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python: typing = discussion

Nate Stemen

Unitary Fund

Quantum Wednesday

3 May 2023

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types

Q: What is a type?

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A: a thing that has certain properties

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Q: what’s the point?

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A: constrains the possible values an expression can have.

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Types in python

Python 1.0

January 1994

PEP 484 - Type Hints

29-Sep-2014

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Type Constructors

  • Build new types from existing ones

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Subtyping

  • Suppose A is a subtype of B: A ≤ B
    • Anywhere B is needed, A can be substituted without issue.
  • Let C be a type constructor
  • How do C<A> relate to C<B>?
    • C<A> ≤ C<B> [covariant]
    • C<B> ≤ C<A> [contravariant]
    • C<A> ≡ C<B> [bivariant]
    • None of the above [invariant]
  • int ≤ float
  • How does list[int] relate to list[float]?

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Best practices

  • Function arguments should be typed generally
  • Return types should be typed specifically
  • reveal_type is your friend!