Dovetail - Sync / Async wrapper for Python.

A lightweight helper for bridging sync and async code, built on Asyncio & ThreadPoolExecutor.

Table of Contents

• Features
• Why Dovetail?
• Design Goals
• Installing Dovetail
• Quick Start
• API Summary & Guide
• Benchmarks
• Development
• License

Features

• Run blocking functions from async applications using the threadpool.
• Run async functions from synchronous applications.
• Bounded threadpool execution.
• Ordered parallel mapping.
• Built-in retries and exponential backoff.
• Per-task timeout support.
• Token-bucket rate limiting.
• Lifecycle event hooks.
• Structured tracing.
• Automatic instance registry and coordinated shutdown.
• Sync and async context-manager support.

Why Dovetail?

Asyncio is powerful, but it has a cascading problem: the moment you want to await something, that function has to become async, which means everything calling it has to become async, and so on until your entire codebase has been rewritten around it.

If you're adding parallelism to an existing project, or only want to optimise one bottleneck, rewriting the entire call chain is often unnecessary. Dovetail solves this by allowing synchronous applications to use asynchronous execution patterns without forcing an async migration.

Dovetail is not intended to outperform native asyncio. Native asyncio gives the most direct control and lowest overhead when an application can be written around async/await. Dovetail's advantage is reducing migration cost: it lets existing synchronous applications introduce concurrency incrementally.

Before Dovetail

Imagine an existing synchronous application:

def process_report(urls):
    results = []

    for url in urls:
        results.append(fetch_url(url))

    return summarise(results)

This works, but every request runs sequentially. If 100 requests each take 100ms:

100 requests × 100ms = ~10 seconds

The obvious asyncio solution is to rewrite everything:

async def process_report(urls):
    tasks = [
        fetch_url(url)
        for url in urls
    ]

    results = await asyncio.gather(*tasks)

    return summarise(results)

But now every caller must also become async:

async def application():
    report = await process_report(urls)

Which means the async boundary spreads:

application()
    ↓
process_report()
    ↓
fetch_url()
    ↓
database()
    ↓
filesystem()

For a large existing codebase, this migration can be expensive.

After Dovetail

Dovetail lets you add concurrency only where you need it:

from pydovetail import Dovetail

with Dovetail(max_workers=20) as dvt:
    def process_report(urls):
        results = dvt.task.map_blocking(fetch_url, urls)
        return summarise(results)

The rest of the application remains synchronous:

def application():
    report = process_report(urls)

No async migration. No rewriting the call stack. No changing unrelated code.

The execution model becomes:

application()
    ↓
process_report()
    ↓
Dovetail threadpool
    ├── fetch_url()
    ├── fetch_url()
    ├── fetch_url()
    └── fetch_url()

The same synchronous interface now benefits from parallel execution.

Design Goals

Dovetail prioritises:

• Minimal adoption cost.
• Incremental concurrency.
• Safe lifecycle management.
• Observable execution.

Dovetail intentionally does not replace asyncio. It provides a bridge for projects where a full async migration is impractical.

Installing Dovetail

Requirements:

• Python 3.9+
• CPython recommended
• asyncio
• ThreadPoolExecutor (standard library)

Dovetail has no runtime dependencies outside Python's standard library.

Install

pip install pydovetail

Quick Start

Preferred usage: create and manage a Dovetail instance with a context manager.

Sync (recommended for most users):

from pydovetail import Dovetail

with Dovetail(max_workers=8) as dvt:
  # Sync caller -> sync function (blocks)
  result = dvt.task.run_blocking(fetch_data)

  # Sync caller -> sync function in threadpool (blocks)
  result = dvt.task.to_thread_blocking(fetch_data, "target.json")

  # Sync caller -> batch map in threadpool (blocks, bounded concurrency)
  results = dvt.task.map_blocking(fetch_data, ["a.json", "b.json"], max_concurrency=4)

  # run an async coroutine from sync code
  value = dvt.task.run_blocking(fetch_data_async())

Async (use async with in async applications):

async with Dovetail() as dvt:
  # schedule coroutines and await
  tasks = [dvt.task.schedule(fetch_async(i)) for i in items]
  results = await asyncio.gather(*tasks)

Manual / library patterns (caller-managed ownership):

# Manual creation: by default Dovetail will attempt to shutdown on
# interpreter exit and on SIGINT/SIGTERM. You can opt out by passing
# `shutdown_on_exit=False` when constructing the instance.
dvt = Dovetail()  # shutdown_on_exit enabled by default
try:
  results = dvt.task.map_blocking(fetch, items)
finally:
  # explicit shutdown is still allowed and idempotent
  dvt.shutdown()

# Library-author pattern (prefer caller-managed instance when possible)
def process(items, dvt: Optional[Dovetail] = None):
  if dvt is None:
    with Dovetail() as _dvt:
      return _dvt.task.map_blocking(worker, items)
  return dvt.task.map_blocking(worker, items)

Development

Run tests locally:

python -m pytest -q

Contributions are welcome, please open an issue or PR.

License

Dovetail is released under the MIT License.

See LICENSE for details.