How to Record Last N Errors with a Ring Buffer in Python
Use collections.deque with maxlen to keep only the most recent N error messages while discarding older entries automatically.
Python code
17 linesimport collections
class ErrorRecorder:
def __init__(self, size):
self.buffer = collections.deque(maxlen=size)
def record_error(self, message):
self.buffer.append(message)
def get_errors(self):
return list(self.buffer)
if __name__ == "__main__":
recorder = ErrorRecorder(3)
for i in range(5):
recorder.record_error(f"Error #{i}")
print(recorder.get_errors())Output
['Error #2', 'Error #3', 'Error #4']How it works
The collections.deque with a maxlen acts as a ring buffer: when the deque reaches its maximum size, appending a new element automatically discards the oldest one. This makes it perfect for keeping a fixed-size rolling window of the last N errors. The record_error method appends each message, and get_errors returns a list copy so callers cannot mutate the internal state. The demo records five errors into a buffer of size three, so the final output contains only the last three messages.
Common mistakes
- Using a plain list and forgetting to trim it, causing unbounded memory growth
- Confusing deque.append with deque.popleft when trying to manually manage capacity
- Returning the deque directly instead of a list, which exposes mutable internal state
Variations
- Use a helper function with a closure that wraps the deque for a lightweight solution
- Store timestamps along with messages in a deque of tuples for diagnostics
Real-world use cases
- Keeping a rolling log of recent exceptions in a long-running service for quick on-demand diagnostics.
- Bounding memory usage in an error-collection agent that forwards the latest N failures to a monitoring endpoint.
- Storing the most recent validation failures in an ETL pipeline to reset processing without persisting the full history.
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