Consider the following code:
try:
raise Exception("a")
except:
try:
raise Exception("b")
finally:
raise
This will raise Exception: a. I expected it to raise Exception: b (need I explain why?). Why does the final raise raise the original exception rather than (what I thought) was the last exception raised?
Raise is re-raising the last exception you caught, not the last exception you raised
(reposted from comments for clarity)
On python2.6
I guess, you are expecting the finally block to be tied with the "try" block where you raise the exception "B". The finally block is attached to the first "try" block.
If you added an except block in the inner try block, then the finally block will raise exception B.
try:
raise Exception("a")
except:
try:
raise Exception("b")
except:
pass
finally:
raise
Output:
Traceback (most recent call last):
File "test.py", line 5, in <module>
raise Exception("b")
Exception: b
Another variation that explains whats happening here
try:
raise Exception("a")
except:
try:
raise Exception("b")
except:
raise
Output:
Traceback (most recent call last):
File "test.py", line 7, in <module>
raise Exception("b")
Exception: b
If you see here, replacing the finally block with except does raise the exception B.
Related
Let's assume you have some simple code that you don't control (eg: it's in a module you're using):
def example():
try:
raise TypeError("type")
except TypeError:
raise Exception("device busy")
How would I go about accessing the TypeError in this traceback in order to handle it?
Traceback (most recent call last):
File "/usr/local/google/home/dthor/dev/pyle/pyle/fab/visa_instrument/exception_helpers.py", line 3, in example
raise TypeError("type")
TypeError: type
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/usr/local/google/home/dthor/dev/pyle/pyle/fab/visa_instrument/exception_helpers.py", line 7, in <module>
example()
File "/usr/local/google/home/dthor/dev/pyle/pyle/fab/visa_instrument/exception_helpers.py", line 5, in example
raise Exception("device busy")
Exception: device busy
I can do the below, but i'm not happy with it because I'm doing string comparison - meaning things would break if the underlying module changes what string they raise (I don't control example()):
try:
example()
except Exception as err:
if "device busy" in str(err):
# do the thing
pass
# But raise any other exceptions
raise err
I'd much rather have:
try:
example()
except Exception as err:
if TypeError in err.other_errors: # magic that doesn't exist
# do the thing
pass
raise err
or even
try:
example()
except TypeError in exception_stack: # Magic that doesn't exist
# do the thing
pass
except Exception:
I'm investigating the traceback module and sys.exc_info(), but don't have anything concrete yet.
Followup: would things be different if the exception was chained? Eg: raise Exception from the_TypeError_exception
Check the __context__ of the exception:
>>> try:
... example()
... except Exception as e:
... print(f"exception: {e!r}")
... print(f"context: {e.__context__!r}")
...
exception: Exception('device busy')
context: TypeError('type')
If you use a chained exception, the original exception will also be accessible via the __cause__ attribute.
my_dictionary = {'a':1}
try:
my_dictionary['b']
except KeyError as e:
raise KeyError('Bad key:' + str(e))
That code, obviously, will raise a KeyError:
Traceback (most recent call last):
File "----.py", line 11, in <module>
my_dictionary['b']
KeyError: 'b'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "----.py", line 13, in <module>
raise KeyError('Bad key:' + str(e))
KeyError: "Bad key:'b'"
While I understand the need for Python to state how the except part created its own error, I'd like for that first KeyError not to be shown. A workaround I came up with is this:
my_dictionary = {'a':1}
err_msg = None
try:
my_dictionary['b']
except KeyError as e:
err_msg = str(e)
if type(err_msg) != type(None):
raise KeyError('Bad key:' + err_msg)
which shortens the error message to this:
Traceback (most recent call last):
File "----.py", line 23, in <module>
raise KeyError('Bad key:' + err_msg)
KeyError: "Bad key:'b'"
Is there a more Pythonic way of doing this?
From this answer, you need to add from None to your exception.
my_dictionary = {'a':1}
try:
my_dictionary['b']
except KeyError as e:
raise KeyError('Bad key:' + str(e)) from None
If you have a specific thing that you want to print instead of showing the stack trace, you can do that by just using print and then sys.exit(1), which is probably as pythonic as you can get. Another thing you can do is to raise KeyError('Bad key:' + str(e)) from e, which replaces "During handling of the above exception, another exception occurred:" with "The above exception was the direct cause of the following exception:". I don't think there's a way to do exactly what you asked, because the whole reason that error messages are generated like that is because you should be given all the information unless you explicitly specify some other behaviour. Uncaught exceptions generally represent some mistake in your code - they aren't supposed to happen normally, so there wouldn't be a need to make the formatting of the error messages too customizable.
I have defined a custom Exception object and would like to get the line number of the exception.
class FlowException(Exception):
pass
def something():
print 2/3
print 1/2
print 2/0
try:
something()
except Exception as e:
raise FlowException("Process Exception", e)
Now, if there is a exception in something() it throws the FlowException but it does not give me the exact line number, How can I get the line number from FlowException(ie; it failed when it executed 2/0)?
Here is the output:--
raise FlowException("Process Exception", e)
__main__.FlowException: ('Process Exception', ZeroDivisionError('integer division or modulo by zero',))
[Finished in 0.4s with exit code 1]
The traceback object holds that info in the tb_lineno attribute:
import sys
# ...
except Exception as e:
trace_back = sys.exc_info()[2]
line = trace_back.tb_lineno
raise FlowException("Process Exception in line {}".format(line), e)
Tested on Python 3.6
class FlowException(Exception):
pass
def something():
raise ValueError
try:
something()
except Exception as e:
raise FlowException("Process Exception", e)
Output has line numbers:
Traceback (most recent call last):
File "/Users/diman/PycharmProjects/invites/test.py", line 8, in <module>
something()
File "/Users/diman/PycharmProjects/invites/test.py", line 5, in something
raise ValueError
ValueError
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/Users/diman/PycharmProjects/invites/test.py", line 10, in <module>
raise FlowException("Process Exception", e)
__main__.FlowException: ('Process Exception', ValueError())
For Python 2 try using internal logger ".exception()" method instead of using monstrous "sys" module.
import logging
logger = logging.getLogger()
try:
something()
except Exception as e:
logger.exception(e)
raise FlowException("Process Exception", e)
Consider the simple example:
def f():
try:
raise TypeError
except TypeError:
raise ValueError
f()
I want to catch TypeError object when ValueError is thrown after f() execution. Is it possible to do it?
If I execute function f() then python3 print to stderr all raised exceptions of exception chain (PEP-3134) like
Traceback (most recent call last):
File "...", line 6, in f
raise TypeError
TypeError
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "...", line 11, in <module>
f()
File "...", line 8, in f
raise ValueError
ValueError
So I would get the list of all exceptions of exception chain or check if exception of some type (TypeError in the above example) exists in exception chain.
Python 3 has a beautiful syntactic enhancement on exceptions handling. Instead of plainly raising ValueError, you should raise it from a caught exception, i.e.:
try:
raise TypeError('Something awful has happened')
except TypeError as e:
raise ValueError('There was a bad value') from e
Notice the difference between the tracebacks. This one uses raise from version:
Traceback (most recent call last):
File "/home/user/tmp.py", line 2, in <module>
raise TypeError('Something awful has happened')
TypeError: Something awful has happened
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "/home/user/tmp.py", line 4, in <module>
raise ValueError('There was a bad value') from e
ValueError: There was a bad value
Though the result may seem similar, in fact it is rather different! raise from saves the context of the original exception and allows one to trace all the exceptions chain back - which is impossible with simple raise.
To get the original exception, you simply have to refer to new exception's __context__ attribute, i.e.
try:
try:
raise TypeError('Something awful has happened')
except TypeError as e:
raise ValueError('There was a bad value') from e
except ValueError as e:
print(e.__context__)
>>> Something awful has happened
Hopefully that is the solution you were looking for.
For more details, see PEP 3134 -- Exception Chaining and Embedded Tracebacks
I'm learning to use python. I just came across this article:
http://nedbatchelder.com/blog/200711/rethrowing_exceptions_in_python.html
It describes rethrowing exceptions in python, like this:
try:
do_something_dangerous()
except:
do_something_to_apologize()
raise
Since you re-throw the exception, there should be an "outer catch-except" statement. But now, I was thinking, what if the do_something_to_apologize() inside the except throws an error. Which one will be caught in the outer "catch-except"? The one you rethrow or the one thrown by do_something_to_apologize() ?
Or will the exception with the highest priotiry be caught first?
Try it and see:
def failure():
raise ValueError, "Real error"
def apologize():
raise TypeError, "Apology error"
try:
failure()
except ValueError:
apologize()
raise
The result:
Traceback (most recent call last):
File "<pyshell#14>", line 10, in <module>
apologize()
File "<pyshell#14>", line 5, in apologize
raise TypeError, "Apology error"
TypeError: Apology error
The reason: the "real" error from the original function was already caught by the except. apologize raises a new error before the raise is reached. Therefore, the raise in the except clause is never executed, and only the apology's error propagates upward. If apologize raises an error, Python has no way of knowing that you were going to raise a different exception after apologize.
Note that in Python 3, the traceback will mention both exceptions, with a message explaining how the second one arose:
Traceback (most recent call last):
File "./prog.py", line 9, in <module>
File "./prog.py", line 2, in failure
ValueError: Real error
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "./prog.py", line 11, in <module>
File "./prog.py", line 5, in apologize
TypeError: Apology error
However, the second exception (the "apology" exception) is still the only one that propagates outward and can be caught by a higher-level except clause. The original exception is mentioned in the traceback but is subsumed in the later one and can no longer be caught.
The exception thrown by do_something_to_apologize() will be caught. The line containing raise will never run, because of the exception thrown by do_something_to_apologize. Also, I don't believe there is any idea of "priority" in python exceptions.
I believe a better idea is to use
raise NewException("Explain why") from CatchedException
pattern. In particular, considering Python 3 and the example given by #BrenBarn I use following
def failure():
raise ValueError("Real error")
try:
failure()
except ValueError as ex:
raise TypeError("Apology error") from ex
which yields
--------- ValueError----
Traceback (most recent call last)
4 try:
----> 5 failure()
6 except ValueError as ex:
1 def failure():
----> 2 raise ValueError("Real error")
3
ValueError: Real error
The above exception was the direct cause of the following exception:
-----TypeError-----
Traceback (most recent call last)
5 failure()
6 except ValueError as ex:
----> 7 raise TypeError("Apology error") from ex
TypeError: Apology error