Lazyline: Zero-Config Line-Level Python Profiler

Zero-config, deterministic, line-level Python profiler. Find slow Python code and profile it line by line — no @profile decorators, no code changes. Point it at a package or script and get exact hit counts and timing for every line. Subprocesses and multiprocessing pools profiled automatically. Find the lazy lines.

Quick Start

pip install lazyline
# or: uvx lazyline

# Profile a package while running its tests:
lazyline run my_package -- pytest tests/

# Profile a script:
lazyline run script.py -- python script.py

Line 14 burned 99.8% of deduplicate checking membership in a list on every iteration — that's your lazy line. Change seen to a set and it drops from O(n²) to O(n).

[!TIP] Using an AI coding assistant? Install the lazyline plugin for Claude Code, or copy skills/lazyline/SKILL.md into any assistant that supports markdown skill files.

Key Features

Need to find performance bottlenecks in your Python code without modifying a single file? Lazyline wraps line_profiler and adds everything needed to go from "I want to profile this package" to "here are the bottlenecks" in a single command:

• Zero configuration — point at a package name, directory, or .py file. Every function is discovered and instrumented automatically. No @profile decorators, no code changes — be lazy, let the tool do the work.

• Subprocess and multiprocessing — ProcessPoolExecutor, multiprocessing.Pool, and child Python processes (Celery workers, Airflow tasks) are profiled automatically. Results are merged into a single report.

• Deterministic precision — exact hit counts and timing for every line, not statistical estimates. "This line ran 47,382 times and took 3.2s." When you need to distinguish O(n) from O(n²), exact counts are the difference.

• Focused scope, clean output — you choose exactly which package to profile. Unlike tools that profile everything in your working directory, lazyline keeps output relevant and overhead contained.

When to Use What

No tool is best for everything. Pick the right one for the job:

Feature comparison

Lazyline trades overhead for precision. Deterministic tracing fires a callback on every line execution. For functions with real work (>0.1ms per call), overhead is negligible (~1.2x). For tight loops calling tiny functions millions of times, it can reach ~7x. Relative rankings are always reliable — use lazyline to find which code is lazy, not to measure how fast it runs. See benchmarks for detailed measurements.

Usage

# Profile a package during its test suite
lazyline run my_package -- pytest tests/

# Profile while running a script
lazyline run my_package -- python evaluate.py

# Profile a CLI tool (hyphenated console scripts work too)
lazyline run my_package -- my-tool run-all

# Export results, view later
lazyline run -o results.json my_package -- pytest tests/
lazyline show results.json --top 10

# Multiple scopes in one run
lazyline run utils.py my_package -- python script.py

Requires Python 3.10+. The target package must be importable in the same environment.

See the full usage guide for all CLI options, scope formats, command resolution, output details, and more examples.

Claude Code Plugin

Lazyline ships as a Claude Code plugin. Install it and Claude will know how to profile your code, interpret results, and suggest optimizations:

/plugin marketplace add TomasVenkrbec/lazyline
/plugin install lazyline@lazyline

Then use /lazyline my_package -- python main.py or let Claude invoke it automatically when you ask about performance.

The skill also works with any AI coding assistant that supports markdown skill files — copy skills/lazyline/SKILL.md into your assistant's configuration.

Documentation

• Usage Guide — CLI reference, scope formats, output details
• How It Works — architecture, overhead, limitations
• Benchmarks — overhead measurements and methodology
• Contributing — development setup, tests, code style
• Changelog

If lazyline helped you find a bottleneck, consider giving it a star — it helps others discover the project. Found a problem? Open an issue.

License

MIT — see LICENSE.