Python: accessing exception message of original exception

Question

I have the following two functions:

>>> def spam():
...     raise ValueError('hello')
...
>>> def catch():
...     try:
...         spam()
...     except ValueError:
...         raise ValueError('test')

Trying to catch the second ValueError exception works just fine and prints the exception's error message:

>>> try:
...     catch()
... except ValueError as e:
...     print(e)
...
test

Is there however any way to access the original exception's error message (i.e. 'hello')? I know I can print the full traceback with:

>>> try:
...     catch()
... except ValueError as e:
...     import traceback
...     print(traceback.format_exc())
...
Traceback (most recent call last):
  File "<stdin>", line 3, in catch
  File "<stdin>", line 2, in spam
ValueError: hello

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "<stdin>", line 5, in catch
ValueError: test

but I don't exactly want to parse the hello from that string. Is there a way to access the list of exceptions and their respective messages, from which I would simply take the first one?

What Python version are you running?

Simeon Visser
– Simeon Visser

2017-07-18 15:18:02 +00:00
Commented Jul 18, 2017 at 15:18 — Simeon Visser
– Simeon Visser, Commented Jul 18, 2017 at 15:18
Using Python 3.4.

mart1n
– mart1n

2017-07-18 15:40:14 +00:00
Commented Jul 18, 2017 at 15:40 — mart1n
– mart1n, Commented Jul 18, 2017 at 15:40

mart1n · Accepted Answer · 2017-07-18 15:18:52Z

4

Figured it out: the original exception is available via e.__cause__.

answered Jul 18, 2017 at 15:18

mart1n

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5 Comments

vaultah Over a year ago

Unless the exception was raised via the raise ... from ... statement, the original exception is stored in __context__, not __cause__.

mart1n Over a year ago

@vaultah Ha, interesting. So using print(e.__cause__ or e.__context__) should be the way to go.

simpleuser Over a year ago

any way to do it without accessing a private attribute?

Nuno André Over a year ago

@simpleuser Python hasn't private attributes. You are probably mistaking them for name-mangled attributes (with form __attr: two leading underscores and none trailing), but neither are. If they were they'd have to be called as e._Exception__cause__, which is the most private access that Python has.

Christian O'Reilly Over a year ago

@simpleuser What about str(e) to get yout error message? str(ValueError("Some error message")) returns "Some error message".

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