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PEP 646: Add note of alternatives that exist (#1884)

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@ -698,6 +698,31 @@ between old unannotated code and new versions of libraries which do use
these type annotations.
Alternatives
============
It should be noted that the approach outlined in this PEP to solve the
issue of shape checking in numerical libraries is *not* the only approach
possible. Examples of lighter-weight alternatives based on *runtime* checking include
ShapeGuard [#shapeguard]_, tsanley [#tsanley]_, and PyContracts [#pycontracts]_.
While these existing approaches improve significantly on the default
situation of shape checking only being possible through lengthy and verbose
assert statements, none of them enable *static* analysis of shape correctness.
As mentioned in `Motivation`_, this is particularly desirable for
machine learning applications where, due to library and infrastructure complexity,
even relatively simple programs must suffer long startup times; iterating
by running the program until it crashes, as is necessary with these
existing runtime-based approaches, can be a tedious and frustrating
experience.
Our hope with this PEP is to begin to codify generic type annotations as
an official, language-supported way of dealing with shape correctness.
With something of a standard in place, in the long run, this will
hopefully enable a thriving ecosystem of tools for analysing and verifying
shape properties of numerical computing programs.
Backwards Compatibility
=======================
@ -809,6 +834,12 @@ References
.. [#arbitrary_len] Discussion on Python typing-sig mailing list:
https://mail.python.org/archives/list/typing-sig@python.org/thread/SQVTQYWIOI4TIO7NNBTFFWFMSMS2TA4J/
.. [#tsanley] tsanley: https://github.com/ofnote/tsanley
.. [#pycontracts] PyContracts: https://github.com/AndreaCensi/contracts
.. [#shapeguard] ShapeGuard: https://github.com/Qwlouse/shapeguard
.. _cpython/23527: https://github.com/python/cpython/pull/24527
.. _mrahtz/cpython/pep637+646: https://github.com/mrahtz/cpython/tree/pep637%2B646