Need type annotation for variable in python 3.5 code

Question

I am using mypy on my python 3.5 code, and I got a lot of messages which look like this:

file:line number: error: Need type annotation for variable

But I read about the new features in python 3.6 that it introduced the syntax for variable annotations only in python 3.6:

PEP 484 introduced the standard for type annotations of function parameters, a.k.a. type hints. This PEP adds syntax to Python for annotating the types of variables including class variables and instance variables...

And if I am trying to add variable type annotations to my variables in the python 3.5 program, It throws SyntaxError.

What should I do? Ignore this messages? Update to python 3.6? Why the mypy compiles my code like it's written in python 3.6?

Answer 1

Use comments to annotate variable type

x = 5  # type: int
my_list = []  # type: List[str]

Check cheat sheet

https://mypy.readthedocs.io/en/latest/cheat_sheet_py3.html

Answer 2

Your code is confusing the type inference that mypy tries to do. For example, redefining a name as in the following snippet, doesn't allow mypy to deduce the type of f:

f = []
f = {}

Since it can't understand what the type of f is supposed to be, it complains and tells you that it needs an annotation for the variable. You can explicitly provide a type-hint with:

• A type comment for Python 3.5.
• A variable annotation for Python 3.6

mypy isn't compiling in 3.6, this error exists in both versions. The difference is in how you can tackle it.