Python-patterns – A collection of design patterns/idioms in Python

Re:Python-patterns – A collection of design patterns/idioms ... by d0mine on Wednesday June 01, 2022 @02:48AM (#31578913)

It looks like a distinction without a difference. We can consider `None and f()` pattern as explicit syntax that unwraps objects as necessary without infecting other code.

Background: an object that either None or true in a boolean context (true unless overriden for custom objects). Given such object, we can consider it in a virtual Maybe container/box. When we want to use it, we have to unwrap it using `obj and obj.method()` syntax. Then `obj.method()` is ordinary "unwrapped" call.

Just to remind you. Here's how "ergonomic" Maybe variant from the article look like:

  # Type hint here is optional, it only helps the reader here:
  discount_program: Maybe['DiscountProgram'] = Maybe.from_optional(
      user,
  ).bind_optional(  # This won't be called if `user is None`
      lambda real_user: real_user.get_balance(),
  ).bind_optional(  # This won't be called if `real_user.get_balance()` is None
      lambda balance: balance.credit_amount(),
  ).bind_optional(  # And so on!
      lambda credit: choose_discount(credit) if credit > 0 else None,
  )

https://github.com/dry-python/returns#maybe-container

You can decide for yourself what is more readable: all these lambdas or the `None and f()` code.

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Re:Python-patterns – A collection of design patterns/idioms ... by kortex on Wednesday June 01, 2022 @04:34PM (#31587702)

The dry-python lambda soup is awful, I totally agree there. That's just there mostly for demonstration purposes though. Generally you'd actually use the flow construct (monads) to compose methods.

But `obj and obj.method()` really is not the same thing as `obj.map(method)`. "virtual Maybe container/box" is a nice idea, and does actually type-check with mypy (mostly), but you cannot actually compose it with other functions. The problem is, each time you do `obj and obj.method()`, you end up union-ing type(obj) and type(obj.method).

https://mypy-play.net/?gist=5b03d81a64453997984e448df9b889e7

    main.py:40: note: Revealed type is "Union[__main__.User, None]"
    main.py:41: note: Revealed type is "Union[__main__.User, None, builtins.float]"
    main.py:42: note: Revealed type is "Union[__main__.User, None, builtins.float]"
    main.py:43: note: Revealed type is "Union[__main__.User, None, builtins.float, builtins.str]"

True Maybe types are more precise. Maybe if the Mypy engine could be retooled to recognise the `obj and obj.method()` idiom as tantamount to `obj.map(method)`, this could be avoided.

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