[Python-ideas] Backward-incompatible changes for Python 4

Jelle Zijlstra jelle.zijlstra at gmail.com
Mon Apr 1 16:05:30 EDT 2019

El lun., 1 abr. 2019 a las 7:28, Antoine Pietri (<antoine.pietri1 at gmail.com>)

> While the switch to Python 3 did an excellent job in removing some of
> the old inconsistencies we had in the language, pretty much everyone
> agrees that some other backwards-incompatible changes could be made to
> remove some old warts and bring even more consistency to Python.
> Since Python 4 is getting closer and closer, I think it’s time to
> finally discuss some of the most obvious changes we should do for
> Python 4. Here is the list I compiled:
> - The / operator returns floats, which loses information when both of
> the operands are integer. In Python 4, “1 / 2” should return a
> decimal.Decimal. To ease the transition, we propose to add a new “from
> __future__ import decimal_division” in Python 3.9 to enable this
> behavior.
More broadly, one of the best changes in Python 3 was the sanitization of
the string/unicode logic: in Python 2 str and unicode were
mostly-but-not-always interchangeable, but not always, and that led to a
lot of hard to debug errors. Python 3 fixed this by separating the two more
cleanly. Python 4 has the opportunity to do something similar to separate
out another pair of easily confused types: int and float.

Broadly speaking, we should use float for human-understandable numbers, and
int for things that map directly to memory offsets in the computer, and we
should avoid mixing them. This suggests the following changes:
- int + float (and generally any mixed operation between ints and floats)
should throw a TypeError
- len() should return a float
- list.__getitem__ should only accepts ints, not floats
- integer overflow should use two's complement wraparound instead of
infinite precision

> - As most of the Python ecosystem is moving towards async, some of the
> old I/O-blocking APIs should be progressively migrated to an async by
> default model. The most obvious candidate to start this transition is
> the print function, which blocks on the I/O of flushes. We propose to
> make “print” an async coroutine. In Python 3.9, this feature could be
> optionally enabled with “from __future__ import print_coroutine”.
> - To ease compatibility with the Windows API, the PyUnicode* objects
> should be internally represented as an array of uint16_t, as it would
> avoid the conversion overhead from UCS. CPython migration details are
> left as an exercise for the developer.
> We think more changes are obviously warranted (e.g adding a new string
> formatting module, changing the semantic of the import system, using
> := in with statements...), but these changes will need specific
> threads of their own.
> So, can you think of other backward-incompatible changes that should
> be done in Python 4? Don't hesitate to add your own ideas :-)
> Thanks,
> --
> Antoine Pietri
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