[Python-Dev] PEP 460 reboot (original) (raw)

Jim Jewett jimjjewett at gmail.com
Wed Jan 15 05:08:48 CET 2014

On Tue, Jan 14, 2014 at 3:06 PM, Guido van Rossum <guido at python.org> wrote:

Personally I wouldn't add any words suggesting or referring to the option of creation another class for this purpose. You wouldn't recommend subclassing dict for constraining the types of keys or values, would you?

Yes, and it is so clear that I suspect I'm missing some context for your question.

Do I recommend that each individual application should create new concrete classes instead of just using the builtins? No.

When trying to understand (learn about) the text/binary distinction, I do recommend pretending that they are represented by separate classes. Limits on the values in a bytearray are NOT the primary reason for this; the primary reason is that operations like the literal representation or the capitalize method are arbitrary nonsense unless the data happens to be representing ASCII.

sound_sample.capitalize()  -- syntactically valid, but semantic garbage
header.capitalize() -- OK, which implies that data is an instance

of something more specific than bytes.

Would I recommend subclassing dict if I wanted to constrain the key types? Yes -- though MutableMapping (fewer gates to guard) or the upcoming TransformDict would probably be better still.

The existing dict implementation itself effectively uses (hidden, quasi-)subclasses to restrict types of keys strictly for efficiency. (lookdict* variants)

-jJ

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