[Python-Dev] Re: Critique of PEP 622 (Structural Pattern Matching)

On Mon, Aug 17, 2020 at 8:11 AM Mark Shannon mark@hotpy.org wrote:

On 17/08/2020 3:08 pm, Henk-Jaap Wagenaar wrote:

...

Thanks for having a look! The example now looks like (looking at int case only, same applies to UID):

     case int():
         if value < 0:
             try:
                 self._fp.write(struct.pack('>Bq', 0x13, value))
             except struct.error:
                 raise OverflowError(value) from None
         elif value < 1 << 8:
             self._fp.write(struct.pack('>BB', 0x10, value))
         ...
         elif value < 1 << 64:
             self._fp.write(b'\x14' + value.to_bytes(16, 'big',

signed=True))

...

         else:
             raise OverflowError(value)

I was more thinking it would read/look something like:

     case int() if value < 0:
             try:
                 self._fp.write(struct.pack('>Bq', 0x13, value))
             except struct.error:
                 raise OverflowError(value) from None
     case int() if value < 1 << 8:
             self._fp.write(struct.pack('>BB', 0x10, value))
     ...
     case int() if value < 1 << 64:
             self._fp.write(b'\x14' + value.to_bytes(16, 'big',

signed=True))

...

     case int():
         raise OverflowError(value)

Which I think works as expected under the current PEP622?

That would work, but would be slower for the reference implementation due to the repeated `isinstance(value, int)` checks.

The PEP allows the compiler to generate optimized code that only checks once.

I think the repeated `int()` cases do not help readability. Which form do you think is more readable?

I find Henk-Jaap's version better, because the case blocks show the structure of the code better.