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?

jOOsko · Accepted Answer · 2021-09-27 07:32:28Z

43

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

edited Sep 27, 2021 at 7:32

answered Jun 18, 2017 at 13:52

jOOsko

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2 Comments

edesz Over a year ago

Exactly what I was looking for. One note, on Python 3.6, flake8 says that 2 spaces are needed before an inline comment, so (for example) I had to change x = 5 # type: int (single space before #) to x = 5 # type: int (double space before #).

demberto Over a year ago

Pylance asks for the same

Dimitris Fasarakis Hilliard · Accepted Answer · 2017-06-18 14:01:32Z

11

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.

answered Jun 18, 2017 at 14:01

Dimitris Fasarakis Hilliard

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1 Comment

Dimitris Fasarakis Hilliard Over a year ago

@YuvalPruss If you plan on using variable annotations, take a look here stackoverflow.com/questions/39971929/… for a bit more on it.

porto · Accepted Answer · 2019-09-16 13:12:39Z

3

If you are having blank value, you must define the type of variable. For example:

my_val: str = ""
my_val1: dict = {}
my_val2: list = []

etc. In your case, I will consider changing the version of python to 3.6 and update code is required.

edited Sep 16, 2019 at 13:12

answered Sep 16, 2019 at 13:02

porto

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Comments

Medhat Gayed · Accepted Answer · 2021-08-13 01:23:32Z

1

The mypy docs mention that empty collections often need to have type annotations for certain complex cases.

https://mypy.readthedocs.io/en/stable/common_issues.html#types-of-empty-collections

answered Aug 13, 2021 at 1:23

Medhat Gayed

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