What is a race condition? Give an example and its symptom.

Question

IT Interview Practice · Accepted Answer

**A race condition is a bug where the result depends on the unpredictable timing of concurrent operations accessing shared state.** The classic case is a read-modify-write that isn't atomic.

## How it works

`counter += 1` looks atomic but is really three steps: read, add, write. Two threads can interleave so an update is lost.

```text
counter = 0
Thread A: read 0 ─────────── write 1
Thread B:      read 0 ─ write 1
Result: 1  (should be 2 — one increment lost)
```

## Example

```python
counter = 0
def inc():
    global counter
    for _ in range(100_000):
        counter += 1          # NOT atomic: read, +1, write

# Run two threads, expect 200_000...
# ...but you often get less — a lost update
```

The fix is to make the section atomic with a lock:

```python
lock = threading.Lock()
def inc():
    global counter
    for _ in range(100_000):
        with lock:            # only one thread in here at a time
            counter += 1
```

## Why it matters

The **symptom** is what makes races dangerous: intermittent, non-reproducible wrong results that pass in tests and fail under load. They hide until timing changes — a faster machine, more traffic, a new core. Recognizing shared mutable state as the root cause is the first step; interviewers want to see you spot the unprotected read-modify-write.

How it works

Example

Why it matters

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