[Python知识库]python while-else / for-else

在python信号量P操作源码中，有一段while-else格式的代码：while self._value == 0:…

    def acquire(self, blocking=True, timeout=None):
        if not blocking and timeout is not None:
            raise ValueError("can't specify timeout for non-blocking acquire")
        rc = False
        endtime = None
        with self._cond:
            while self._value == 0:
                if not blocking:
                    break
                if timeout is not None:
                    if endtime is None:
                        endtime = _time() + timeout
                    else:
                        timeout = endtime - _time()
                        if timeout <= 0:
                            break
                self._cond.wait(timeout)
            else:
                self._value -= 1
                rc = True
        return rc

自行举了几个例子测试逻辑：

1、不符合循环条件，执行else

count = 6
while count <= 5:
    count += 1
    if count == 4:
        continue
    print("Loop", count)

else:
    print("while正常执行完或不符合while循环条件")
print("-----out of while loop ------")

结果：

while正常执行完或不符合while循环条件
-----out of while loop ------

2、循环中途break，不执行else

count = 0  ##
while count <= 5:
    count += 1
    if count == 4:
        break  ##
    print("Loop", count)

else:
    print("while正常执行完或不符合while循环条件")
print("-----out of while loop ------")
....

结果：

Loop 1
Loop 2
Loop 3
-----out of while loop ------

3、while正常执行完毕，执行else

count = 0 ##
while count <= 5:
    count += 1
    if count == 4:
        continue ##
    print("Loop", count)

else:
    print("while正常执行完或不符合while循环条件")
print("-----out of while loop ------")
....

结果：

Loop 1
Loop 2
Loop 3
Loop 5
Loop 6
while正常执行完或不符合while循环条件
-----out of while loop ------