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Precise Type Hints with Literals and Generics

Python

Check a value's type before you use it, limit what a value can be with Literal, Final and NewType, and write reusable functions and classes with generics. Learn why mypy will not pass a list[int] where a list[object] is expected, and finish by modernizing a typed module.

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23 exercises

3 articles

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Your learning sequence

  1. 1 Not solved yet. Check a Value Before You Use It Practice · Easy
  2. 2 Not solved yet. Narrow a Union Without Treating Zero as Missing Practice · Medium
  3. 3 Not solved yet. Turn an Unknown Value into an int Practice · Medium
  4. 4 Not solved yet. Limit a Parameter to a Few Exact Values with Literal Practice · Medium
  5. 5 Not solved yet. Make mypy Report an Unhandled Event with assert_never Practice · Medium
  6. 6 Not solved yet. Lock Down a Rate Limiter with Final and @final Practice · Easy
  7. 7 Not solved yet. Give Each Task an ID from a ClassVar Counter Practice · Easy
  8. 8 Not solved yet. Keep Cents and Millis Apart with NewType Practice · Medium
  9. 9 Not solved yet. Describe Nested JSON with a Recursive Type Alias Practice · Medium
  10. 10 Generic Functions from Zero Read
  11. 11 Not solved yet. Partition and Chunk a List with Generic Helpers Practice · Medium
  12. 12 Not solved yet. Build a Generic Result Class Practice · Medium
  13. 13 Not solved yet. Limit a Type Parameter with Bounds and Constraints Practice · Hard
  14. 14 Not solved yet. Why a list[Dog] Is Not a list[Animal] Practice · Medium
  15. 15 Not solved yet. Accept Read-Only Inputs and Return Concrete Results Practice · Medium
  16. 16 Not solved yet. Control Variance in Your Own Generic Classes Practice · Hard
  17. 17 Not solved yet. Type the Callbacks of a Retry Helper Practice · Hard
  18. 18 Not solved yet. Catch a Renamed Method with @override Practice · Easy
  19. 19 Not solved yet. Keep the Subclass Type in a Chained Builder with Self Practice · Medium
  20. 20 Not solved yet. Write Your Own Narrowing Check with TypeIs Practice · Hard
  21. 21 Not solved yet. Find Out What isinstance Checks on a Protocol Practice · Hard
  22. 22 Not solved yet. List the Members of a Union at Runtime Practice · Easy
  23. 23 Not solved yet. Rewrite Old Typing Spellings Such as List and Optional Practice · Easy
  24. 24 Not solved yet. Capstone: Modernize a Typed Inventory Module Practice · Hard
  25. 25 Give a Type Parameter a Default Type Read
  26. 26 Read Annotations at Runtime in Python 3.14 Read

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7 questions · not saved while you're signed out

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0 / 7 correct

What does mypy catch in this call that a plain str annotation would let through?

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from typing import Literal

def lit(mode: Literal["r", "w"]) -> None: ...
lit("x")
Question 1 of 7