UnboundLocalError: local variable 'min_diff' referenced before assignment

UnboundLocalError是一个常见的Python错误，当你在一个函数内部引用了一个局部变量（也就是在函数体中声明的变量），但在引用它之前没有对其进行初始化赋值，就会抛出这个错误。例如，在下面的代码中：

def find_min_diff(z, x):
    min_diff = None  # 忘记了给 min_diff 赋初值
    for i in z:
        diff = abs(i - x)
        if not min_diff or diff < min_diff:
            min_diff = diff
    print(min_diff)  # 此处会报错，因为尝试访问未赋值的 min_diff

z = [1, 2, 3]
find_min_diff(z, 0)

在这个例子中，`min_diff`在第一次迭代前并没有被赋值，所以在计算 `diff < min_diff` 的条件时就引发了错误。

解决这个问题的方法是在引用 `min_diff` 之前先给它一个默认初始值，比如 `None` 或者 `float('inf')`：

def find_min_diff(z, x):
    min_diff = float('inf')
    for i in z:
        diff = abs(i - x)
        if diff < min_diff:
            min_diff = diff
    print(min_diff)

这样就不会再出现 `UnboundLocalError` 了。