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know the features of your language (lemmy.world)

submitted 2 years ago by jroid8@lemmy.world to c/programmerhumor@lemmy.ml

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[-] rwhitisissle@lemmy.ml 1 points 2 years ago

For newer python people, they see return a or b and typically think it returns a boolean if either is True. Nope. Returns a if a is truthy and then checks if b is truthy. If neither are truthy, it returns b.

[-] cypherpunks@lemmy.ml 7 points 2 years ago* (last edited 2 years ago)

Returns a if a is truthy and then checks if b is truthy. If neither are truthy, it returns b.

Not quite. If a is not truthy, then the expression a or b will always return b.

So, there is never any reason to check the truthiness of b.

you can paste this in your repl to confirm it does not.

class C:
 def __repr__(self): return [k for k, v in globals().items() if v is self][0]
 def __bool__(self):
  print(f"{self}.__bool__() was called")
  return False

a, b = C(), C()
print(f"result: {a or b}")

spoiler output

a.__bool__() was called
result: b

:::

[-] rwhitisissle@lemmy.ml 3 points 2 years ago

Ah, good catch.

this post was submitted on 08 Dec 2023

622 points (96.4% liked)

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