Python Parallelization

Python Parallelization Skill

Transform sequential Python code to leverage parallel and concurrent execution patterns.

Workflow

1. Analyze the code to identify parallelization candidates
2. Classify the workload type (CPU-bound, I/O-bound, or data-parallel)
3. Select the appropriate parallelization strategy
4. Transform the code with proper synchronization and error handling
5. Verify correctness and measure expected speedup

Parallelization Decision Tree

Is the bottleneck CPU-bound or I/O-bound?

CPU-bound (computation-heavy):
├── Independent iterations? → multiprocessing.Pool / ProcessPoolExecutor
├── Shared state needed? → multiprocessing with Manager or shared memory
├── NumPy/Pandas operations? → Vectorization first, then consider numba/dask
└── Large data chunks? → chunked processing with Pool.map

I/O-bound (network, disk, database):
├── Many independent requests? → asyncio with aiohttp/aiofiles
├── Legacy sync code? → ThreadPoolExecutor
├── Mixed sync/async? → asyncio.to_thread()
└── Database queries? → Connection pooling + async drivers

Data-parallel (array/matrix ops):
├── NumPy arrays? → Vectorize, avoid Python loops
├── Pandas DataFrames? → Use built-in vectorized methods
├── Large datasets? → Dask for out-of-core parallelism
└── GPU available? → Consider CuPy or JAX

Transformation Patterns

Pattern 1: Loop to ProcessPoolExecutor (CPU-bound)

Before:

results = []
for item in items:
    results.append(expensive_computation(item))

After:

from concurrent.futures import ProcessPoolExecutor

with ProcessPoolExecutor() as executor:
    results = list(executor.map(expensive_computation, items))

Pattern 2: Sequential I/O to Async (I/O-bound)

Before:

import requests

def fetch_all(urls):
    return [requests.get(url).json() for url in urls]

After:

import asyncio
import aiohttp

async def fetch_all(urls):
    async with aiohttp.ClientSession() as session:
        tasks = [fetch_one(session, url) for url in urls]
        return await asyncio.gather(*tasks)

async def fetch_one(session, url):
    async with session.get(url) as response:
        return await response.json()

Pattern 3: Nested Loops to Vectorization

Before:

result = []
for i in range(len(a)):
    row = []
    for j in range(len(b)):
        row.append(a[i] * b[j])
    result.append(row)

After:

import numpy as np
result = np.outer(a, b)

Pattern 4: Mixed CPU/IO with asyncio

import asyncio
from concurrent.futures import ProcessPoolExecutor

async def hybrid_pipeline(data, urls):
    loop = asyncio.get_event_loop()

    # CPU-bound in process pool
    with ProcessPoolExecutor() as pool:
        processed = await loop.run_in_executor(pool, cpu_heavy_fn, data)

    # I/O-bound with async
    results = await asyncio.gather(*[fetch(url) for url in urls])

    return processed, results

Parallelization Candidates

Look for these patterns in code:

Safety Requirements

Always preserve correctness when parallelizing:

1. Identify shared state - variables modified across iterations break parallelism
2. Check dependencies - iteration N depending on N-1 requires sequential execution
3. Handle exceptions - wrap parallel code in try/except, use executor.submit() for granular error handling
4. Manage resources - use context managers, limit worker count to avoid exhaustion
5. Preserve ordering - use map() over submit() when order matters

Common Pitfalls

• GIL trap: Threading doesn't help CPU-bound Python code—use multiprocessing
• Pickle failures: Lambda functions and nested classes can't be pickled for multiprocessing
• Memory explosion: ProcessPoolExecutor copies data to each process—use shared memory for large data
• Async in sync: Can't just add async to existing code—requires restructuring call chain
• Over-parallelization: Parallel overhead exceeds gains for small workloads (<1000 items typically)

Verification Checklist

Before finalizing transformed code:

• Output matches sequential version for test inputs
• No race conditions (shared mutable state properly synchronized)
• Exceptions are caught and handled appropriately
• Resources are properly cleaned up (pools closed, connections released)
• Worker count is bounded (default or explicit limit)
• Added appropriate imports

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