局部变量和全局变量之间的差异？

Question

Possible Duplicate:
Python passing list as argument

我找了很多关于这个的话题，但我不明白到底发生了什么。在

我有这个代码：

def altera(L1, L2):
    for elemento in L2:
        L1.append(elemento)
    L2 = L2 + [4]
    L1[-1] = 10
    del L2[0]
    return L2[:]

Lista1 = [1,2,3]
Lista2 = [1,2,3]

Lista3 = altera(Lista1, Lista2)

print Lista1
print Lista2
print Lista3

结果是：

^{pr2}$

我不明白Lista1是如何被修改的，而{}是如何被修改的。但是在测试代码之前，我认为Lista1和{}将保持不变，因为它们是全局变量。在

Answer 1

当您执行L1.append(elemento)时，您正在调用一个方法，该方法实际上更改了由变量L1命名的列表。其他所有设置L1和L2值的命令实际上只是为新变量创建新名称。在

此版本不会更改任何内容：

def altera(L1, L2):
    for elemento in L2:
        # create a new list and assign name L1
        L1 = L1 + [elemento]
    # create a new list and assign name L2
    L2 = L2 + [4]
    return L2

Lista1 = [1,2,3]
Lista2 = [1,2,3]

Lista3 = altera(Lista1, Lista2)

print Lista1
print Lista2
print Lista3

而这一次：

^{pr2}$

但是L += [2]有一个棘手的问题，它与L = L + 2不完全相同。关于扩充赋值语句的Python Language Reference部分解释了这一区别：

An augmented assignment expression like x += 1 can be rewritten as x = x + 1 to achieve a similar, but not exactly equal effect. In the augmented version, x is only evaluated once. Also, when possible, the actual operation is performed in-place, meaning that rather than creating a new object and assigning that to the target, the old object is modified instead.”