case EXPR_2:
        SUITE
    case in EXPR_3, EXPR_4, ...:
        SUITE
    ...
    else:
        SUITE

by:

var_name = EXPR
if var_name == EXPR_1:
        SUITE
elif var_name == EXPR_2:
        SUITE
elif var_name in EXPR_3, EXPR_4, ...:
        SUITE
else:
        SUITE
del var_name

Limitation: switch blocks cannot be part of a SUITE of another switch block.