from bash

mkdir test
cd test
python3 -m venv .env
source .env/bin/activate
python -m pip install flake8
python -m pip install flake8-mypy
code ./

install plugin

then install this in VSCode https://marketplace.visualstudio.com/items?itemName=donjayamanne.python
and config

settings

./.vscode/settings.json

{
    "python.envFile": "${workspaceRoot}/.env",
    "python.pythonPath": ".env/bin/python",
    "python.linting.flake8Enabled": true,
    "python.linting.pylintEnabled": false,
    "python.linting.mypyEnabled": true,
}

./.vscode/launch.json

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "Python",
            "type": "python",
            "request": "launch",
            "stopOnEntry": false,
            "pythonPath": "${config:python.pythonPath}",
            "program": "${file}",
            "cwd": "${workspaceRoot}",
            "env": {},
            "envFile": "${workspaceRoot}/.env",
            "debugOptions": [
                "WaitOnAbnormalExit",
                "WaitOnNormalExit",
                "RedirectOutput"
            ]
        }
    ]
}

OMG, this is Python 3 only!

https://pypi.python.org/pypi/flake8-mypy/17.3.3

Yes, so is mypy. Relax, you can run Flake8 with all popular plugins
as a tool perfectly fine under Python 3.5+ even if you want to analyze
Python 2 code. This way you’ll be able to parse all of the new syntax
supported on Python 3 but also effectively all the Python 2 syntax at
the same time.

By making the code exclusively Python 3.5+, I’m able to focus on the
quality of the checks and re-use all the nice features of the new
releases (check out pathlib) instead of wasting cycles on Unicode
compatibility, etc.

IDE & Linter Integrations

https://github.com/python/mypy#ide--linter-integrations

IDE & Linter Integrations

Mypy can be integrated into popular IDEs:

• Vim: vim-mypy
• Emacs: using Flycheck and Flycheck-mypy
• Sublime Text: SublimeLinter-contrib-mypy
• Atom: linter-mypy
• PyCharm: PyCharm integrates its own implementation of PEP 484.

Mypy can also be integrated into Flake8 using flake8-mypy.

o ,-

• https://github.com/python/mypy
• https://pypi.org/project/flake8-mypy
• https://docs.python.org/3/library/typing.html

This happens because you already annotated the name parameter as str, so it seems silly to narrow the type again after that with isinstance.

I personally find the error message a bit misleading because from a runtime perspective name could of course be something else (up until the isinstance check), i.e. it is technically not "always an instance of str". But from a static type checker's perspective that complaint makes sense.

Python is a dynamically typed language and that philosophy carries over to type annotations. There is a reason they are also referred to as "type hints". It is extremely un-pythonic to strictly check the argument types inside a function.

That is what the annotations are for. They tell the user of the function: "These are the types I expect in the body of this function. You can pass something else, but it might cause errors, unexpected behavior or break everything. Do it at your own peril."

I assume there is more going on in your actual function because right now it seems semantically redundant. But my suggestion is to get rid of the isinstance check. Did you decide to annotate the name parameter with the type str? Good. Then proceed in the function body under the assumption that that is what the argument will be. It is not your job, to check the type. It is the caller's job to call your function properly (or handle the consequences).

If you are using Pylance, you can add a new line to your settings.json (you need to restart VS Code after updating the file):

"python.analysis.typeCheckingMode": "basic"

The default value is off, the other possible values are basic and strict.

The following screenshot shows warnings for different situations: wrong variable type, wrong function return type and wrong parameter type.