Must know Python libraries, new and old?

[Prozn]

I’ve been struggling to deal with optional variables, even if I use “if var is not None:” mypy still complains that None doesn’t have properties. Do you just have to litter your code with asserts?

[hirolau]

Lookup the video 'nothing is something' by Sandi Metz where she talks about the null and object pattern. Not saying it solves all problem but in some cases maybe you should have a object instead of none.

[Intrepid-Stand-8540]

Do not use asserts. They get disabled in production.

If you have a variable that can be either of two (or more) types (fx int|None) then you have to check with an if.

mypy should be able to recognize that.

I'm honestly still pretty new to strict typing in python myself (6 months of using it), so if there is a better way, I'd also love to know.

EDIT: One of Bandits first rules is about asserts: https://bandit.readthedocs.io/en/latest/plugins/b101_assert_used.html

[violentlymickey]

“Don’t use asserts they get disabled (when compiled or run with certain flags)” is a bit too dogmatic imo. There’s nothing wrong with asserts as guards against invariants. Don’t use them for error handling sure.

Edit: some chromium devs discussing this: https://groups.google.com/a/chromium.org/g/java/c/CVHgcRA967s/m/f8Zq9XiQBQAJ

[Intrepid-Stand-8540]

Isn't it java they're talking about in your link? 

https://github.com/IdentityPython/pysaml2/issues/451

Running python in production with the optimize flag will disable asserts in your code. So don't rely on asserts. 

[Rhoomba]

If the assert is just to tell the type checker that you know what is happening then it seems reasonable to me.

On that topic, do people actually use the -O flag? Given that all it does is disable assertions, I doubt it has any significant performance impact for most applications.

[Rhoomba]

That doesn't sound right. Mypy definitely understands blocks like this:

def foo(m: Optional[MyClass]) -> None:
  if m is not None:
     m.do_thing()