Python代码结构的基石之main函数实例详解

# math_tools.py
def add(a, b):
    return a + b
 
if __name__ == '__main__':
    print(add(2,3))  # 直接执行时输出
python
# 其他文件导入时
from math_tools import add  # 不会执行print语句

def complex_calculation(x):
    # 复杂计算逻辑
    return x*2
 
if __name__ == '__main__':
    # 单元测试
    assert complex_calculation(3) == 6

import argparse
 
def main(args):
    # 主逻辑
    print(f"Hello {args.name}")
 
if __name__ == '__main__':
    parser = argparse.ArgumentParser()
    parser.add_argument("--name", default="World")
    args = parser.parse_args()
    main(args)

my_project/
├── main.py
├── utils/
│   ├── __init__.py
│   └── helpers.py
└── tests/
    └── test_main.py

import sys
 
def main():
    if len(sys.argv) < 2:
        print("Usage: python script.py <name>")
        sys.exit(1)
    print(f"Hello {sys.argv[1]}")
 
if __name__ == '__main__':
    main()

import os
 
def main():
    db_url = os.getenv("DATABASE_URL", "sqlite:///default.db")
    # 初始化数据库连接
 
if __name__ == '__main__':
    main()

import logging
 
def setup_logging():
    logging.basicConfig(
        level=logging.INFO,
        format='%(asctime)s - %(levelname)s - %(message)s'
    )
 
def main():
    logging.info("Program started")
    # 主逻辑
 
if __name__ == '__main__':
    setup_logging()
    main()

# 错误示范
x = 10
def func():
    print(x)
x = 20  # 意外修改全局变量
 
# 正确做法
def main():
    x = 10
    def func():
        print(x)
    x = 20  # 仅在main作用域内修改

# a.py
from b import func_b  # 错误：循环导入
 
def main():
    func_b()
 
# b.py
from a import main  # 错误：循环导入

# 错误：主逻辑直接写在全局作用域
print("This will run during tests!")
 
# 正确：封装在main函数中
def main():
    print("This only runs when executed directly")

def main():
    # 只在需要时导入大模块
    import pandas as pd
    df = pd.read_csv("large_data.csv")
 
if __name__ == '__main__':
    main()

from multiprocessing import Pool
 
def process_task(task):
    # 耗时任务
    return task * 2
 
def main():
    with Pool(4) as p:
        results = p.map(process_task, range(10))
    print(results)
 
if __name__ == '__main__':
    main()

from typing import List
 
def process_data(data: List[float]) -> List[float]:
    return [x*2 for x in data]
 
def main() -> None:
    input_data = [1.5, 2.5, 3.5]
    output = process_data(input_data)
    print(output)

import click
 
@click.command()
@click.option('--name', default='World')
def main(name):
    """Simple greeting program"""
    click.echo(f"Hello {name}")
 
if __name__ == '__main__':
    main()

from fastapi import FastAPI
 
app = FastAPI()
 
@app.get("/")
def main():
    return {"message": "Hello World"}

import asyncio
 
async def main():
    print("Starting async tasks")
    await asyncio.sleep(1)
    print("Async tasks completed")
 
if __name__ == '__main__':
    asyncio.run(main())