Python中捕获异常并引发另一个异常的正确方法是什么？也就是说，我想捕获一个类型为1的异常，操作exception对象并无条件地引发第二个类型为2的异常，这样调用函数就不会看到类型1的exception，而是绝对看到类型2的异常(其构造依赖于从类型1的异常访问的数据(。
这是我尝试过的不起作用的代码。
def d(wrt):
try:
return rt.derivative(wrt).canonicalize()
except CannotComputeDerivative as e:
msg = "n".join([f"When generating derivatives from {self}",
f" when computing edges of {rt}",
f" which canonicalizes to {self.canonicalize()}",
f" computing derivative of {e.rte}",
f" wrt={e.wrt}",
f" derivatives() reported: {e.msg}"])
raise CannotComputeDerivatives(msg=msg,
rte=rt,
wrt=wrt,
first_types=fts,
mdtd=wrts)
我认为它不起作用的原因是，我得到以下消息作为输出：
Error
Traceback (most recent call last):
File "/Users/jnewton/Repos/python-rte/pyrte/rte/r_rte.py", line 95, in d
return rt.derivative(wrt).canonicalize()
File "/Users/jnewton/Repos/python-rte/pyrte/rte/r_singleton.py", line 68, in derivative
return super().derivative(wrt)
File "/Users/jnewton/Repos/python-rte/pyrte/rte/r_rte.py", line 72, in derivative
return self.derivative_down(wrt)
File "/Users/jnewton/Repos/python-rte/pyrte/rte/r_singleton.py", line 91, in derivative_down
raise CannotComputeDerivative(
rte.r_rte.CannotComputeDerivative
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/Users/jnewton/Repos/python-rte/pyrte/tests/rte_tests.py", line 615, in test_derivatives
self.assertTrue(rt.derivatives())
File "/Users/jnewton/Repos/python-rte/pyrte/rte/r_rte.py", line 111, in derivatives
return trace_graph(self, edges)
File "/Users/jnewton/Repos/python-rte/pyrte/genus/utils.py", line 199, in trace_graph
es = edges(v0) # List[(L,V)]
File "/Users/jnewton/Repos/python-rte/pyrte/rte/r_rte.py", line 109, in edges
return [(td, d(td)) for td in wrts]
File "/Users/jnewton/Repos/python-rte/pyrte/rte/r_rte.py", line 109, in <listcomp>
return [(td, d(td)) for td in wrts]
File "/Users/jnewton/Repos/python-rte/pyrte/rte/r_rte.py", line 103, in d
raise CannotComputeDerivatives(msg=msg,
rte.r_rte.CannotComputeDerivatives
根据宫城先生的建议，我在raise CannotComputeDerivatives(...)之后添加了from None。这取得了进展，但unittest环境似乎并不喜欢它。单元测试期间显示的消息如下所示。看起来unittest不幸地截断了消息。
Error
Traceback (most recent call last):
File "/Users/jnewton/Repos/python-rte/pyrte/tests/rte_tests.py", line 615, in test_derivatives
rt.derivatives()
File "/Users/jnewton/Repos/python-rte/pyrte/rte/r_rte.py", line 111, in derivatives
return trace_graph(self, edges)
File "/Users/jnewton/Repos/python-rte/pyrte/genus/utils.py", line 199, in trace_graph
es = edges(v0) # List[(L,V)]
File "/Users/jnewton/Repos/python-rte/pyrte/rte/r_rte.py", line 109, in edges
return [(td, d(td)) for td in wrts]
File "/Users/jnewton/Repos/python-rte/pyrte/rte/r_rte.py", line 109, in <listcomp>
return [(td, d(td)) for td in wrts]
File "/Users/jnewton/Repos/python-rte/pyrte/rte/r_rte.py", line 103, in d
raise CannotComputeDerivatives(msg=msg,
rte.r_rte.CannotComputeDerivatives
=============要跟进解决方案：最初的问题是CannotComputeDerivatives类的__init__函数调用super().__init__()时没有传递消息字符串。我更新了类定义如下。
class CannotComputeDerivatives(Exception):
def __init__(self, msg, rte, wrt, first_types, mdtd):
self.msg = msg
self.rte = rte
self.wrt = wrt
self.first_types = first_types
self.mdtd = mdtd
super().__init__(msg)
结果是，我在单元测试期间收到了漂亮的错误消息：
Error
Traceback (most recent call last):
File "/Users/jnewton/Repos/python-rte/pyrte/tests/rte_tests.py", line 615, in test_derivatives
self.assertTrue(rt.derivatives())
File "/Users/jnewton/Repos/python-rte/pyrte/rte/r_rte.py", line 111, in derivatives
return trace_graph(self, edges)
File "/Users/jnewton/Repos/python-rte/pyrte/genus/utils.py", line 199, in trace_graph
es = edges(v0) # List[(L,V)]
File "/Users/jnewton/Repos/python-rte/pyrte/rte/r_rte.py", line 109, in edges
return [(td, d(td)) for td in wrts]
File "/Users/jnewton/Repos/python-rte/pyrte/rte/r_rte.py", line 109, in <listcomp>
return [(td, d(td)) for td in wrts]
File "/Users/jnewton/Repos/python-rte/pyrte/rte/r_rte.py", line 103, in d
raise CannotComputeDerivatives(msg=msg,
rte.r_rte.CannotComputeDerivatives: When generating derivatives from Singleton(SOr(odd?, SAtomic(Test2)))
when computing edges of Singleton(SOr(odd?, SAtomic(Test2)))
which canonicalizes to Singleton(SOr(odd?, SAtomic(Test2)))
computing derivative of Singleton(SOr(odd?, SAtomic(Test2)))
wrt=SOr(SAtomic(Test2), odd?)
derivatives() reported: Singleton.derivative_down cannot compute derivative of Singleton(SOr(odd?, SAtomic(Test2)))
wrt=SOr(SAtomic(Test2), odd?)
disjoint=False
subtypep=None
感谢@MisterMiyagi建议在raise ...之后添加from None。这避免了系统在异常中抱怨异常的问题。