I came across a Python program that defines a countdown class, which should print out countdown numbers when iterated over. Here's the code:
```python
class countdown:
def __init__(self, n):
self.n = n
def __getitem__(self, k):
if v := self.n - k:
return print(v),
print("rocket launching 🚀") in countdown(10)
```
Can someone explain what this outputs and why it does that?
2 Answers
This is pretty wild! The reason it works this way is due to how Python handles iteration when using `__getitem__`. Essentially, when you try to iterate over your `Countdown` class, it uses the indices starting from 0 until it hits a `None` value. The walrus operator (:=) is introduced for assignment while checking a condition. So, when you call `print("rocket launching 🚀") in Countdown(10)`, Python prints countdown values until it hits a point where it returns `None`, which stops the iteration. Pretty neat, right?
Exactly! Python will keep iterating until it can’t anymore, and if everything is fine in the iterator, it will just continue. If the last value returned is `None`, that's when it stops. So, in essence, it continuously subtracts from your countdown until it reaches zero!
Right! It's fascinating how the `Countdown` class is set up to leverage iterator behavior without explicit methods like `__iter__`. Thanks for pointing that out!