深入探讨Python推导式的边界场景与常见错误避免指南

[expression for item in iterable if condition]

squares = [x**2 for x in range(10) if x % 2 == 0]
print(squares)  # [0, 4, 16, 36, 64]

# 错误示例：变量名冲突
matrix = [[1, 2], [3, 4], [5, 6]]
result = [x for x in row for row in matrix]  # ❌ 报错！

for row in matrix:
    for x in row:
        yield x

# 正确写法：外层变量必须先于内层出现
matrix = [[1, 2], [3, 4], [5, 6]]
result = [x for row in matrix for x in row]
print(result)  # [1, 2, 3, 4, 5, 6]

# 错误：if 放在表达式前面
numbers = [1, 2, 3, 4, 5]
result = [x if x % 2 == 0 for x in numbers]  # ❌ SyntaxError

# ✅ 正确：使用三元表达式
result = [x if x % 2 == 0 else 0 for x in numbers]
print(result)  # [0, 2, 0, 4, 0]

# ✅ 正确：用 if 过滤（非三元）
result = [x for x in numbers if x % 2 == 0]
print(result)  # [2, 4]

# 错误：引用了局部作用域外的变量
value = 10
result = [x + y for x in range(5) if x > 3 for y in [value, value*2]]
print(result)  # ✅ 本应正常？等等...

def make_multiplier(n):
    return [x * n for x in range(5)]

print(make_multiplier(2))  # [0, 2, 4, 6, 8] ✅

def make_multipliers():
    factors = [2, 3, 5]
    return [x * f for x in range(3) for f in factors]

result = make_multipliers()
print(result)  # [0, 0, 0, 3, 6, 9, 10, 15, 20] ✅

def create_handlers():
    handlers = []
    for i in range(3):
        handlers.append(lambda: print(i))  # ❌ 捕获的是最后的 i
    return handlers

handlers = create_handlers()
for h in handlers:
    h()  # 打印 2, 2, 2 ❌ 不是预期的 0,1,2

def create_handlers_fixed():
    handlers = []
    for i in range(3):
        handlers.append(lambda i=i: print(i))  # ✅ 固定当前 i
    return handlers

handlers = create_handlers_fixed()
for h in handlers:
    h()  # 打印 0, 1, 2 ✅

# 推导式中也需注意变量捕获
factors = [2, 3, 5]
result = [x * f for f in factors for x in range(3)]
print(result)  # [0, 1, 2, 0, 2, 4, 0, 3, 6] ✅

empty_list = []
result = [x * 2 for x in empty_list]
print(result)  # [] ✅ 正常，但容易忽略

data = []
default_result = [x * 2 for x in data] or [0]
print(default_result)  # [0] ✅

result = [x * 2 for x in data] if data else [0]

# 危险！处理百万级数据
huge_data = range(1_000_000)
squares = [x**2 for x in huge_data]  # 占用大量内存！

# ✅ 使用生成器表达式（推荐）
squares_gen = (x**2 for x in huge_data)

# 只在需要时才计算
for sq in squares_gen:
    if sq > 1000000:
        print(f"第一个大于100万的平方数是: {sq}")
        break

words = ['hello', 'world', 'python']
result = ''.join([w.upper() for w in words])  # ✅ 可行，但不高效

result = ''.join(w.upper() for w in words)  # ✅ 推荐！
print(result)  # HELLOWORLDPYTHON

mixed = [1, '2', 3.0, '4']
# 错误：试图对字符串进行数学运算
result = [x * 2 for x in mixed]  # ❌ 有些会报错

from typing import Union

def safe_multiply(items: list, factor: Union[int, float]) -> list:
    result = []
    for item in items:
        try:
            if isinstance(item, (int, float)):
                result.append(item * factor)
            else:
                # 尝试转换为数字
                num = float(item)
                result.append(num * factor)
        except (ValueError, TypeError):
            continue  # 忽略无法转换的项
    return result

mixed = ['1', '2', 'abc', 3, '4.5']
print(safe_multiply(mixed, 2))  # [2.0, 4.0, 6.0, 9.0]

results = []
for i in range(10):
    for j in range(5):
        if i % 2 == 0:
            results.append(i * j)