Constant MAX_FORMULA_CHARS 

pub const MAX_FORMULA_CHARS: usize = 8_192;

Excel’s limit on the characters of one formula. A Python cell’s code must fit in it once it is written as the PY formula of the function’s syntax (PyCode::to_file_formula): quotes are doubled and every 255 characters add "&", so the limit on the code itself depends on what the code holds.