Python: raise / 3 key elements of an exception

1. raise-statement in Python 2

形式定义是 raise [arg1[, arg2[, arg3]]]，分这么几种情况：

1.1 raise nothing（也就是 raise 后面什么都不接）

• In this case, raise statement re-raises the last exception that was active in the current scope.
• If no exception is active in the current scope, a TypeError exception is raised indicating that this is an error.
  • If running under IDLE, a Queue.Empty exception is raised instead.

所谓 “last exception that was active in the current scope”，大概的场景是：

try:
    # do something dangerous
except SomeException as e:  # 即使你没有 `as e`，`raise` 也是可以定位到你当前的 exception instance
    raise 
    # ----- IS EQUIVALENT TO ----- #
    # raise e

此外，这个当前 scope 下 active 的 exception 可以通过 sys.exc_info() 来获取。这个函数返回一个 tuple，(exc_type, exc_value, tb_obj)，也就是我们的异常三要素。

如果 raise 后面有参数，它一定会 evaluate 出一个 (exc_type, exc_value, tb_obj)，空缺的值用 None 填充。

• 注意形式定义里的 arg3 即是 tb_obj（一个 built-in 类型 Traceback 的对象）
  • 它的作用是 be substituted as the place where the exception occurre instead of the current location
  • 如果你 arg3 不是 Traceback 类型，系统会抛一个 TypeError
• (arg1, arg2) 并不严格对应 (exc_type, exc_value)，因为 raise 语法的性质导致（所以我不喜欢 python 2！）
  • 而且我一直以为 exc_value 指 error msg，其实不是的，它本质一定是一个 exception instance

1.2 raise arg1

单独抛出一个 exception instance e，此时 (exc_type, exc_value) == (type(e), e)。

tb_obj 在异常发生时系统会自动创建一个给你，所以我估计是很少会需要去自己管理这个对象，所以这里我们也忽略这个对象的值。

import sys

ve = ValueError()

try:
    raise ve
except ValueError: 
    print(sys.exc_info())

# Output: (<type 'exceptions.ValueError'>, ValueError(), <traceback object at 0x7fb8d03d97a0>)

import sys
    
ve = ValueError("1 != 2")

try:
    raise ve
except ValueError: 
    print(sys.exc_info())

# Output: (<type 'exceptions.ValueError'>, ValueError('1 != 2',), <traceback object at 0x7fb8d03d97a0>)

1.3 raise arg1, arg2

涉及到异常类型继承关系时，这一句的 evaluation 有点微妙。

用法一： raise exc_type, constructor_param 或者 raise exc_type, (constructor_params...)

此时 exc_type = arg1；arg2 可以是单个值或是一个 tuple，用来传递给 exc_type 的 constructor，从而 exc_value = arg1(arg2)

import sys

try:
    raise ValueError, "1 != 2"
except ValueError: 
    print(sys.exc_info())

# Output: (<type 'exceptions.ValueError'>, ValueError('1 != 2',), <traceback object at 0x7fb8d03d9758>)

try:
    raise ValueError, ('1', '!=', '2')
except ValueError: 
    print(sys.exc_info())

# Output: (<type 'exceptions.ValueError'>, ValueError('1', '!=', '2'), <traceback object at 0x7fb8d03d9a28>)

用法二： raise exc_type, exc_value

此时 exc_type = arg1；如果 isinstance（arg2, arg1) == True，则 exc_value = arg2，否则 fall back 到用法一。

import sys

class MyValueError(ValueError):
    pass

class MyIOError():
    pass

try:
    raise ValueError, MyValueError()
except ValueError: 
    print(sys.exc_info())

# Output: (<class '__main__.MyValueError'>, MyValueError(), <traceback object at 0x7fb8d03d9b90>)
    
try:
    raise ValueError, MyIOError()
except ValueError: 
    print(sys.exc_info())

# Output: (<type 'exceptions.ValueError'>, ValueError(<__main__.MyIOError instance at 0x7fb8d03d98c0>,), <traceback object at 0x7fb8d03d95a8>)

2. raise-statement in Python 3

形式定义是 raise [arg1 [from arg2]]，而且在没有参数对应 tb_obj，这是因为 python 3 的 exception 自带 setter __traceback__()，而且是 return self，所以可以这么连写：

raise Exception("foo occurred").with_traceback(tb_obj) [from Xxx]

2.1 raise nothing

基本等同于 python 2 的情况，不同之处在于：

• If no exception is active in the current scope, a RuntimeError exception is raised indicating that this is an error.
  • python 2 是抛 TypeError

2.2 raise arg1

arg1 must be either a subclass or an instance of BaseException.

• If it is a class, exc_type = arg1 and exc_value = arg1() (一定是使用无参 constructor).
• If it is an instance, exc_type = type(arg1) and exc_value = arg1

2.3 raise arg1 from arg2

The from clause is used for exception chaining: if given, arg2 must be another exception class or instance, which will then be attached to the raised exception as the __cause__ attribute (which is writable). If the raised exception is not handled, both exceptions will be printed:

>>> try:
...     print(1 / 0)
... except Exception as exc:
...     raise RuntimeError("Something bad happened") from exc
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: division by zero

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
RuntimeError: Something bad happened

A similar mechanism works implicitly if an exception is raised inside an except or a finally clause: the previous exception is then attached as the new exception’s __context__ attribute:

>>> try:
...     print(1 / 0)
... except:
...     raise RuntimeError("Something bad happened")
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
RuntimeError: Something bad happened

Exception chaining can be explicitly suppressed by specifying from None:

>>> try:
...     print(1 / 0)
... except:
...     raise RuntimeError("Something bad happened") from None
...
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
RuntimeError: Something bad happened