[Python-Dev] PEP 526 (variable annotations) accepted provisionally

Guido van Rossum guido at python.org
Wed Sep 7 14:18:34 EDT 2016

I'm accepting PEP 526 provisionally.

I am personally confident that this PEP is adding a useful new feature
to the language: annotations that can be used by a wide variety of
tools, whether off-line type checkers or frameworks that add runtime
checking (e.g. traits or traitlets).

The provisional status reflects the understanding that minor details
of the proposed syntax and its runtime effects may still have to
change based on experience during the 3.6 life cycle. (For example,
maybe we end up not liking ClassVar, or maybe we'll decide we'll want
to support `x, y, z: T` after all.)

There's been some quite contentious discussion about the PEP, on and
off python-dev, regarding how the mere presence of annotation syntax
in the language will change the way people will see the language. My
own experience using mypy and PyCharm has been quite different:
annotations are a valuable addition for large code bases, and it's
worth the effort to add them to large legacy code bases (think
millions of lines of Python 2.7 code that needs to move to Python 3 by
2020). The effect of this has been that engineers using Python are
happier and more confident that their code works than before, have an
easier time spelunking code they don't know, and are less afraid of
big refactorings (where conversion to Python 3 can be seen as the
ultimate refactoring).

I should blog about our experience at Dropbox; I hope the Zulip open
source folks (not at Dropbox) will also blog about their experience.
In the meantime you can read Daniel F. Moisset's three-part blog about
adding annotations to pycodestyle (formerly pep8) here:


If you want to see a large open source code base that's annotated for
mypy (with 97% coverage), I recommend looking at Zulip:

Finally, some of us are starting a new (informational) PEP to set
expectations for how type checkers should make use of the annotation
syntax standardized by PEP 484 and PEP 526. This is going to take more
time, and new collaborators are welcome here:
https://github.com/ilevkivskyi/peps/blob/new-pep/pep-0555.txt. (Mark,
I really hope you'll accept the invitation to participate. Your
experience would be most welcome.)

--Guido van Rossum (python.org/~guido)

More information about the Python-Dev mailing list