Question

给出一个函数调用和一个紧随其后的try块，在任何情况下该调用都能正常返回，但引发了异常并且没有被try块捕获？

例如：

# example 1
resource = acquire_a_resource()
try:
    resource.do_something()
    # some more code...
finally:
    resource.close()

acquire_a_resource()是否可以正常返回，但不会调用resource.close()？

换句话说，是否存在以下情况：

# example 2
resource = None
try:
    resource = acquire_a_resource()
    resource.do_something()
    # some more code...
finally:
    if resource:
        resource.close()

比示例1更安全吗？

也许是因为与KeyboardInterrupt /线程/信号有关？

Answer 1

是的，至少在理论上是这样，尽管在CPython中不是（有关详细信息，请参见脚注）。线程并不是特别相关，但是您的KeyboardInterrupt场景恰恰是正确的：

resource = acquire_a_resource()

调用该函数。该函数获取资源并返回该句柄，然后在分配给变量¹的过程中发生键盘中断。所以：

try:

不运行-发生KeyboardInterrupt异常，保留当前函数并取消绑定变量。

第二个版本通过finally子句，因此假设if resource找到了布尔truth-y，resource.close()就会被调用。

（请注意，实际上触发此操作通常非常困难：您必须将中断时间定为正确。您可以通过例如在time.sleep(1)之前添加try来大大增加比赛窗口。）

在许多情况下，with语句效果很好：

with acquire_a_resource() as resource:
    resource.do_something()

close内置到__exit__方法中。即使通过异常转义了块，该方法也会运行。

¹通常，该实现必须完成将获取的资源与变量的绑定，否则将发生不可恢复的竞争。在CPython中，发生这种情况的原因是解释器检查语句之间的中断，并偶尔检查源代码中的重要位置。

CPython实际上增加了另一种特殊情况：

    /* Do periodic things.  Doing this every time through
       the loop would add too much overhead, so we do it
       only every Nth instruction.  We also do it if
       ``pendingcalls_to_do'' is set, i.e. when an asynchronous
       event needs attention (e.g. a signal handler or
       async I/O handler); see Py_AddPendingCall() and
       Py_MakePendingCalls() above. */

    if (_Py_atomic_load_relaxed(&_PyRuntime.ceval.eval_breaker)) {
        opcode = _Py_OPCODE(*next_instr);
        if (opcode == SETUP_FINALLY ||
            opcode == SETUP_WITH ||
            opcode == BEFORE_ASYNC_WITH ||
            opcode == YIELD_FROM) {
            /* Few cases where we skip running signal handlers and other
               pending calls:
               - If we're about to enter the 'with:'. It will prevent
                 emitting a resource warning in the common idiom
                 'with open(path) as file:'.
               - If we're about to enter the 'async with:'.
               - If we're about to enter the 'try:' of a try/finally (not
                 *very* useful, but might help in some cases and it's
                 traditional)
               - If we're resuming a chain of nested 'yield from' or
                 'await' calls, then each frame is parked with YIELD_FROM
                 as its next opcode. If the user hit control-C we want to
                 wait until we've reached the innermost frame before
                 running the signal handler and raising KeyboardInterrupt
                 (see bpo-30039).
            */
            goto fast_next_opcode;
        }

（Python/ceval.c，第1000行附近）。

实际上，try行确实在运行，因为这里有SETUP_FINALLY。我还不清楚其他Python实现是否也做同样的事情。

在python中，是否可能在调用之后但紧随其后的try块之前发生异常？

1 个答案: