python - 为什么删除 else 会导致递归函数重复结果？

Question

当我使用以下语句在 Python 控制台上执行以下代码时，

for L in permute([12, 32, 3]):
    print(L)

对于下面的代码，

def permute(L):
    if len(L) <= 1:
        yield L
    else:
        for i in range(0, len(L)):
            L[0], L[i] = L[i], L[0]
            for L1 in permute(L[1:]):
                yield [L[0]] + L1

每个结果仅出现一次。但是，如果我删除 else 并删除其下方代码的关联缩进，我会收到每个结果两次。为什么会发生这种情况？

Answer 1

来自docs :

The big difference between yield and a return statement is that on reaching a yield the generator’s state of execution is suspended and local variables are preserved. On the next call to the generator’s __next__() method, the function will resume executing.

发生这种情况是因为 if 主体不会中断执行，因此它会前进到下一个 yield 语句，并且 with else 子句到达函数结束并隐式返回。

检查

def counter(step):
    assert step >= 0, ('Can count only from non-negative number, but found '
                       + str(step))
    if step == 0:
        print('Reached end')
        yield step
    print('Step', step)
    for sub_step in counter(step - 1):
        yield sub_step

>>> list(counter(1))

Step 1
Reached end
Traceback (most recent call last):
  File "<input>", line 13, in <module>
Step 0
  File "<input>", line 8, in counter
  File "<input>", line 8, in counter
  File "<input>", line 3, in counter
AssertionError: Can count only from non-negative number, but found -1

和

def counter(step):
    assert step >= 0, ('Can count only from non-negative number, but found '
                       + str(step))
    if step == 0:
        print('Reached end')
        yield step
    else:
        print('Step', step)
        for sub_step in counter(step - 1):
            yield sub_step
    # implicitly returns here

>>> list(counter(1))
Step 1
Reached end

正如我们所见，如果没有else，执行将继续并调用counter，step等于-1。

因此，您可以保留 else 或重构您的 if 子句，并在其中添加 return 语句以显式完成执行，例如

def permute(L):
    if len(L) <= 1:
        yield L
        return
    for i in range(0, len(L)):
        L[0], L[i] = L[i], L[0]
        for L1 in permute(L[1:]):
            yield [L[0]] + L1

进一步阅读

• Generators .
• return和 yield声明。