[英]in python, is it possible for an exception to occur after a call but “before” the try block that follows it?
给定一个函数调用和一个紧随其后的try块,是否存在调用正常返回但引发了异常且没有被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()
是否有可能acquire_a_resource()
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()
会比示例1更安全吗?
也许是因为与KeyboardInterrupt
/ threads / signals有关?
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场景恰好是正确的:
resource = acquire_a_resource()
调用函数。 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:
不会运行-而是发生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
发现它为boolean-truth-y,则将调用resource.close()
。
(请注意,实际上触发此操作通常非常困难:您必须恰好计时中断。您可以通过例如在try
之前添加time.sleep(1)
来大大增加竞赛窗口。)
在许多情况下, with
语句可以很好地工作:
with acquire_a_resource() as resource:
resource.do_something()
close
内置在__exit__
方法中。 即使通过异常转义了该块,该方法也会运行。
1通常,实现必须完成所获取资源与变量的绑定,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
,在1000行附近)。
实际上, try
行确实有效,因为这里有一个SETUP_FINALLY
。 我还不清楚其他Python实现是否也做同样的事情。
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