How to get the last exception object after an error is raised at a Python prompt?

Question

When debugging Python code at the interactive prompt (REPL), often I’ll write some code which raises an exception, but I haven’t wrapped it in a try/except, so once the error is raised, I’ve forever lost the exception object.

Often the traceback and error message Python prints out isn’t enough. For example, when fetching a URL, the server might return a 40x error, and you need the content of the response via error.read() … but you haven’t got the error object anymore. For example:

>>> import urllib2
>>> f = urllib2.urlopen('http://example.com/api/?foo=bad-query-string')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  ...
urllib2.HTTPError: HTTP Error 400: Bad Request

Drat, what did the body of the response say? It probably had valuable error information in it…

I realize it’s usually easy to re-run your code wrapped in a try/except, but that’s not ideal. I also realize that in this specific case if I were using the requests library (which doesn’t raise for HTTP errors), I wouldn’t have this problem … but I’m really wondering if there’s a more general way to get the last exception object at a Python prompt in these cases.

Asked By: Ben Hoyt

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Source

Answer 1

The sys module provides some functions for post-hoc examining of exceptions: sys.last_type, sys.last_value, and sys.last_traceback.

sys.last_value is the one you’re looking for.

Answered By: Cairnarvon

Answer 2

As @Cairnarvon mentioned, I didn’t find any last_valuemember is sys module.

sys.exc_info() did the trick for me. sys.exc_info() returns a tuple with three values (type, value, traceback).

So sys.exc_info()[1] will give the readable error. Here is the example,

import sys
list = [1,2,3,4]
try:
    del list[8]
except Exception:
    print(sys.exc_info()[1])

will output list assignment index out of range

Also, traceback.format_exc() from traceback module can be used to print out the similar information.

Below is the output if format_exec() is used,

Traceback (most recent call last):
  File "python", line 6, in <module>
IndexError: list assignment index out of range

Answered By: Kumar Sambhav Pandey

Answer 3

The sys.last_type, sys.last_value, and sys.last_traceback variables are not always defined. Actually, they are intended to be used in an interactive session. I was able to get the last raised exception using sys.exc_info(). Usage:

import sys
ex_type, ex_value, traceback = sys.exc_info()

The last raised exception object will be stored in ex_value

Answered By: fraverta

How to get the last exception object after an error is raised at a Python prompt?

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