control flow.py
文件信息
- 📄 原文件：
02_control_flow.py - 🔤 语言：python
Python 控制流 本文件介绍 Python 中的条件语句、循环、异常处理等控制流结构。
完整代码
python
def main01_if_else():
"""
============================================================
1. 条件语句 if/elif/else
============================================================
"""
print("=" * 60)
print("1. 条件语句 if/elif/else")
print("=" * 60)
# 【基本 if 语句】
age = 18
if age >= 18:
print(f"年龄 {age}：成年人")
# 【if-else】
score = 75
if score >= 60:
print(f"分数 {score}：及格")
else:
print(f"分数 {score}：不及格")
# 【if-elif-else】
score = 85
if score >= 90:
grade = 'A'
elif score >= 80:
grade = 'B'
elif score >= 70:
grade = 'C'
elif score >= 60:
grade = 'D'
else:
grade = 'F'
print(f"分数 {score} 的等级是: {grade}")
# 【三元表达式】（条件表达式）
x = 10
result = "正数" if x > 0 else "非正数"
print(f"\n三元表达式: x={x}, 结果: {result}")
# 嵌套三元表达式（不推荐，可读性差）
x = 0
result = "正数" if x > 0 else ("零" if x == 0 else "负数")
print(f"嵌套三元: x={x}, 结果: {result}")
# 【海象运算符 :=】（Python 3.8+）
# 在表达式中同时赋值和使用
print("\n--- 海象运算符 := ---")
data = [1, 2, 3, 4, 5]
if (n := len(data)) > 3:
print(f"列表长度 {n} 大于 3")
# 【技巧】利用短路求值
print("\n--- 短路求值 ---")
name = ""
display_name = name or "匿名用户" # 空字符串为 False
print(f"显示名称: {display_name}")
# and 返回第一个 False 值或最后一个值
result = 1 and 2 and 3
print(f"1 and 2 and 3 = {result}") # 3
# or 返回第一个 True 值或最后一个值
result = 0 or "" or "default"
print(f"0 or '' or 'default' = {result}") # default
def main02_match():
"""
============================================================
2. 模式匹配 match（Python 3.10+）
============================================================
【重要】match 是 Python 3.10 新增的结构化模式匹配
比 switch/case 更强大！
"""
print("\n" + "=" * 60)
print("2. 模式匹配 match（Python 3.10+）")
print("=" * 60)
# 【基本 match】
def get_day_type(day):
match day:
case "Saturday" | "Sunday": # 使用 | 匹配多个值
return "周末"
case "Monday" | "Tuesday" | "Wednesday" | "Thursday" | "Friday":
return "工作日"
case _: # 通配符，匹配所有
return "未知"
print(f"Saturday: {get_day_type('Saturday')}")
print(f"Monday: {get_day_type('Monday')}")
# 【序列模式匹配】
def process_point(point):
match point:
case (0, 0):
return "原点"
case (0, y):
return f"Y轴上，y={y}"
case (x, 0):
return f"X轴上，x={x}"
case (x, y):
return f"点 ({x}, {y})"
case _:
return "不是有效的点"
print(f"\n(0, 0): {process_point((0, 0))}")
print(f"(0, 5): {process_point((0, 5))}")
print(f"(3, 4): {process_point((3, 4))}")
# 【字典模式匹配】
def process_command(command):
match command:
case {"action": "quit"}:
return "退出程序"
case {"action": "move", "direction": direction}:
return f"移动方向: {direction}"
case {"action": "attack", "target": target, "damage": damage}:
return f"攻击 {target}，伤害 {damage}"
case _:
return "未知命令"
print(f"\n命令1: {process_command({'action': 'quit'})}")
print(f"命令2: {process_command({'action': 'move', 'direction': 'north'})}")
print(f"命令3: {process_command({'action': 'attack', 'target': 'dragon', 'damage': 100})}")
# 【守卫条件】使用 if 添加额外条件
def check_number(n):
match n:
case x if x < 0:
return "负数"
case x if x == 0:
return "零"
case x if x < 10:
return "个位正数"
case _:
return "大于等于10"
print(f"\n-5: {check_number(-5)}")
print(f"0: {check_number(0)}")
print(f"7: {check_number(7)}")
print(f"15: {check_number(15)}")
def main03_for_loop():
"""
============================================================
3. for 循环
============================================================
Python 的 for 循环是 for-each 风格，遍历可迭代对象
"""
print("\n" + "=" * 60)
print("3. for 循环")
print("=" * 60)
# 【遍历列表】
fruits = ["apple", "banana", "cherry"]
print("遍历列表:")
for fruit in fruits:
print(f" {fruit}")
# 【遍历字符串】
print("\n遍历字符串:")
for char in "Hello":
print(f" {char}")
# 【range 函数】
print("\n使用 range:")
print("range(5):", list(range(5))) # [0, 1, 2, 3, 4]
print("range(2, 5):", list(range(2, 5))) # [2, 3, 4]
print("range(0, 10, 2):", list(range(0, 10, 2))) # [0, 2, 4, 6, 8]
print("range(5, 0, -1):", list(range(5, 0, -1))) # [5, 4, 3, 2, 1]
# 【enumerate】同时获取索引和值
print("\n使用 enumerate:")
for i, fruit in enumerate(fruits):
print(f" {i}: {fruit}")
# 指定起始索引
print("\nenumerate 指定起始:")
for i, fruit in enumerate(fruits, start=1):
print(f" {i}: {fruit}")
# 【zip】并行遍历多个序列
print("\n使用 zip:")
names = ["Alice", "Bob", "Charlie"]
ages = [25, 30, 35]
for name, age in zip(names, ages):
print(f" {name}: {age}岁")
# 【技巧】zip_longest 处理不等长序列
from itertools import zip_longest
a = [1, 2, 3]
b = ['a', 'b']
print("\nzip_longest:")
for x, y in zip_longest(a, b, fillvalue='N/A'):
print(f" ({x}, {y})")
# 【遍历字典】
print("\n遍历字典:")
person = {"name": "Alice", "age": 25, "city": "Beijing"}
print("遍历键:")
for key in person:
print(f" {key}")
print("遍历键值对:")
for key, value in person.items():
print(f" {key}: {value}")
print("遍历值:")
for value in person.values():
print(f" {value}")
# 【列表推导式】（更 Pythonic！）
print("\n列表推导式:")
squares = [x**2 for x in range(5)]
print(f"平方: {squares}")
evens = [x for x in range(10) if x % 2 == 0]
print(f"偶数: {evens}")
# 嵌套推导式
matrix = [[i*j for j in range(1, 4)] for i in range(1, 4)]
print(f"乘法表: {matrix}")
# 【字典推导式】
print("\n字典推导式:")
squares_dict = {x: x**2 for x in range(5)}
print(f"平方字典: {squares_dict}")
# 【集合推导式】
print("\n集合推导式:")
squares_set = {x**2 for x in range(-3, 4)}
print(f"平方集合: {squares_set}")
# 【生成器表达式】（节省内存）
print("\n生成器表达式:")
gen = (x**2 for x in range(5))
print(f"生成器: {gen}")
print(f"转为列表: {list(gen)}")
def main04_while_loop():
"""
============================================================
4. while 循环
============================================================
"""
print("\n" + "=" * 60)
print("4. while 循环")
print("=" * 60)
# 【基本 while】
print("基本 while:")
count = 0
while count < 5:
print(f" count = {count}")
count += 1
# 【while-else】
# 【重要】else 在循环正常结束（没有 break）时执行
print("\nwhile-else (正常结束):")
n = 0
while n < 3:
print(f" n = {n}")
n += 1
else:
print(" 循环正常结束！")
print("\nwhile-else (break 退出):")
n = 0
while n < 10:
if n == 3:
print(" 找到 3，退出！")
break
n += 1
else:
print(" 这行不会执行")
# 【无限循环】
print("\n无限循环示例:")
count = 0
while True:
count += 1
if count >= 3:
print(f" 达到 {count}，退出")
break
def main05_break_continue():
"""
============================================================
5. break 和 continue
============================================================
"""
print("\n" + "=" * 60)
print("5. break 和 continue")
print("=" * 60)
# 【break】跳出整个循环
print("break 示例:")
for i in range(10):
if i == 5:
print(f" i={i}, 跳出循环")
break
print(f" i={i}")
# 【continue】跳过本次迭代
print("\ncontinue 示例:")
for i in range(5):
if i == 2:
print(f" i={i}, 跳过")
continue
print(f" i={i}")
# 【for-else】
print("\nfor-else (查找示例):")
def find_item(items, target):
for item in items:
if item == target:
print(f" 找到 {target}!")
break
else:
print(f" 未找到 {target}")
find_item([1, 2, 3, 4, 5], 3)
find_item([1, 2, 3, 4, 5], 10)
def main06_pass():
"""
============================================================
6. pass 语句
============================================================
pass 是空操作，用作占位符
"""
print("\n" + "=" * 60)
print("6. pass 语句")
print("=" * 60)
# 【用于空函数】
def not_implemented_yet():
pass # TODO: 以后实现
# 【用于空类】
class EmptyClass:
pass
# 【用于空的条件分支】
x = 10
if x > 0:
pass # 暂时不处理
else:
print("负数")
# 【技巧】使用 ... 代替 pass（更现代的写法）
def another_placeholder():
... # 等价于 pass
print("pass 和 ... 都可用作占位符")
def main07_exception():
"""
============================================================
7. 异常处理 try/except
============================================================
"""
print("\n" + "=" * 60)
print("7. 异常处理 try/except")
print("=" * 60)
# 【基本 try-except】
print("基本异常处理:")
try:
result = 10 / 0
except ZeroDivisionError:
print(" 捕获到除零错误！")
# 【捕获多种异常】
print("\n捕获多种异常:")
try:
# 可能引发不同类型的异常
num = int("abc")
except ValueError:
print(" 捕获到 ValueError")
except TypeError:
print(" 捕获到 TypeError")
# 【捕获多种异常（合并）】
print("\n合并捕获多种异常:")
try:
num = int("abc")
except (ValueError, TypeError) as e:
print(f" 捕获到异常: {type(e).__name__}: {e}")
# 【获取异常信息】
print("\n获取异常信息:")
try:
result = 10 / 0
except ZeroDivisionError as e:
print(f" 异常类型: {type(e).__name__}")
print(f" 异常信息: {e}")
# 【try-except-else】
# else 在没有异常时执行
print("\ntry-except-else:")
try:
result = 10 / 2
except ZeroDivisionError:
print(" 除零错误")
else:
print(f" 计算成功: {result}")
# 【try-except-finally】
# finally 无论如何都执行
print("\ntry-except-finally:")
try:
result = 10 / 2
except ZeroDivisionError:
print(" 除零错误")
finally:
print(" 清理工作（finally 总是执行）")
# 【完整结构】
print("\n完整的异常处理结构:")
try:
result = 10 / 2
except ZeroDivisionError:
print(" 除零错误")
except Exception as e:
print(f" 其他错误: {e}")
else:
print(f" 成功: {result}")
finally:
print(" 清理完成")
# 【raise】主动抛出异常
print("\nraise 抛出异常:")
def check_age(age):
if age < 0:
raise ValueError("年龄不能为负数")
return age
try:
check_age(-5)
except ValueError as e:
print(f" 捕获到: {e}")
# 【自定义异常】
print("\n自定义异常:")
class MyError(Exception):
"""自定义异常类"""
def __init__(self, message, code):
super().__init__(message)
self.code = code
try:
raise MyError("自定义错误", code=404)
except MyError as e:
print(f" 异常: {e}, 错误码: {e.code}")
# 【异常链】
print("\n异常链:")
try:
try:
result = 10 / 0
except ZeroDivisionError as e:
raise ValueError("计算失败") from e
except ValueError as e:
print(f" 捕获: {e}")
print(f" 原因: {e.__cause__}")
def main08_context_manager():
"""
============================================================
8. 上下文管理器 with
============================================================
with 语句用于资源管理，确保资源被正确释放
"""
print("\n" + "=" * 60)
print("8. 上下文管理器 with")
print("=" * 60)
# 【文件操作】
print("文件操作示例:")
# 传统方式（需要手动关闭）
# f = open('file.txt')
# try:
# content = f.read()
# finally:
# f.close()
# 使用 with（推荐）
# with open('file.txt') as f:
# content = f.read()
# 文件自动关闭
print(" with open('file.txt') as f: ...")
# 【多个上下文管理器】
print("\n多个上下文管理器:")
# with open('input.txt') as fin, open('output.txt', 'w') as fout:
# fout.write(fin.read())
print(" with open('a') as f1, open('b') as f2: ...")
# 【自定义上下文管理器】
print("\n自定义上下文管理器:")
class MyContext:
def __enter__(self):
print(" 进入上下文")
return self
def __exit__(self, exc_type, exc_val, exc_tb):
print(" 退出上下文")
if exc_type:
print(f" 处理异常: {exc_val}")
return False # 不抑制异常
def do_something(self):
print(" 执行操作")
with MyContext() as ctx:
ctx.do_something()
# 【使用 contextlib】
print("\n使用 contextlib:")
from contextlib import contextmanager
@contextmanager
def my_context():
print(" 设置资源")
try:
yield "资源对象"
finally:
print(" 清理资源")
with my_context() as resource:
print(f" 使用: {resource}")
if __name__ == "__main__":
main01_if_else()
main02_match()
main03_for_loop()
main04_while_loop()
main05_break_continue()
main06_pass()
main07_exception()
main08_context_manager()
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