Python Operator Chaining… and why you still can’t trust ChatGPT

A colleague of mine recently posted the following on a Teams channel which we’ve appropriately named “Something cool I learned today!”:

"a" in "abc" is True will evaluate to False (!)

This is because the [Python] interpreter reads this as "a" in ("abc" is True), which is the same as "a" in False.

The first problem with this, which I did not catch until later, is that "a" in False does not in fact evaluate to False. It raises an exception because False is not iterable so the in operator does not work:

"a" in False

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: argument of type 'bool' is not iterable

Having overlooked that issue, I was about to respond with this link giving more details about operator precedence in Python. However, after looking more closely at the explanation there, I noticed something else: in and is have the same precedence and have left-to-right associativity (they get processed left-to-right). Following that, I would have expected the example to evaluate to True:

"a" in "abc" is True # -> ("a" in "abc") is True
"a" in "abc" # -> True
True is True # -> True

But when I actually ran the code, it returned False.

Something wasn't lining up, so I decided to do some more digging. I started with a conversation with ChatGPT that went something like this (full thread here - https://chatgpt.com/share/67c629cb-38e8-8013-8daf-290abec2600a):

Me: Hey, ChatGPT, what's going on here?
ChatGPT: Simple. It’s operator precedence.
Me: But don’t is and in have the same precedence?
ChatGPT: Good point. This should evaluate to True.
Me: But it doesn’t.
ChatGPT: Oooohhh. Riiiighht. It’s operator chaining.

This was a good reminder that ChatGPT is still not a Python expert. I would have expected it to catch both of the points I mentioned above, unravel the incorrect assumptions, and then give the correct explanation. Instead, it gave a misleading answer and only corrected itself after two rounds of prodding. Oh well.

Needless to say, my conversation with ChatGPT was not the most confidence-inspiring one I’ve had in my life, so I went to Google next. I found a Stack Overflow post with a similar question and the same explanation that ChatGPT eventually gave: https://stackoverflow.com/questions/31487806/a-in-abc-true-evaluates-to-false

Essentially, because of "operator chaining" the example above, "a" in "abc" is True, translates to the following:

'a' in 'abc' and 'abc' is True

Since "abc" is not True, the right half of the expression, and thereby all of it, evaluates to False.

This is similar to how Python evaluates 1 < 2 < 3 as (1 < 2) and (2 < 3). Alternatively, using the example from the Python docs:

x < y <= z is equivalent to x < y and y <= z

I had known about this rule generally, but did not realize it applied in a case like this.

Lessons learned:

1. If two or more comparison operators are "chained" together, Python inserts ands between them to interpret what's going on.
   • "Comparison operators" include: <, <=, >, >=, ==, !=, is, is not, in, and not in
2. ChatGPT is not a Python expert... yet.

For more on this, check out these links:

• GeeksForGeeks on operator precedence: https://www.geeksforgeeks.org/precedence-and-associativity-of-operators-in-python/
• Stack Overflow on the example covered here: https://stackoverflow.com/questions/31487806/a-in-abc-true-evaluates-to-false
• Python docs on operators: https://docs.python.org/3/reference/expressions.html#comparisons
• PEP 535 – Rich comparison chaining: https://peps.python.org/pep-0535/