Token bucket rate limiter
N tokens refill at rate R; each request costs 1. Allows controlled bursts and steady-state throughput.
Python Code Vault
Hand-picked production patterns. Each entry pairs the canonical Python implementation with WHY it exists and WHEN to reach for it. Free, copy-ready, no signup.
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18 of 18 patterns
Distributed systems
Idempotency-Key cache
Same key → same result; retry-safe across network flakes. Stripe / PayPal / Square use this verbatim.
Circuit breaker
Open after N failures, short-circuit to fallback while OPEN, probe-once via HALF_OPEN to recover.
Retry with exponential backoff + jitter
Wait 1s, 2s, 4s, 8s… with jitter; cap attempts and total wait. Survives transient flakes without thundering herd.
Cache-aside (lazy cache)
Read cache → miss → read DB → populate cache. The 90% case for application caching.
Concurrency & async
asyncio.gather with bounded concurrency
Run N coroutines but never more than K at once via Semaphore. Prevents flooding downstream.
Producer-consumer with queue.Queue
Producers push, workers pop, sentinel signals shutdown. Thread-safe, FIFO, no manual locking.
Timeout any awaitable
Bound a coroutine's runtime with asyncio.wait_for — never await something that has no time ceiling.
Data structures
Top-K with heapq
Maintain the k largest items in O(n log k) using a min-heap. Beats sort-then-slice on large streams.
Counter for frequency tables
Counter + .most_common(n) replaces 10 lines of dict-counting. Sorted by count, ties broken by insertion order.
Cursor pagination (not OFFSET)
Keyset / cursor pagination — the next page is built from the last row's sort key, not row N.
Production patterns
Structured logging
Emit JSON, not free-text. Datadog / Splunk / Grafana parse the fields directly — searchable, filterable.
Feature flag with overrides + bucket rollout
Override map for VIPs / kill-switches, deterministic hash-bucket for percentage rollouts. Sub-millisecond eval.
Graceful SIGTERM shutdown
Catch SIGTERM, stop accepting work, drain in-flight tasks before exit — survives k8s rolling deploys.
Security & privacy
Row-level multi-tenancy + leak audit
Always-applied tenant filter + audit_leaks() that pages SRE if a query forgets the WHERE clause.
PII scrubber (email / phone / card)
Three regex passes catch the obvious shapes before user-supplied text leaves your process boundary.
Performance
__slots__ for memory-dense classes
Skip per-instance __dict__; 40-50% memory cut for many-instance classes. Zero API change.
functools.lru_cache for memoization
Decorator turns any pure function into a memoised one. Hit rate visible via .cache_info().