Find specific subdir, open files and find specific lines that are missing from a file

[Gribouillis]

Could you explain the code please?

Well the expresssion re.search(r"CELL\-(\d+)", line) returns either None or a MatchObject in the sense of the re module. There is a match object if a substring such as CELL-8 was found in the line. With the match object, one can get the number 8, that's the value returned by the expression int(match.group(1)). Thus the sequence

sequence = (re.search(r"CELL\-(\d+)", line) for line in cells_file)

is a sequence such as None, None, match, None, match,... with one item per line of the file.

The expression

inum = set(int(match.group(1)) for match in sequence if match)

computes the set of all integers found in the above matches, hence all the integers i such that CELL-i was found in the file (actually, only the first occurrence on each line is taken into account).
Then there is a loop, equivalent to

for i in [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]:
    if i in inum:
        print(f"CELL-{i} infile")
    else:
        print(f"CELL-{i} missing")