📄 Parent resource: My general Python notes

Resources

• One page cheatsheet
• https://github.com/microsoft/pyright/blob/main/docs/type-concepts.md
• Official PEPs
  • PEP 484 – Type Hints
  • PEP 526 – Syntax for Variable Annotations
• PlainEnglish Python: Python for TypeScript Developers
• My docs: Using Python Type-Checking Features in VSCode
• Benita: Exhaustiveness Checking with Mypy

Strongly-Typed Dictionaries and Objects

Typed-Dicts

A TypedDict is really just a type-annotation layer over a standard dictionary.

Syntax:

from typing import TypedDict

# Class declaration
class Snack(TypedDict, total=True):
	name: str
	calories: int

apple: Snack = {
	'name': 'Medium Apple',
	'calories': 95
}

# Alternate syntax
Snack = TypedDict('Snack', {'name': str, 'calories': int}, total=True)
apple: Snack = {
	'name': 'Medium Apple',
	'calories': 95
}

Currently, inline / anonymous TypedDicts are not supported, although there is discussion around supporting them

There is are known issue (#10759 and #7981) with mypy and TypedDict.values() returning the wrong types

Miscellaneous Questions and TypeScript Equivalents

• NotRequired[T] vs Optional[T]?
  • NotRequired[T] can be used to mark a property as being completely optional
  • Optional[T] means the property is still required, but can be None (type is T | None / Union[T, None])
• Null or undefined types?
  • Python uses None instead of Null or Undefined.
  • Typing a possibly unset variable?
    • Although you can use possible_string: Union[None, other_type] to represent the type of None | other_type, a shorter way is with Optional[other_type]
  • Checking for None?
    • if my_var is None or even just if not my_var works for checking if variable == None, but not for checking if a variable exists period (this will throw an exception is variable is not defined)
• Equivalent to TS ReturnType<T>?
  • Does not exist - see issue #769
• Equivalent to TS typeof {MY_VAR}?
  • Nope, same as above
• Equivalent to @ts-ignore?
  • # type: ignore
    • If you get an Invalid "type: ignore" comment error, a possible cause is adding your own text after it. E.g., # type: ignore blah blah
• Equivalent to TS assertions (like myVar as myType)?
  • Most efficient: myVar as myType -> typing.cast(my_type, my_var)
  • Less efficient (but perhaps safer): Using assert
  • https://stackoverflow.com/a/57931741/11447682
• Why don't if type() checks narrow the type?
  • They can, depending on checker. Try using isinstance() to narrow instead though.
• Using isinstance() is not working for narrowing
  • Try different syntax approaches. E.g., if not isinstance(my_obj, MyClass): instead of if isinstance(my_obj, MyClass) is not True:

Troubleshooting

A word of warning; stubs can (and often are) shipped separately from the actual library; for example, VSCode's pylance extension provides out-of-the-box type stubs for Django, but these might not match the actual version of Django you happen to have installed in your project!

• Intellisense isn't kicking in when adding elements in an array, dict, etc.
  • Try adding a comma / separator before actually writing out the element. For some reason in VSCode, it won't work unless you do this when adding elements to a list.