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Yuval Pruss
Yuval Pruss

Reputation: 9826

Need type annotation for variable in python 3.5 code

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?

Upvotes: 26

Views: 47384

Answers (4)

jOOsko
jOOsko

Reputation: 668

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

Upvotes: 43

Medhat Gayed
Medhat Gayed

Reputation: 2813

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

Upvotes: 1

porto
porto

Reputation: 585

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.

Upvotes: 3

Dimitris Fasarakis Hilliard
Dimitris Fasarakis Hilliard

Reputation: 160417

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.

Upvotes: 11

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