pycasher 0.5.9

Cache function results and side effects (stdout, stderr, file writes) with automatic file I/O discovery via strace or audit hooks

pycasher

Cache Python function results and their side effects — stdout, stderr, and filesystem writes — with automatic invalidation.

uv add pycasher

If you want the casher CLI outside a project environment, install it as a tool instead:

uv tool install pycasher

What makes it different

Most caching libraries cache return values. casher also captures and replays:

• stdout/stderr printed during execution
• Files written by the function (restored from cache on hit)
• Files read by the function (used as cache keys — change a true upstream input file, cache auto-invalidates)

No manual file declarations needed. casher discovers file I/O automatically via strace (subprocess mode) or wrapped Python file handles (in-process mode). Files that are written before they are first read during one invocation are treated as generated outputs, not cache inputs.

Usage

from casher import cached, expand_input_dir

@cached
def train(data_path: str, output_path: str, lr: float = 0.01) -> dict:
    df = read_csv(data_path)
    model = fit(df, lr=lr)
    save(model, output_path)
    return {"accuracy": model.score}

# First call — runs function, traces file I/O, caches everything
result = train("train.csv", "model.pkl")

# Second call — instant replay from cache (model.pkl restored too)
result = train("train.csv", "model.pkl")

# Change train.csv — casher detects it, re-runs automatically

For directory-shaped inputs, keep the argument semantics explicit instead of making every directory Path recursive by magic:

from pathlib import Path

from casher import cached, expand_input_dir


@cached(input_files=lambda data_dir: expand_input_dir(data_dir, "*.csv"))
def build_dataset(data_dir: Path) -> int:
    return len(list(data_dir.glob("*.csv")))

Path arguments that point to files are hashed by file content for the function-argument portion of the cache key. Auto-discovered input files remain path-sensitive and content-sensitive.

Workflow-style functions can declare output directories explicitly to keep reads under those roots out of input_files:

from pathlib import Path

from casher import cached


work_dir = Path("work")


@cached(output_roots=[work_dir], replay_outputs="if-missing")
def assemble_workset() -> Path:
    generated = work_dir / "reference" / "mworld.par"
    generated.parent.mkdir(parents=True, exist_ok=True)
    generated.write_text("patched content")
    generated.read_text()
    return generated

On cache hit, unchanged output files are not restored again. You can also set replay_outputs=False or replay_outputs="if-missing" to control file replay.

Cache any shell command without code changes:

casher -- python train.py --data train.csv

Key features

• Automatic file tracking: strace (kernel-level, catches C extensions) or wrapped file handles (Python-only)
• Generated-output awareness: files written before their first read are excluded from input_files
• Dependency invalidation: changes to imported .py files invalidate the cache
• Narrow dependency overrides: use dep_files=[...] when only specific modules should invalidate a function
• File-hash memoization: unchanged files reuse cached content hashes from a small SQLite metadata store
• LRU eviction: configurable via max_cache_bytes or CASHER_MAX_CACHE_BYTES env var (default 32 GB)
• Faster hits for large artifacts: output replay skips files whose current hash already matches the cached output
• DataFrame support: polars and pandas DataFrames serialized via Arrow IPC
• Environment-aware: include env vars in cache key with env_vars=["MY_VAR"]
• Miss diagnostics: diagnose_misses=True logs which recorded input changed or disappeared
• Structured logging: loguru INFO for config changes, enablement, hit/miss, mode, eviction
• Explicit directory expansion helper: expand_input_dir() for stable input_files lists

Configuration

Env var	Default	Description
CASHER_CACHE_DIR	unset	Cache storage directory. Caching stays disabled until this is set.
CASHER_MAX_CACHE_BYTES	34359738368 (32 GB)	Max cache size before LRU eviction

Or set programmatically (takes priority over env vars):

from casher import configure, get_config

configure(cache_dir="/data/my_cache", max_cache_bytes=10 * 1024**3)
print(get_config())  # effective config

If no cache directory is configured via CASHER_CACHE_DIR, configure(cache_dir=...), @cached(cache_dir=...), or casher --cache-dir ..., casher runs transparently without caching and emits a one-time warning.

Platform support

Full caching on Linux only (requires strace for subprocess mode, fcntl for locking). On macOS and Windows the decorator is a transparent pass-through — functions execute normally, caching is skipped with a one-time warning.

Documentation

See documentation/ for detailed docs:

• Introduction — architecture, limitations, in-process vs subprocess comparison
• Quick Start — installation, decorator options, CLI usage
• API Reference — full API surface

License

MIT