DISCLAIMER: This post is mainly just curious thoughts, it has nothing to do with real-life application or good practice. So please don't actually use any examples provided.

Python is (or at least was) rather famous for its possibilities for one-liners (programs occupying only a single line of code) some time ago. A lot of things can be achieved like this, but among the most puzzling things must be infinite loops; they aren't exactly easy to implement with the tools we have available.

An infinite loop usually requires the use of a while-loop, because for-loops have a beginning and an end. Using a while-loop in one-liners is problematic, though, because you may only use it once, on the top level. This is due to how Python restricts block structures to either be separated by whitespace (and proper indentation), or to only have a single depth level following it. In other words,

while True: print("This works!")

is valid Python, but

while True: if 1 == 1: print("But this doesn't...)

is not.

We do have another "kind" of loop, though; list comprehensions. They are unique in that they may be nested as we see fit, all while using only a single line.

[["Order pizza." for _ in range(6)] for _ in range(42)]

But this doesn't give us an infinite loop; even if we simply input a ridiculously large number to range, it's still technically finite no matter what kind of hardware we're using. Thus, a different approach is required. I mentioned how infinite loops usually require the use of while-loops in Python. We can, however, utilise a certain property of Python to create an infinite loop with for-loops.

nums = [1, 2, 3, 4]
for num in nums:
    print(num)

Okay, that prints out four numbers. Not exactly infinite. But if we tweak our approach a little...

nums = [1]
for num in nums:
    print(num)
    nums.append(num + 1)

We actually get... as many numbers as the computer's memory allows. With this, we can essentially get something like this to work:

nums=[1];[(print(num) and nums.append(num+1)) for num in nums]

(Disclaimer; I never tested if that actually runs.)

It's not a pure one-liner, because it still technically requires two lines (fused together with a semicolon), but it's a proof-of-concept. I initially tried to make it work without having to define a variable, but failed to find a way.

I hope this was mildly interesting, I don't usually write stuff like this. Just found it curious myself, so why not share the thought? Maybe someone can even improve on this.