python-performance-optimization

Python 性能优化

全面介绍如何分析、评估和优化 Python 代码以提升性能，包括 CPU 分析、内存优化和实现最佳实践。

何时使用此技能

• 识别 Python 应用程序中的性能瓶颈
• 降低应用程序延迟和响应时间
• 优化 CPU 密集型操作
• 降低内存消耗并排查内存泄漏
• 提升数据库查询性能
• 优化 I/O 操作
• 加快数据处理流水线
• 实现高性能算法
• 分析生产环境中的应用程序

核心概念

1. 分析类型

• CPU 分析：识别耗时较长的函数
• 内存分析：跟踪内存分配和泄漏
• 逐行分析：按行粒度进行分析
• 调用图：可视化函数调用关系

2. 性能指标

• 执行时间：操作所需的时间
• 内存用量：内存消耗的峰值和平均值
• CPU 利用率：处理器使用模式
• I/O 等待：I/O 操作所耗费的时间

3. 优化策略

• 算法层面：采用更好的算法和数据结构
• 实现层面：采用更高效的代码模式
• 并行化：多线程／多进程
• 缓存：避免重复计算
• 原生扩展：在关键路径中使用 C／Rust

快速入门

基本计时

import time

def measure_time():
    """Simple timing measurement."""
    start = time.time()

    # Your code here
    result = sum(range(1000000))

    elapsed = time.time() - start
    print(f"Execution time: {elapsed:.4f} seconds")
    return result

# Better: use timeit for accurate measurements
import timeit

execution_time = timeit.timeit(
    "sum(range(1000000))",
    number=100
)
print(f"Average time: {execution_time/100:.6f} seconds")

分析工具

模式 1：cProfile — CPU 分析

import cProfile
import pstats
from pstats import SortKey

def slow_function():
    """Function to profile."""
    total = 0
    for i in range(1000000):
        total += i
    return total

def another_function():
    """Another function."""
    return [i**2 for i in range(100000)]

def main():
    """Main function to profile."""
    result1 = slow_function()
    result2 = another_function()
    return result1, result2

# Profile the code
if __name__ == "__main__":
    profiler = cProfile.Profile()
    profiler.enable()

    main()

    profiler.disable()

    # Print stats
    stats = pstats.Stats(profiler)
    stats.sort_stats(SortKey.CUMULATIVE)
    stats.print_stats(10)  # Top 10 functions

    # Save to file for later analysis
    stats.dump_stats("profile_output.prof")

命令行分析：

# Profile a script
python -m cProfile -o output.prof script.py

# View results
python -m pstats output.prof
# In pstats:
# sort cumtime
# stats 10

模式 2：line_profiler — 逐行分析

# Install: pip install line-profiler

# Add @profile decorator (line_profiler provides this)
@profile
def process_data(data):
    """Process data with line profiling."""
    result = []
    for item in data:
        processed = item * 2
        result.append(processed)
    return result

# Run with:
# kernprof -l -v script.py

手动逐行分析：

from line_profiler import LineProfiler

def process_data(data):
    """Function to profile."""
    result = []
    for item in data:
        processed = item * 2
        result.append(processed)
    return result

if __name__ == "__main__":
    lp = LineProfiler()
    lp.add_function(process_data)

    data = list(range(100000))

    lp_wrapper = lp(process_data)
    lp_wrapper(data)

    lp.print_stats()

模式 3：memory_profiler — 内存用量

# Install: pip install memory-profiler

from memory_profiler import profile

@profile
def memory_intensive():
    """Function that uses lots of memory."""
    # Create large list
    big_list = [i for i in range(1000000)]

    # Create large dict
    big_dict = {i: i**2 for i in range(100000)}

    # Process data
    result = sum(big_list)

    return result

if __name__ == "__main__":
    memory_intensive()

# Run with:
# python -m memory_profiler script.py

模式 4：py-spy — 生产环境分析

# Install: pip install py-spy

# Profile a running Python process
py-spy top --pid 12345

# Generate flamegraph
py-spy record -o profile.svg --pid 12345

# Profile a script
py-spy record -o profile.svg -- python script.py

# Dump current call stack
py-spy dump --pid 12345

优化模式

模式 5：列表推导式与循环

import timeit

# Slow: Traditional loop
def slow_squares(n):
    """Create list of squares using loop."""
    result = []
    for i in range(n):
        result.append(i**2)
    return result

# Fast: List comprehension
def fast_squares(n):
    """Create list of squares using comprehension."""
    return [i**2 for i in range(n)]

# Benchmark
n = 100000

slow_time = timeit.timeit(lambda: slow_squares(n), number=100)
fast_time = timeit.timeit(lambda: fast_squares(n), number=100)

print(f"Loop: {slow_time:.4f}s")
print(f"Comprehension: {fast_time:.4f}s")
print(f"Speedup: {slow_time/fast_time:.2f}x")

# Even faster for simple operations: map
def faster_squares(n):
    """Use map for even better performance."""
    return list(map(lambda x: x**2, range(n)))

模式 6：使用生成器表达式节省内存

import sys

def list_approach():
    """Memory-intensive list."""
    data = [i**2 for i in range(1000000)]
    return sum(data)

def generator_approach():
    """Memory-efficient generator."""
    data = (i**2 for i in range(1000000))
    return sum(data)

# Memory comparison
list_data = [i for i in range(1000000)]
gen_data = (i for i in range(1000000))

print(f"List size: {sys.getsizeof(list_data)} bytes")
print(f"Generator size: {sys.getsizeof(gen_data)} bytes")

# Generators use constant memory regardless of size

模式 7：字符串拼接

import timeit

def slow_concat(items):
    """Slow string concatenation."""
    result = ""
    for item in items:
        result += str(item)
    return result

def fast_concat(items):
    """Fast string concatenation with join."""
    return "".join(str(item) for item in items)

def faster_concat(items):
    """Even faster with list."""
    parts = [str(item) for item in items]
    return "".join(parts)

items = list(range(10000))

# Benchmark
slow = timeit.timeit(lambda: slow_concat(items), number=100)
fast = timeit.timeit(lambda: fast_concat(items), number=100)
faster = timeit.timeit(lambda: faster_concat(items), number=100)

print(f"Concatenation (+): {slow:.4f}s")
print(f"Join (generator): {fast:.4f}s")
print(f"Join (list): {faster:.4f}s")

模式 8：字典查找与列表搜索

import timeit

# Create test data
size = 10000
items = list(range(size))
lookup_dict = {i: i for i in range(size)}

def list_search(items, target):
    """O(n) search in list."""
    return target in items

def dict_search(lookup_dict, target):
    """O(1) search in dict."""
    return target in lookup_dict

target = size - 1  # Worst case for list

# Benchmark
list_time = timeit.timeit(
    lambda: list_search(items, target),
    number=1000
)
dict_time = timeit.timeit(
    lambda: dict_search(lookup_dict, target),
    number=1000
)

print(f"List search: {list_time:.6f}s")
print(f"Dict search: {dict_time:.6f}s")
print(f"Speedup: {list_time/dict_time:.0f}x")

模式 9：局部变量访问

import timeit

# Global variable (slow)
GLOBAL_VALUE = 100

def use_global():
    """Access global variable."""
    total = 0
    for i in range(10000):
        total += GLOBAL_VALUE
    return total

def use_local():
    """Use local variable."""
    local_value = 100
    total = 0
    for i in range(10000):
        total += local_value
    return total

# Local is faster
global_time = timeit.timeit(use_global, number=1000)
local_time = timeit.timeit(use_local, number=1000)

print(f"Global access: {global_time:.4f}s")
print(f"Local access: {local_time:.4f}s")
print(f"Speedup: {global_time/local_time:.2f}x")

模式 10：函数调用开销

import timeit

def calculate_inline():
    """Inline calculation."""
    total = 0
    for i in range(10000):
        total += i * 2 + 1
    return total

def helper_function(x):
    """Helper function."""
    return x * 2 + 1

def calculate_with_function():
    """Calculation with function calls."""
    total = 0
    for i in range(10000):
        total += helper_function(i)
    return total

# Inline is faster due to no call overhead
inline_time = timeit.timeit(calculate_inline, number=1000)
function_time = timeit.timeit(calculate_with_function, number=1000)

print(f"Inline: {inline_time:.4f}s")
print(f"Function calls: {function_time:.4f}s")

高级优化

模式 11：使用 NumPy 进行数值运算

import timeit
import numpy as np

def python_sum(n):
    """Sum using pure Python."""
    return sum(range(n))

def numpy_sum(n):
    """Sum using NumPy."""
    return np.arange(n).sum()

n = 1000000

python_time = timeit.timeit(lambda: python_sum(n), number=100)
numpy_time = timeit.timeit(lambda: numpy_sum(n), number=100)

print(f"Python: {python_time:.4f}s")
print(f"NumPy: {numpy_time:.4f}s")
print(f"Speedup: {python_time/numpy_time:.2f}x")

# Vectorized operations
def python_multiply():
    """Element-wise multiplication in Python."""
    a = list(range(100000))
    b = list(range(100000))
    return [x * y for x, y in zip(a, b)]

def numpy_multiply():
    """Vectorized multiplication in NumPy."""
    a = np.arange(100000)
    b = np.arange(100000)
    return a * b

py_time = timeit.timeit(python_multiply, number=100)
np_time = timeit.timeit(numpy_multiply, number=100)

print(f"\nPython multiply: {py_time:.4f}s")
print(f"NumPy multiply: {np_time:.4f}s")
print(f"Speedup: {py_time/np_time:.2f}x")

模式 12：使用 functools.lru_cache 进行缓存

from functools import lru_cache
import timeit

def fibonacci_slow(n):
    """Recursive fibonacci without caching."""
    if n < 2:
        return n
    return fibonacci_slow(n-1) + fibonacci_slow(n-2)

@lru_cache(maxsize=None)
def fibonacci_fast(n):
    """Recursive fibonacci with caching."""
    if n < 2:
        return n
    return fibonacci_fast(n-1) + fibonacci_fast(n-2)

# Massive speedup for recursive algorithms
n = 30

slow_time = timeit.timeit(lambda: fibonacci_slow(n), number=1)
fast_time = timeit.timeit(lambda: fibonacci_fast(n), number=1000)

print(f"Without cache (1 run): {slow_time:.4f}s")
print(f"With cache (1000 runs): {fast_time:.4f}s")

# Cache info
print(f"Cache info: {fibonacci_fast.cache_info()}")

模式 13：使用 slots 节省内存

import sys

class RegularClass:
    """Regular class with __dict__."""
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z

class SlottedClass:
    """Class with __slots__ for memory efficiency."""
    __slots__ = ['x', 'y', 'z']

    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z

# Memory comparison
regular = RegularClass(1, 2, 3)
slotted = SlottedClass(1, 2, 3)

print(f"Regular class size: {sys.getsizeof(regular)} bytes")
print(f"Slotted class size: {sys.getsizeof(slotted)} bytes")

# Significant savings with many instances
regular_objects = [RegularClass(i, i+1, i+2) for i in range(10000)]
slotted_objects = [SlottedClass(i, i+1, i+2) for i in range(10000)]

print(f"\nMemory for 10000 regular objects: ~{sys.getsizeof(regular) * 10000} bytes")
print(f"Memory for 10000 slotted objects: ~{sys.getsizeof(slotted) * 10000} bytes")

模式 14：使用多进程处理 CPU 密集型任务

import multiprocessing as mp
import time

def cpu_intensive_task(n):
    """CPU-intensive calculation."""
    return sum(i**2 for i in range(n))

def sequential_processing():
    """Process tasks sequentially."""
    start = time.time()
    results = [cpu_intensive_task(1000000) for _ in range(4)]
    elapsed = time.time() - start
    return elapsed, results

def parallel_processing():
    """Process tasks in parallel."""
    start = time.time()
    with mp.Pool(processes=4) as pool:
        results = pool.map(cpu_intensive_task, [1000000] * 4)
    elapsed = time.time() - start
    return elapsed, results

if __name__ == "__main__":
    seq_time, seq_results = sequential_processing()
    par_time, par_results = parallel_processing()

    print(f"Sequential: {seq_time:.2f}s")
    print(f"Parallel: {par_time:.2f}s")
    print(f"Speedup: {seq_time/par_time:.2f}x")

模式 15：使用异步 I/O 处理 I/O 密集型任务

import asyncio
import aiohttp
import time
import requests

urls = [
    "https://httpbin.org/delay/1",
    "https://httpbin.org/delay/1",
    "https://httpbin.org/delay/1",
    "https://httpbin.org/delay/1",
]

def synchronous_requests():
    """Synchronous HTTP requests."""
    start = time.time()
    results = []
    for url in urls:
        response = requests.get(url)
        results.append(response.status_code)
    elapsed = time.time() - start
    return elapsed, results

async def async_fetch(session, url):
    """Async HTTP request."""
    async with session.get(url) as response:
        return response.status

async def asynchronous_requests():
    """Asynchronous HTTP requests."""
    start = time.time()
    async with aiohttp.ClientSession() as session:
        tasks = [async_fetch(session, url) for url in urls]
        results = await asyncio.gather(*tasks)
    elapsed = time.time() - start
    return elapsed, results

# Async is much faster for I/O-bound work
sync_time, sync_results = synchronous_requests()
async_time, async_results = asyncio.run(asynchronous_requests())

print(f"Synchronous: {sync_time:.2f}s")
print(f"Asynchronous: {async_time:.2f}s")
print(f"Speedup: {sync_time/async_time:.2f}x")

数据库优化

模式 16：批量执行数据库操作

import sqlite3
import time

def create_db():
    """Create test database."""
    conn = sqlite3.connect(":memory:")
    conn.execute("CREATE TABLE users (id INTEGER PRIMARY KEY, name TEXT)")
    return conn

def slow_inserts(conn, count):
    """Insert records one at a time."""
    start = time.time()
    cursor = conn.cursor()
    for i in range(count):
        cursor.execute("INSERT INTO users (name) VALUES (?)", (f"User {i}",))
        conn.commit()  # Commit each insert
    elapsed = time.time() - start
    return elapsed

def fast_inserts(conn, count):
    """Batch insert with single commit."""
    start = time.time()
    cursor = conn.cursor()
    data = [(f"User {i}",) for i in range(count)]
    cursor.executemany("INSERT INTO users (name) VALUES (?)", data)
    conn.commit()  # Single commit
    elapsed = time.time() - start
    return elapsed

# Benchmark
conn1 = create_db()
slow_time = slow_inserts(conn1, 1000)

conn2 = create_db()
fast_time = fast_inserts(conn2, 1000)

print(f"Individual inserts: {slow_time:.4f}s")
print(f"Batch insert: {fast_time:.4f}s")
print(f"Speedup: {slow_time/fast_time:.2f}x")

模式 17：查询优化

# Use indexes for frequently queried columns
"""
-- Slow: No index
SELECT * FROM users WHERE email = '[email protected]';

-- Fast: With index
CREATE INDEX idx_users_email ON users(email);
SELECT * FROM users WHERE email = '[email protected]';
"""

# Use query planning
import sqlite3

conn = sqlite3.connect("example.db")
cursor = conn.cursor()

# Analyze query performance
cursor.execute("EXPLAIN QUERY PLAN SELECT * FROM users WHERE email = ?", ("[email protected]",))
print(cursor.fetchall())

# Use SELECT only needed columns
# Slow: SELECT *
# Fast: SELECT id, name

内存优化

模式 18：检测内存泄漏

import tracemalloc
import gc

def memory_leak_example():
    """Example that leaks memory."""
    leaked_objects = []

    for i in range(100000):
        # Objects added but never removed
        leaked_objects.append([i] * 100)

    # In real code, this would be an unintended reference

def track_memory_usage():
    """Track memory allocations."""
    tracemalloc.start()

    # Take snapshot before
    snapshot1 = tracemalloc.take_snapshot()

    # Run code
    memory_leak_example()

    # Take snapshot after
    snapshot2 = tracemalloc.take_snapshot()

    # Compare
    top_stats = snapshot2.compare_to(snapshot1, 'lineno')

    print("Top 10 memory allocations:")
    for stat in top_stats[:10]:
        print(stat)

    tracemalloc.stop()

# Monitor memory
track_memory_usage()

# Force garbage collection
gc.collect()

模式 19：迭代器与列表

import sys

def process_file_list(filename):
    """Load entire file into memory."""
    with open(filename) as f:
        lines = f.readlines()  # Loads all lines
        return sum(1 for line in lines if line.strip())

def process_file_iterator(filename):
    """Process file line by line."""
    with open(filename) as f:
        return sum(1 for line in f if line.strip())

# Iterator uses constant memory
# List loads entire file into memory

模式 20：使用 weakref 管理缓存

import weakref

class CachedResource:
    """Resource that can be garbage collected."""
    def __init__(self, data):
        self.data = data

# Regular cache prevents garbage collection
regular_cache = {}

def get_resource_regular(key):
    """Get resource from regular cache."""
    if key not in regular_cache:
        regular_cache[key] = CachedResource(f"Data for {key}")
    return regular_cache[key]

# Weak reference cache allows garbage collection
weak_cache = weakref.WeakValueDictionary()

def get_resource_weak(key):
    """Get resource from weak cache."""
    resource = weak_cache.get(key)
    if resource is None:
        resource = CachedResource(f"Data for {key}")
        weak_cache[key] = resource
    return resource

# When no strong references exist, objects can be GC'd

基准测试工具

自定义基准测试装饰器

import time
from functools import wraps

def benchmark(func):
    """Decorator to benchmark function execution."""
    @wraps(func)
    def wrapper(*args, **kwargs):
        start = time.perf_counter()
        result = func(*args, **kwargs)
        elapsed = time.perf_counter() - start
        print(f"{func.__name__} took {elapsed:.6f} seconds")
        return result
    return wrapper

@benchmark
def slow_function():
    """Function to benchmark."""
    time.sleep(0.5)
    return sum(range(1000000))

result = slow_function()

使用 pytest-benchmark 进行性能测试

# Install: pip install pytest-benchmark

def test_list_comprehension(benchmark):
    """Benchmark list comprehension."""
    result = benchmark(lambda: [i**2 for i in range(10000)])
    assert len(result) == 10000

def test_map_function(benchmark):
    """Benchmark map function."""
    result = benchmark(lambda: list(map(lambda x: x**2, range(10000))))
    assert len(result) == 10000

# Run with: pytest test_performance.py --benchmark-compare

最佳实践

1. 先分析，再优化 - 通过测量找出真正的瓶颈
2. 关注热点路径 - 优化运行频率最高的代码
3. 使用合适的数据结构 - 查找用 dict，成员检测用 set
4. 避免过早优化 - 先保证清晰易读，再进行优化
5. 使用内置函数 - 它们由 C 实现
6. 缓存开销较大的计算 - 使用 lru_cache
7. 批量执行 I/O 操作 - 减少系统调用
8. 对大型数据集使用生成器
9. 数值运算可考虑使用 NumPy
10. 分析生产环境代码 - 使用 py-spy 分析运行中的系统

常见问题

• 未经分析就进行优化
• 不必要地使用全局变量
• 未使用合适的数据结构
• 创建不必要的数据副本
• 数据库未使用连接池
• 忽视算法复杂度
• 过度优化很少执行的代码路径
• 未考虑内存用量

资源

• cProfile：内置 CPU 性能分析器
• memory_profiler：内存使用情况分析
• line_profiler：逐行性能分析
• py-spy：用于生产环境的采样分析器
• NumPy：高性能数值计算
• Cython：将 Python 编译为 C
• PyPy：带有 JIT 的替代 Python 解释器

性能检查清单

• 对代码进行性能分析，找出瓶颈
• 使用合适的数据结构
• 在有益的情况下实现缓存
• 优化数据库查询
• 对大型数据集使用生成器
• 对 CPU 密集型任务考虑使用多进程
• 对 I/O 密集型任务使用异步 I/O
• 尽量减少热点循环中的函数调用开销
• 检查是否存在内存泄漏
• 在优化前后进行基准测试