[Python-ideas] ideas for type hints for variable: beyond comments

ia n ian.team.python at gmail.com
Tue Sep 1 06:03:33 CEST 2015


mypy currently inspects the comment on the line of first assignment for the 
variables to be type hinted.

It is logical that at some time python language will add support to allow 
these type hints to move from comments to the code as has happened for 
'def' signatures.

One logical syntax would be to move from

i = 1           # Infer type int for i


i:int = 1       # no comment needed, but does not look attractive

The first question that arises is 'is the type inference legal for the additional uses.  Having a 'second use' flagged by warning or error by either an external typechecker or even the language itself could pick up on accidental reuse of a name, but in practice accidentally creating a new variable through a typo can be more common.

In python today the first use is the same as every other, so this change just does not feel comfortable.

The other question is 'what about globals and nonlocals?'.  Currently globals and nonlocals need a 'global' or 'nonlocal' statement to allow assignment, but what if these values are not assigned in scope?

What if we allowed 
global i:int


nonlocal i:int

and even

local i:int

Permitting a new keyword 'local' to me might bring far more symmetry 
between different cases.

It would also allow type hinting to be collected near the function 
definition and keep the type hinting clear of the main code.

Use of the 'local' keyword in the global namespace could indicate a value 
not accessible in other namespaces.

Personally I would like to go even further and allow some syntax to allow 
(or disable) flagging the use of new variables without type hinting as 
possible typos

I have a syntax in mind, but the idea is the discussion point, not the 
specific syntax.

Possibly what is here already is too much of a change of direction to 
consider for ideas already in progress?

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