structlog

structlog is a structured logging library for Python that produces machine-readable, structured log output instead of traditional free-form text messages. Created and maintained by Hynek Schlawack, structlog addresses a fundamental limitation of traditional logging: unstructured text logs are difficult to parse, search, filter, and aggregate in centralized logging systems. By emitting logs as structured key-value pairs (typically rendered as JSON), structlog enables powerful log analysis, correlation, and observability workflows when combined with tools like Elasticsearch, Loki, Datadog, or Splunk. The library is designed to be pragmatic and non-intrusive, meaning it can be used alongside the standard library logging module without conflict. Key design principles include: structured event objects that carry contextual key-value data throughout the logging pipeline, a processor chain architecture where each log event passes through a configurable sequence of processor functions that can add, modify, or filter the event data, and pluggable renderers that determine the final output format (JSON, console, or custom). Features include: automatic context binding via bind_context for attaching request IDs, user IDs, or session data to all subsequent log entries, lazy evaluation of expensive log fields, standard library integration via structlog.stdlib for seamless interoperation with existing logging configurations, performance-optimized processors that avoid unnecessary string formatting, support for asyncio context variables, and template-free logging via positional and keyword arguments. structlog also supports custom log levels, exception logging with structured tracebacks, and optional coloring for terminal output. As of 2026, structlog has over 3,700 GitHub stars and is licensed under the Apache 2.0 license.