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学习目标

本章节你将学习:

• 条件语句 (if/elif/else)
• 循环语句 (for/while)
• 模式匹配 (match-case，Python 3.10+)
• 异常处理 (try/except)

条件语句

if/elif/else

score = 85

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "D"

print(f"Grade: {grade}")  # Grade: B

// JavaScript 对比
const score = 85
let grade

if (score >= 90) {
  grade = 'A'
} else if (score >= 80) {
  grade = 'B'
} else if (score >= 70) {
  grade = 'C'
} else {
  grade = 'D'
}

关键差异:

• Python 用 elif，JavaScript 用 else if
• Python 无括号 ()，用 : 结尾
• Python 用缩进代替 {}

三元表达式

# Python 三元表达式
age = 20
status = "adult" if age >= 18 else "minor"

// JavaScript 三元运算符
const age = 20
const status = age >= 18 ? 'adult' : 'minor'

逻辑运算符

# Python 用英文单词
if x > 0 and y > 0:
    print("Both positive")

if x > 0 or y > 0:
    print("At least one positive")

if not is_admin:
    print("Not admin")

// JavaScript 用符号
if (x > 0 && y > 0) {
  console.log('Both positive')
}

if (x > 0 || y > 0) {
  console.log('At least one positive')
}

if (!isAdmin) {
  console.log('Not admin')
}

JavaScript	Python
&&	and
||	or
!	not

布尔表达式规范 (PEP 8)

# ✅ 推荐: 直接使用真值判断
if items:
    print("列表非空")

if not items:
    print("列表为空")

if name:
    print("名字非空")

if is_valid:
    print("有效")

# ❌ 不推荐: 与 True/False 比较
if items == True:      # 不好
    pass

if is_valid == True:   # 不好
    pass

if len(items) > 0:     # 不好，直接用 if items:
    pass

if len(items) == 0:    # 不好，直接用 if not items:
    pass

# ✅ 推荐: None 检查使用 is
if result is None:
    print("无结果")

if result is not None:
    print("有结果")

# ❌ 不推荐: 使用 == 检查 None
if result == None:     # 不好
    pass

# ✅ 推荐: 空容器检查
data = []
if not data:           # 推荐
    print("空列表")

# ❌ 不推荐
if data == []:         # 不好
    pass

if len(data) == 0:     # 不好
    pass

// JavaScript 对比 - 也推荐类似风格
if (items.length) {
  console.log('数组非空')
}

if (!items.length) {
  console.log('数组为空')
}

// 但 JS 的 null/undefined 检查通常用 ===
if (result === null) {
  console.log('无结果')
}

规范总结：

场景	✅ 推荐	❌ 不推荐
检查非空容器	if items:	if len(items) > 0:
检查空容器	if not items:	if len(items) == 0:
检查 None	if x is None:	if x == None:
检查非 None	if x is not None:	if x != None:
检查布尔值	if flag:	if flag == True:
检查假布尔值	if not flag:	if flag == False:

for 循环

遍历序列

# 遍历列表
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

# 遍历字符串
for char in "hello":
    print(char)

// JavaScript for...of 对比
const fruits = ['apple', 'banana', 'cherry']
for (const fruit of fruits) {
  console.log(fruit)
}

range() 函数

# range(stop) - 从 0 到 stop-1
for i in range(5):
    print(i)  # 0, 1, 2, 3, 4

# range(start, stop)
for i in range(2, 5):
    print(i)  # 2, 3, 4

# range(start, stop, step)
for i in range(0, 10, 2):
    print(i)  # 0, 2, 4, 6, 8

# 倒序
for i in range(5, 0, -1):
    print(i)  # 5, 4, 3, 2, 1

// JavaScript 对比
for (let i = 0; i < 5; i++) {
  console.log(i)
}

带索引遍历

# 使用 enumerate() 获取索引
fruits = ["apple", "banana", "cherry"]
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")
# 0: apple
# 1: banana
# 2: cherry

# 指定起始索引
for index, fruit in enumerate(fruits, start=1):
    print(f"{index}: {fruit}")
# 1: apple
# 2: banana
# 3: cherry

// JavaScript 对比
const fruits = ['apple', 'banana', 'cherry']
fruits.forEach((fruit, index) => {
  console.log(`${index}: ${fruit}`)
})

遍历字典

user = {"name": "Alice", "age": 25, "city": "Beijing"}

# 遍历键
for key in user:
    print(key)

# 遍历键值对
for key, value in user.items():
    print(f"{key}: {value}")

# 遍历值
for value in user.values():
    print(value)

// JavaScript 对比
const user = { name: 'Alice', age: 25, city: 'Beijing' }

for (const key in user) {
  console.log(`${key}: ${user[key]}`)
}

// 或使用 Object.entries()
for (const [key, value] of Object.entries(user)) {
  console.log(`${key}: ${value}`)
}

while 循环

# 基本 while 循环
count = 0
while count < 5:
    print(count)
    count += 1

# while-else (Python 特有)
count = 0
while count < 5:
    print(count)
    count += 1
else:
    print("Loop completed normally")

Python 特色: while 和 for 循环都可以有 else 子句，当循环正常结束（非 break）时执行。

break 和 continue

# break - 跳出循环
for i in range(10):
    if i == 5:
        break
    print(i)  # 0, 1, 2, 3, 4

# continue - 跳过本次迭代
for i in range(5):
    if i == 2:
        continue
    print(i)  # 0, 1, 3, 4

# for-else 与 break
for i in range(5):
    if i == 10:  # 永远不会触发
        break
else:
    print("Loop completed without break")  # 会执行

match-case (Python 3.10+)

Python 3.10 引入了结构化模式匹配，类似 JavaScript 的 switch-case，但功能更强大。

基本用法

def http_status(status):
    match status:
        case 200:
            return "OK"
        case 404:
            return "Not Found"
        case 500:
            return "Server Error"
        case _:  # 默认情况，相当于 default
            return "Unknown"

print(http_status(200))  # OK
print(http_status(999))  # Unknown

// JavaScript switch 对比
function httpStatus(status) {
  switch (status) {
    case 200:
      return 'OK'
    case 404:
      return 'Not Found'
    case 500:
      return 'Server Error'
    default:
      return 'Unknown'
  }
}

模式匹配进阶

# 匹配多个值
match status:
    case 200 | 201 | 204:  # 或运算
        return "Success"
    case 400 | 401 | 403 | 404:
        return "Client Error"
    case _:
        return "Unknown"

# 解构匹配
point = (3, 4)
match point:
    case (0, 0):
        print("Origin")
    case (x, 0):
        print(f"On x-axis at {x}")
    case (0, y):
        print(f"On y-axis at {y}")
    case (x, y):
        print(f"Point at ({x}, {y})")

# 带守卫条件
match point:
    case (x, y) if x == y:
        print("On diagonal")
    case (x, y):
        print(f"Point at ({x}, {y})")

异常处理

try/except

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero!")
except Exception as e:
    print(f"Error: {e}")
else:
    print(f"Result: {result}")  # 无异常时执行
finally:
    print("Cleanup")  # 总是执行

// JavaScript 对比
try {
  const result = 10 / 0
} catch (e) {
  console.log(`Error: ${e}`)
} finally {
  console.log('Cleanup')
}

关键差异:

• Python 用 except，JavaScript 用 catch
• Python 可以捕获特定类型的异常
• Python 有 else 子句（无异常时执行）

常见异常类型

# ValueError - 值错误
int("hello")

# TypeError - 类型错误
"hello" + 42

# KeyError - 键不存在
d = {}
d["missing"]

# IndexError - 索引越界
lst = [1, 2, 3]
lst[10]

# FileNotFoundError - 文件不存在
open("nonexistent.txt")

抛出异常

def divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

try:
    divide(10, 0)
except ValueError as e:
    print(e)  # Cannot divide by zero

// JavaScript 对比
function divide(a, b) {
  if (b === 0) {
    throw new Error('Cannot divide by zero')
  }
  return a / b
}

对前端开发者

常见误区

# 1. 没有 switch-case（Python 3.10 之前）
# 使用 if-elif-else 或字典映射代替

# 字典映射模式
def get_status_text(status):
    return {
        200: "OK",
        404: "Not Found",
        500: "Server Error"
    }.get(status, "Unknown")

# 2. 循环中修改列表要小心
# 错误做法
numbers = [1, 2, 3, 4, 5]
for n in numbers:
    if n % 2 == 0:
        numbers.remove(n)  # 危险! 可能跳过元素

# 正确做法 - 创建新列表
numbers = [n for n in numbers if n % 2 != 0]

小结

概念	JavaScript	Python
条件关键字	else if	elif
逻辑与	&&	and
逻辑或	||	or
逻辑非	!	not
遍历	for...of	for...in
带索引遍历	forEach	enumerate()
switch	switch/case	match/case (3.10+)
异常捕获	catch	except
抛出异常	throw	raise