Python Doesn't Require Commas in Lists .. Sorta

Today I helped a colleague with a subtle Python bug. We have a system that queries for data given a list of IDs. The list of IDs looked like this:

ids = [
    '7d38c515-d543-4186-a6a6-e46d4e356a81' # location 1
    'f384fc68-3030-473f-95a8-52d5fee6cfd4' # location 2
    'b27fef7f-9e5d-4af5-8596-a6949dd257a5' # location 3
]

It look us an unfortunate amount of time to realize we were missing commas in that list. Python blissfully will concatenate string elements inside of a list for you.

bad_list = ['a' 'b' 'c']
bad_list[0] == 'abc'
True

This is because ,

"a""b" == "ab"
True

I'm failing to come up with a helpful example of where this behavior is useful though.