Implicit string concatenation

The gotcha

Adjacent string literals are concatenated at parse time. Useful for splitting long strings — devastating when a missing comma in a list silently merges two items.

Minimal repro

# Useful:
msg = ("This is a long message "
       "that spans multiple lines "
       "without explicit + .")

# Bug:
animals = [
    "cat",
    "dog"
    "rabbit",   # missing comma after "dog"
    "fish",
]
print(animals)   # ['cat', 'dograbbit', 'fish']   ← 3 items, not 4
print(len(animals))   # 3

The compiler treats "dog" "rabbit" as "dograbbit" because they’re adjacent string literals. No warning. No error. Just silent bug.

Why it happens

PEP 3101 / language spec: any sequence of string/byte literals separated only by whitespace is concatenated at compile time, like C. This applies to literal strings only — not f-strings combined with regular strings:

"abc" "def"           # "abcdef"
"abc" f"de{f}"        # works in 3.12+, syntax error pre-3.12
"abc" + variable      # runtime concat (different mechanism)

It does not apply to expressions: s = "a"; "abc" s is a SyntaxError, not concatenation.

How to avoid

Use a linter (ruff, pylint) with the rule enabled. ruff flags this as ISC001 (implicit string concat).
Add trailing commas in multi-line lists and tuples — most modern formatters (black, ruff format) enforce this.
For multi-line strings, prefer triple-quoted or explicit + if clarity helps.
Inspect lists if their length is wrong by exactly one — common cause is a swallowed comma.

Interview angle

Show a list of strings with one missing comma and ask the candidate to find the bug. They’ll often miss it; this is exactly what linters catch.