some_function() raises an exception while executing, so the program jumps to the except:
try:
some_function()
except:
print("exception happened!")
How do I see what caused the exception to occur?
The other answers all point out that you should not catch generic exceptions, but no one seems to want to tell you why, which is essential to understanding when you can break the "rule". Here is an explanation. Basically, it's so that you don't hide:
the fact that an error occurred
the specifics of the error that occurred (error hiding antipattern)
So as long as you take care to do none of those things, it's OK to catch the generic exception. For instance, you could provide information about the exception to the user another way, like:
Present exceptions as dialogs in a GUI
Transfer exceptions from a worker thread or process to the controlling thread or process in a multithreading or multiprocessing application
So how to catch the generic exception? There are several ways. If you just want the exception object, do it like this:
try:
someFunction()
except Exception as ex:
template = "An exception of type {0} occurred. Arguments:\n{1!r}"
message = template.format(type(ex).__name__, ex.args)
print message
Make sure message is brought to the attention of the user in a hard-to-miss way! Printing it, as shown above, may not be enough if the message is buried in lots of other messages. Failing to get the users attention is tantamount to swallowing all exceptions, and if there's one impression you should have come away with after reading the answers on this page, it's that this is not a good thing. Ending the except block with a raise statement will remedy the problem by transparently reraising the exception that was caught.
The difference between the above and using just except: without any argument is twofold:
A bare except: doesn't give you the exception object to inspect
The exceptions SystemExit, KeyboardInterrupt and GeneratorExit aren't caught by the above code, which is generally what you want. See the exception hierarchy.
If you also want the same stacktrace you get if you do not catch the exception, you can get that like this (still inside the except clause):
import traceback
print traceback.format_exc()
If you use the logging module, you can print the exception to the log (along with a message) like this:
import logging
log = logging.getLogger()
log.exception("Message for you, sir!")
If you want to dig deeper and examine the stack, look at variables etc., use the post_mortem function of the pdb module inside the except block:
import pdb
pdb.post_mortem()
I've found this last method to be invaluable when hunting down bugs.
Get the name of the class that exception object belongs:
e.__class__.__name__
and using print_exc() function will also print stack trace which is essential info for any error message.
Like this:
from traceback import print_exc
class CustomException(Exception): pass
try:
raise CustomException("hi")
except Exception as e:
print ('type is:', e.__class__.__name__)
print_exc()
# print("exception happened!")
You will get output like this:
type is: CustomException
Traceback (most recent call last):
File "exc.py", line 7, in <module>
raise CustomException("hi")
CustomException: hi
And after print and analysis, the code can decide not to handle exception and just execute raise:
from traceback import print_exc
class CustomException(Exception): pass
def calculate():
raise CustomException("hi")
try:
calculate()
except CustomException as e:
# here do some extra steps in case of CustomException
print('custom logic doing cleanup and more')
# then re raise same exception
raise
Output:
custom logic doing cleanup and more
And interpreter prints exception:
Traceback (most recent call last):
File "test.py", line 9, in <module>
calculate()
File "test.py", line 6, in calculate
raise CustomException("hi")
__main__.CustomException: hi
After raise original exception continues to propagate further up the call stack. (Beware of possible pitfall) If you raise new exception it caries new (shorter) stack trace.
from traceback import print_exc
class CustomException(Exception):
def __init__(self, ok):
self.ok = ok
def calculate():
raise CustomException(False)
try:
calculate()
except CustomException as e:
if not e.ok:
# Always use `raise` to rethrow exception
# following is usually mistake, but here we want to stress this point
raise CustomException(e.ok)
print("handling exception")
Output:
Traceback (most recent call last):
File "test.py", line 13, in <module>
raise CustomException(e.message)
__main__.CustomException: hi
Notice how traceback does not include calculate() function from line 9 which is the origin of original exception e.
You usually should not catch all possible exceptions with try: ... except as this is overly broad. Just catch those that are expected to happen for whatever reason. If you really must, for example if you want to find out more about some problem while debugging, you should do
try:
...
except Exception as ex:
print ex # do whatever you want for debugging.
raise # re-raise exception.
Most answers point to except (…) as (…): syntax (rightly so) but at the same time nobody wants to talk about an elephant in the room, where the elephant is sys.exc_info() function.
From the documentation of sys module (emphasis mine):
This function returns a tuple of three values that give information
about the exception that is currently being handled.
(…)
If no exception is being handled anywhere on the stack, a tuple
containing three None values is returned. Otherwise, the values
returned are (type, value, traceback). Their meaning is: type gets the
type of the exception being handled (a subclass of BaseException);
value gets the exception instance (an instance of the exception type);
traceback gets a traceback object (see the Reference Manual) which
encapsulates the call stack at the point where the exception
originally occurred.
I think the sys.exc_info() could be treated as the most direct answer to the original question of How do I know what type of exception occurred?
These answers are fine for debugging, but for programmatically testing the exception, isinstance(e, SomeException) can be handy, as it tests for subclasses of SomeException too, so you can create functionality that applies to hierarchies of exceptions.
Unless somefunction is a very bad coded legacy function, you shouldn't need what you're asking.
Use multiple except clause to handle in different ways different exceptions:
try:
someFunction()
except ValueError:
# do something
except ZeroDivision:
# do something else
The main point is that you shouldn't catch generic exception, but only the ones that you need to. I'm sure that you don't want to shadow unexpected errors or bugs.
In Python 2, the following are useful
except Exception, exc:
# This is how you get the type
excType = exc.__class__.__name__
# Here we are printing out information about the Exception
print 'exception type', excType
print 'exception msg', str(exc)
# It's easy to reraise an exception with more information added to it
msg = 'there was a problem with someFunction'
raise Exception(msg + 'because of %s: %s' % (excType, exc))
Use type class and as statement
try:#code
except Exception as e:
m=type(e)
#m is the class of the exception
strm=str(m)
#strm is the string of m
Hope this will help a little more
import sys
varExcepHandling, varExcepHandlingZer = 2, 0
try:
print(varExcepHandling/varExcepHandlingZer)
except Exception as ex:
print(sys.exc_info())
'sys.exc_info()' will return a tuple, if you only want the exception class name use 'sys.exc_info()[0]'
Note:- if you want to see all the exception classes just write dir(__builtin__)
Here's how I'm handling my exceptions. The idea is to do try solving the issue if that's easy, and later add a more desirable solution if possible. Don't solve the issue in the code that generates the exception, or that code loses track of the original algorithm, which should be written to-the-point. However, pass what data is needed to solve the issue, and return a lambda just in case you can't solve the problem outside of the code that generates it.
path = 'app.p'
def load():
if os.path.exists(path):
try:
with open(path, 'rb') as file:
data = file.read()
inst = pickle.load(data)
except Exception as e:
inst = solve(e, 'load app data', easy=lambda: App(), path=path)()
else:
inst = App()
inst.loadWidgets()
# e.g. A solver could search for app data if desc='load app data'
def solve(e, during, easy, **kwargs):
class_name = e.__class__.__name__
print(class_name + ': ' + str(e))
print('\t during: ' + during)
return easy
For now, since I don't want to think tangentially to my app's purpose, I haven't added any complicated solutions. But in the future, when I know more about possible solutions (since the app is designed more), I could add in a dictionary of solutions indexed by during.
In the example shown, one solution might be to look for app data stored somewhere else, say if the 'app.p' file got deleted by mistake.
For now, since writing the exception handler is not a smart idea (we don't know the best ways to solve it yet, because the app design will evolve), we simply return the easy fix which is to act like we're running the app for the first time (in this case).
To add to Lauritz's answer, I created a decorator/wrapper for exception handling and the wrapper logs which type of exception occurred.
class general_function_handler(object):
def __init__(self, func):
self.func = func
def __get__(self, obj, type=None):
return self.__class__(self.func.__get__(obj, type))
def __call__(self, *args, **kwargs):
try:
retval = self.func(*args, **kwargs)
except Exception, e :
logging.warning('Exception in %s' % self.func)
template = "An exception of type {0} occured. Arguments:\n{1!r}"
message = template.format(type(e).__name__, e.args)
logging.exception(message)
sys.exit(1) # exit on all exceptions for now
return retval
This can be called on a class method or a standalone function with the decorator:
#general_function_handler
See my blog about for the full example: http://ryaneirwin.wordpress.com/2014/05/31/python-decorators-and-exception-handling/
You can start as Lauritz recommended, with:
except Exception as ex:
and then just to print ex like so:
try:
#your try code here
except Exception as ex:
print ex
Your question is: "How can I see exactly what happened in the someFunction() that caused the exception to happen?"
It seems to me that you are not asking about how to handle unforeseen exceptions in production code (as many answers assumed), but how to find out what is causing a particular exception during development.
The easiest way is to use a debugger that can stop where the uncaught exception occurs, preferably not exiting, so that you can inspect the variables. For example, PyDev in the Eclipse open source IDE can do that. To enable that in Eclipse, open the Debug perspective, select Manage Python Exception Breakpoints in the Run menu, and check Suspend on uncaught exceptions.
Use the below for both Exception type and Exception text
import sys
print(str(sys.exc_info()[0]).split(' ')[1].strip('>').strip("'")+"-"+(str(sys.exc_info()[1])))
if you want only exception type: Use -->
import sys
print(str(sys.exc_info()[0]).split(' ')[1].strip('>').strip("'"))
Thanks Rajeshwar
The actual exception can be captured in the following way:
try:
i = 1/0
except Exception as e:
print e
You can learn more about exceptions from The Python Tutorial.
Just refrain from catching the exception and the traceback that Python prints will tell you what exception occurred.
When I run this code:
i=0
while i<5:
i=i+1;
try:
SellSta=client.get_order(symbol=Symb,orderId=SellOrderNum,recvWindow=Delay)
except client.get_order as e:
print ("This is an error message!{}".format(i))
#End while
I got this error:
TypeError: catching classes that do not inherit from BaseException is not allowed
I read this tread Exception TypeError warning sometimes shown, sometimes not when using throw method of generator and this one Can't catch mocked exception because it doesn't inherit BaseException also read this https://medium.com/python-pandemonium/a-very-picky-except-in-python-d9b994bdf7f0
I kind of fix it with this code:
i=0
while i<5:
i=i+1;
try:
SellSta=client.get_order(symbol=Symb,orderId=SellOrderNum,recvWindow=Delay)
except:
print ("This is an error message!{}".format(i))
#End while
The result it's that ignores the error and go to the next while but I want to catch the error and print it.
I post the question in Spanish Stack with better results.
To translate and sum up:
The error occurs because in the exception clause you must indicate which exception you capture. An exception is a class that inherits (directly or indirectly) from the base class Exception.
Instead I have put client.get_order where python expected the name of the exception, and what you have put is a method of an object, and not a class that inherits from Exception.
The solution goes this way
try:
SellSta=client.get_order(symbol=Symb,orderId=SellOrderNum,recvWindow=Delay)
except Exception as e:
if e.code==-2013:
print ("Order does not exist.");
elif e.code==-2014:
print ("API-key format invalid.");
#End If
You'll need to code for every exception in here
How can I ignore a certain exception to be raised to the caller in python 3?
Example:
def do_something():
try:
statement1
statement2
except Exception as e:
# ignore the exception
logging.warning("this is normal, exception is ignored")
try:
do_something()
except Exception as e:
# this is unexpected control flow, the first exception is already ignored !!
logging.error("unexpected error")
logging.error(e) # prints None
I found someone mentioned that "Because of the last thrown exception being remembered in Python, some of the objects involved in the exception-throwing statement are being kept live indefinitely" and then mentioned to use "sys.exc_clear()" in this case which is not available anymore in python 3. Any clue how can I completely ignore the exception in python3?
There's no need to do this in Python 3, sys.exc_clear() was removed because Python doesn't store the last raised exception internally as it did in Python 2:
For example, in Python 2, the exception is still kept alive when inside a function:
def foo():
try:
raise ValueError()
except ValueError as e:
print(e)
import sys; print(sys.exc_info())
Calling foo now shows the exception is kept:
foo()
(<type 'exceptions.ValueError'>, ValueError(), <traceback object at 0x7f45c57fc560>)
You need to call sys.exc_clear() in order to clear the Exception raised.
In Python 3, on the contrary:
def foo():
try:
raise ValueError()
except ValueError as e:
print(e)
import sys; print(sys.exc_info())
Calling the same function:
foo()
(None, None, None)
try:
...
except (SomeError) as err:
...
else:
...
finally:
if err:
...
This gives an error: 'err not defined'. Because the exception argument - err - is not defined as far as the finally block is concerned. It appears then that the exception argument is local to the exception block.
You can get round it by copying err to another variable defined outside the block:
teleport = ""
try:
...
except (SomeError) as err:
teleport = err
else:
...
finally:
if teleport:
...
But why can't you simply reference the exception argument in the finally block? (Assuming I've not overlooked something else.)
try blocks will execute code that could possibly raise an exception. except block will execute the moment an exception is raised. The else block executes if no except is raised, and the finally block is executed no matter what.
There is no point in checking for an exception in the finally block when you can just do that in the else block.
Aside from that, the variable was likely garbage collected at the end of execution of the except block. It's similar to what happens with with blocks. This is why you can't do if err:
You cannot access just because exception is not raised and so variable is not defined, hence the undefined variable error. Besides there is no point in dealing with exception in your final block, you should do that stuff in except block itself.
Using custom exceptions in python (v2.7.3): don't get a stack trace when call getSub() whereas getSub(True) invokes one, the difference being that it is caused via an additional try...except which I want to avoid (+feels unnecessary) so why/how can it be avoided?
import sys, traceback
class customException(Exception):
def __init__(self, *args):
super(customException,self).__init__(*args)
print "Stack trace within exception", traceback.extract_tb(sys.exc_info()[2])
errTxt = [a for a in args]
print "error text", errTxt
def getFn():
try:
getSub()
except customException as e:
print "customException was raised"
try:
getSub(True)
except customException as e:
print "customException2 was raised"
def getSub(flag=False):
if flag:
try:
1/0
except:
raise customException('test')
else:
raise customException('test')
getFn()
output:
Stack trace within exception []
error text ['test']
customException was raised
Stack trace within exception [('./test3.py', 25, 'getSub', '1/0')]
error text ['test']
customException2 was raised
To put the above in context I have code (pseudo) along the below lines and it wasn’t until I boiled down the code to the above example that I realised why the traceback access was not always working. The purpose of the custom exception class is to collect an aggregate count of exceptions in order to classify downstream the result of each do_something, e.g. fatal, warning etc. The use of the traceback was to record from WHERE the exception was ‘raised’ and therefore the creation of an exception (1/0 = albeit seems out of place) enables that to work. Wondering about using the inspect module rather than thinking about this within the traceback stack?
__main__
With each item in set:
Try:
do_something(item)
except customException()
clean up = log info etc.
end With
end.__main__
do_something(item)
try:
check_something()
except customException
if exception=typeA.1 raise customException(Type1)
if exception=typeB.2 and item = ‘x’
raise customException(Type2)
check_something()
a = getInfo()
unless getInfo()
raise customException(typeA.1)
try:
b = getOtherInfo()
except customException
raise customException(typeB.2)
…
If I understand you right, you're wondering why your print "Stack trace within exception" line prints an empty list for getSub(), but prints some traceback info for getSub(True).
You have code in your exception class's __init__ to look at the most recent exception via sys.exc_info. When you do raise customException('test'), customException('test') is evaluated first on its own, before it "knows" that it is going to be raised as an exception. So when you do that raise in getSub(), there is no most recent exception.
With getSub(True), there is a most recent exception, because the 1/0 raises an exception before you create your customException. Note that when you do 1/0, the most recent exception is the one from that 1/0; you can see that there's nothing about your customException in that traceback.
An exception object has to be created before the exception is raised. So you can't look at "the current exception" in your exception class's __init__ to get info about the stack trace that will be created when it is raised, because it hasn't been raised yet at that time.
If you want, you could use traceback.extract_stack to get the call stack at the time the exception object is created, but there's no guarantee that that has anything to do with when it will be raised. Just because an instance of your exception class is created doesn't even mean it will ever be raised at all. It's perfectly legal (although generally pointless) for someone to just create an exception object with stuff = customException('blah') but never actually raise the exception.
In any case, it's not clear from your question what you're trying to achieve here. It would help if you explained that.
Because you're catching the exception, there will be no traceback unless you either explicitly re-raise
try:
getSub(True)
except customException as e:
print "customException2 was raised"
raise # add this to re-raise, with original traceback
or print it yourself:
try:
getSub(True)
except customException as e:
print "customException2 was raised"
print traceback.format_exc()