在 python 中，是否有可能在调用之后但“之前”跟随它的 try 块发生异常？

Question

给定一个函数调用和紧随其后的 try 块，是否存在调用正常返回但引发异常且未被 try 块捕获的情况？

例如：

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

有没有可能acquire_a_resource()正常返回但resource.close()不会被调用？

或者换句话说，是否有任何场景：

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

会比示例 #1 更安全吗？

也许是因为与KeyboardInterrupt/threads/signals 有关？

Answer 1

是的，至少在理论上是这样，尽管在 CPython 中不是（详见脚注）。线程不是特别相关，但您的 KeyboardInterrupt 场景恰到好处：

resource = acquire_a_resource()

调用函数。函数获取资源并返回句柄，然后在赋值给变量的过程中，¹发生键盘中断。所以：

try:

不运行 - 而是发生 KeyboardInterrupt 异常，离开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 发现它是布尔值，resource.close() 确实会被调用。

（请注意，实际上触发它通常非常困难：您必须恰到好处地安排中断时间。您可以通过例如在try 之前添加time.sleep(1) 来增加竞争窗口。）

在许多情况下，with 语句效果很好：

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

close 内置于 __exit__ 方法中。即使该块通过异常转义，该方法也会运行。

---

¹一般情况下，实现有义务完成获取到的资源到变量的绑定，否则会出现不可恢复的竞态。在 CPython 中，这是因为解释器会检查语句之间的中断，有时还会检查源代码中的关键位置。

CPython 实际上增加了另一个特殊情况：

    /* 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，1000 行附近）。

所以实际上try 行确实 运行，因为这里有一个SETUP_FINALLY。我完全不清楚其他 Python 实现是否做同样的事情。