Unexpected behaviour in test (if) on list when list index out of range

[sbersier]

Bug report

Bug description:

Have a look at the minimal example below. Running this example, I would expect the following error: "IndexError: list index out of range" but it carries on, as if L[30] was a boolean ("False" or "True"). Is it the expected behaviour?
I'm using python 3.10.12

L=[0,1,2,3]
if True or L[30]: 
    print("That's OK despite the fact that I used an out of range index value of 30 for my test.")

But, if I use and "AND" instead of the "OR":

L=[0,1,2,3]
if True and L[30]:
    print("That's OK despite the fact that I used an out of range index value of 30 for my test.")

Then it throws the expected error: "IndexError: list index out of range"

CPython versions tested on:

3.10

Operating systems tested on:

Linux

[gvanrossum]

Thank you for using the Python bug tracker! The behavior you observe is not a bug -- it's by design that the or operator doesn't evaluate its second operand if the first is truthy. This is how boolean operations work in most languages -- e.g. && and || work this way in C, C++, Java, JavaScript, and many other languages. (In fact I have to go as far back as Pascal for a language I know that doesn't do this.)

PS. The same behavior works for and if you write if False and L[30].

[sbersier]

I would prefer the interpreter to tell me that I did something wrong instead of just ignoring my mistake.
Thanks for your answer.

[gvanrossum]

The whole point of shortcut Boolean operators is to prevent avoidable runtime errors. E.g. a common idiom is

If p is not None and p.link:
    print(p.link)