[Python-Dev] Updated version of PEP 526 (Syntax for Variable Annotations) (original) (raw)

Guido van Rossum guido at python.org
Fri Sep 2 18:19:44 EDT 2016

We've prepared an updated version of PEP 526: https://www.python.org/dev/peps/pep-0526/

This changes the title to "Syntax for Variable Annotations", now that we've settled on global, class, instance, and local variables as the things you might annotate.

There is one substantial change: where the previous version supported only

NAME: TYPE TARGET: TYPE = VALUE

the new PEP removes the distinction and just allows

TARGET: TYPE [= VALUE]

This simplifies the explanations a bit and enables type checkers to support separating the annotation from the assignment for instance variables in the init method, e.g.

def init(self): self.name: str if : self.name = else: self.name =

The other changes are all minor editing nits, or clarifications about the scope of the PEP. The scope clarification is important: while I really want the new syntax settled in 3.6, I have no intention to pin down the way type checkers use this syntax, apart from the observation that

TARGET: TYPE = VALUE

is just meant as a cleaner way to write what you'd currently write using PEP 484 as

TARGET = VALUE # type: TYPE

The PEP does not claim that you have to use variable annotations -- in fact we'd prefer that they were unnecessary, but the prevalence of type comments in code we've annotated so far makes it clear that there are plenty of uses for them, and we'd rather have a clean syntax for them that tools can see in the AST.

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

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