I know that if I want to re-raise an exception, I simple use raise without arguments in the respective except block. But given a nested expression like

try:
    something()
except SomeError as e:
    try:
        plan_B()
    except AlsoFailsError:
        raise e  # I'd like to raise the SomeError as if plan_B()
                 # didn't raise the AlsoFailsError

how can I re-raise the SomeError without breaking the stack trace? raise alone would in this case re-raise the more recent AlsoFailsError. Or how could I refactor my code to avoid this issue?

As of Python 3 the traceback is stored in the exception, so a simple raise e will do the (mostly) right thing:

try:
    something()
except SomeError as e:
    try:
        plan_B()
    except AlsoFailsError:
        raise e  # or raise e from None - see below

The traceback produced will include an additional notice that SomeError occurred while handling AlsoFailsError (because of raise e being inside except AlsoFailsError). This is misleading because what actually happened is the other way around – we encountered AlsoFailsError, and handled it, while trying to recover from SomeError. To obtain a traceback that doesn’t include AlsoFailsError, replace raise e with raise e from None.

In Python 2 you’d store the exception type, value, and traceback in local variables and use the three-argument form of raise:

try:
    something()
except SomeError:
    t, v, tb = sys.exc_info()
    try:
        plan_B()
    except AlsoFailsError:
        raise t, v, tb

Even if the accepted solution is right, it’s good to point to the Six library which has a Python 2+3 solution, using six.reraise.

six.reraise(exc_type, exc_value, exc_traceback=None)

Reraise an exception, possibly with a different traceback. […]

So, you can write:

import six


try:
    something()
except SomeError:
    t, v, tb = sys.exc_info()
    try:
        plan_B()
    except AlsoFailsError:
        six.reraise(t, v, tb)

As per Drew McGowen’s suggestion, but taking care of a general case (where a return value s is present), here’s an alternative to user4815162342’s answer:

try:
    s = something()
except SomeError as e:
    def wrapped_plan_B():
        try:
            return False, plan_B()
        except:
            return True, None
    failed, s = wrapped_plan_B()
    if failed:
        raise

Python 3.5+ attaches the traceback information to the error anyway, so it’s no longer necessary to save it separately.

>>> def f():
...   try:
...     raise SyntaxError
...   except Exception as e:
...     err = e
...     try:
...       raise AttributeError
...     except Exception as e1:
...       raise err from None
>>> f()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 9, in f
  File "<stdin>", line 3, in f
SyntaxError: None
>>>