I have code that raises exceptions from other exceptions so that we can see details about eveything that went wrong. In the example below we include information about what we are processing and what specific thing in the processing went wrong.
def process(thing):
try:
process_widgets(thing)
except Exception as e:
raise CouldNotProcess(thing) from e
def process_widgets(thing):
for widget in get_widgets(thing):
raise CouldNotProcessWidget(widget)
def do_processing():
for thing in things_to_process():
process(thing)
I am trying to change this so that process_widgets can raise a specific type of exception and do_processing can change its behaviour based on this exception. However the raise from in process is masking this exception which makes this impossible. Is there a good to let do_processing know about what went wrong in process_widgets while also doing raise from.
Ideas:
Python 3.11 has exception groups. So perhaps there is a way of adding exceptions to group and catching them with the likely confusing except* syntax.
There is a dirty trick where I do raise e from CouldNoPorcess(thing) to get both the helpful logging.
Apparently internally exception chaining works by adding __cause__ property (forming a linked list) so I could manually look through causes in the top most exception to manually implement behaviour like except* with exception groups.
def raised_from(e, type):
while e is not None:
if isinstance(e, Specific):
return True
e = e.__cause__
return False
...
try:
do_processing()
except CouldNotProcess as e:
if raised_from(e, CouldNotProcessWidget):
do_stuff()
You see this pattern quite a lot with status_codes from http.
I could use logging rather than adding information to exceptions. This hides information from the exception handling code, but works for logging. I think this is the work around that I'll use at the moment.
It's noticeable that the PEP says that exception chaining isn't quite designed for adding information to exceptions.
Update
python 3.11 has an add_note method and notes property which can be used to add information - which works for some use cases.
For this use case exception groups might be the way to go, though I am concerned that this might be a little confusing.
This is a hard question to phrase, but here's a stripped-down version of the situation. I'm using some library code that accepts a callback. It has its own error-handling, and raises an error if anything goes wrong while executing the callback.
class LibraryException(Exception):
pass
def library_function(callback, string):
try:
# (does other stuff here)
callback(string)
except:
raise LibraryException('The library code hit a problem.')
I'm using this code inside an input loop. I know of potential errors that could arise in my callback function, depending on values in the input. If that happens, I'd like to reprompt, after getting helpful feedback from its error message. I imagine it looking something like this:
class MyException(Exception):
pass
def my_callback(string):
raise MyException("Here's some specific info about my code hitting a problem.")
while True:
something = input('Enter something: ')
try:
library_function(my_callback, something)
except MyException as e:
print(e)
continue
Of course, this doesn't work, because MyException will be caught within library_function, which will raise its own (much less informative) Exception and halt the program.
The obvious thing to do would be to validate my input before calling library_function, but that's a circular problem, because parsing is what I'm using the library code for in the first place. (For the curious, it's Lark, but I don't think my question is specific enough to Lark to warrant cluttering it with all the specific details.)
One alternative would be to alter my code to catch any error (or at least the type of error the library generates), and directly print the inner error message:
def my_callback(string):
error_str = "Here's some specific info about my code hitting a problem."
print(error_str)
raise MyException(error_str)
while True:
something = input('Enter something: ')
try:
library_function(my_callback, something)
except LibraryException:
continue
But I see two issues with this. One is that I'm throwing a wide net, potentially catching and ignoring errors other than in the scope I'm aiming at. Beyond that, it just seems... inelegant, and unidiomatic, to print the error message, then throw the exception itself into the void. Plus the command line event loop is only for testing; eventually I plan to embed this in a GUI application, and without printed output, I'll still want to access and display the info about what went wrong.
What's the cleanest and most Pythonic way to achieve something like this?
There seems to be many ways to achieve what you want. Though, which one is more robust - I cannot find a clue about. I'll try to explain all the methods that seemed apparent to me. Perhaps you'll find one of them useful.
I'll be using the example code you provided to demonstrate these methods, here's a fresher on how it looks-
class MyException(Exception):
pass
def my_callback(string):
raise MyException("Here's some specific info about my code hitting a problem.")
def library_function(callback, string):
try:
# (does other stuff here)
callback(string)
except:
raise Exception('The library code hit a problem.')
The simplest approach - traceback.format_exc
import traceback
try:
library_function(my_callback, 'boo!')
except:
# NOTE: Remember to keep the `chain` parameter of `format_exc` set to `True` (default)
tb_info = traceback.format_exc()
This does not require much know-how about exceptions and stack traces themselves, nor does it require you to pass any special frame/traceback/exception to the library function. But look at what this returns (as in, the value of tb_info)-
'''
Traceback (most recent call last):
File "path/to/test.py", line 14, in library_function
callback(string)
File "path/to/test.py", line 9, in my_callback
raise MyException("Here's some specific info about my code hitting a problem.")
MyException: Here's some specific info about my code hitting a problem.
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "path/to/test.py", line 19, in <module>
library_function(my_callback, 'boo!')
File "path/to/test.py", line 16, in library_function
raise Exception('The library code hit a problem.')
Exception: The library code hit a problem.
'''
That's a string, the same thing you'd see if you just let the exception happen without catching. Notice the exception chaining here, the exception at the top is the exception that happened prior to the exception at the bottom. You could parse out all the exception names-
import re
exception_list = re.findall(r'^(\w+): (\w+)', tb_info, flags=re.M)
With that, you'll get [('MyException', "Here's some specific info about my code hitting a problem"), ('Exception', 'The library code hit a problem')] in exception_list
Although this is the easiest way out, it's not very context aware. I mean, all you get are class names in string form. Regardless, if that is what suits your needs - I don't particularly see a problem with this.
The "robust" approach - recursing through __context__/__cause__
Python itself keeps track of the exception trace history, the exception currently at hand, the exception that caused this exception and so on. You can read about the intricate details of this concept in PEP 3134
Whether or not you go through the entirety of the PEP, I urge you to at least familiarize yourself with implicitly chained exceptions and explicitly chained exceptions. Perhaps this SO thread will be useful for that.
As a small refresher, raise ... from is for explicitly chaining exceptions. The method you show in your example, is implicit chaining
Now, you need to make a mental note - TracebackException#__cause__ is for explicitly chained exceptions and TracebackException#__context__ is for implicitly chained exceptions. Since your example uses implicit chaining, you can simply follow __context__ backwards and you'll reach MyException. In fact, since this is only one level of nesting, you'll reach it instantly!
import sys
import traceback
try:
library_function(my_callback, 'boo!')
except:
previous_exc = traceback.TracebackException(*sys.exc_info()).__context__
This first constructs the TracebackException from sys.exc_info. sys.exc_info returns a tuple of (exc_type, exc_value, exc_traceback) for the exception at hand (if any). Notice that those 3 values, in that specific order, are exactly what you need to construct TracebackException - so you can simply destructure it using * and pass it to the class constructor.
This returns a TracebackException object about the current exception. The exception that it is implicitly chained from is in __context__, the exception that it is explicitly chained from is in __cause__.
Note that both __cause__ and __context__ will return either a TracebackException object, or None (if you're at the end of the chain). This means, you can call __cause__/__context__ again on the return value and basically keep going till you reach the end of the chain.
Printing a TracebackException object just prints the message of the exception, if you want to get the class itself (the actual class, not a string), you can do .exc_type
print(previous_exc)
# prints "Here's some specific info about my code hitting a problem."
print(previous_exc.exc_type)
# prints <class '__main__.MyException'>
Here's an example of recursing through .__context__ and printing the types of all exceptions in the implicit chain. (You can do the same for .__cause__)
def classes_from_excs(exc: traceback.TracebackException):
print(exc.exc_type)
if not exc.__context__:
# chain exhausted
return
classes_from_excs(exc.__context__)
Let's use it!
try:
library_function(my_callback, 'boo!')
except:
classes_from_excs(traceback.TracebackException(*sys.exc_info()))
That will print-
<class 'Exception'>
<class '__main__.MyException'>
Once again, the point of this is to be context aware. Ideally, printing isn't the thing you'll want to do in a practical environment, you have the class objects themselves on your hands, with all the info!
NOTE: For implicitly chained exceptions, if an exception is explicitly suppressed, it'll be a bad day trying to recover the chain - regardless, you might give __supressed_context__ a shot.
The painful way - walking through traceback.walk_tb
This is probably the closest you can get to the low level stuff of exception handling. If you want to capture entire frames of information instead of just the exception classes and messages and such, you might find walk_tb useful....and a bit painful.
import traceback
try:
library_function(my_callback, 'foo')
except:
tb_gen = traceback.walk_tb(sys.exc_info()[2])
There is....entirely too much to discuss here. .walk_tb takes a traceback object, you may remember from the previous method that the 2nd index of the returned tuple from sys.exec_info is just that. It then returns a generator of tuples of frame object and int (Iterator[Tuple[FrameType, int]]).
These frame objects have all kinds of intricate information. Though, whether or not you'll actually find exactly what you're looking for, is another story. They may be complex, but they aren't exhaustive unless you play around with a lot of frame inspection. Regardless, this is what the frame objects represent.
What you do with the frames is upto you. They can be passed to many functions. You can pass the entire generator to StackSummary.extract to get framesummary objects, you can iterate through each frame to have a look at [0].f_locals (The [0] on Tuple[FrameType, int] returns the actual frame object) and so on.
for tb in tb_gen:
print(tb[0].f_locals)
That will give you a dict of the locals for each frame. Within the first tb from tb_gen, you'll see MyException as part of the locals....among a load of other stuff.
I have a creeping feeling I have overlooked some methods, most probably with inspect. But I hope the above methods will be good enough so that no one has to go through the jumble that is inspect :P
Chase's answer above is phenomenal. For completeness's sake, here's how I implemented their second approach in this situation. First, I made a function that can search the stack for the specified error type. Even though the chaining in my example is implicit, this should be able to follow implicit and/or explicit chaining:
import sys
import traceback
def find_exception_in_trace(exc_type):
"""Return latest exception of exc_type, or None if not present"""
tb = traceback.TracebackException(*sys.exc_info())
prev_exc = tb.__context__ or tb.__cause__
while prev_exc:
if prev_exc.exc_type == exc_type:
return prev_exc
prev_exc = prev_exc.__context__ or prev_exc.__cause__
return None
With that, it's as simple as:
while True:
something = input('Enter something: ')
try:
library_function(my_callback, something)
except LibraryException as exc:
if (my_exc := find_exception_in_trace(MyException)):
print(my_exc)
continue
raise exc
That way I can access my inner exception (and print it for now, although eventually I may do other things with it) and continue. But if my exception wasn't in there, I simply reraise whatever the library raised. Perfect!
Both these snippets do the same thing:
Try/except in function declaration:
def something():
try:
# code goes here
except:
print("Error")
sys.exit(1)
something()
Try/except in function call:
def something():
# code goes here
try:
something()
except:
print("Error")
sys.exit(1)
Is there one that is better/more Pythonic/recommended by PEP8 or is it just up to personal preference? I understand that the second method would get tedious and repetitive if the function needs to be called more than once, but assuming the function is only called once, which one should I use?
the general rule is "only catch exceptions you can handle", see here for an explanation
note that an uncaught exception (in most languages) will cause the program to exit with an unsuccessful status code (i.e. your sys.exit(1)), it will probably also print out a message saying that an exception occurred. your demo therefore is emulating default behaviour, but doing it worse
further, you're catching every exception and this is generally bad style, e.g. you'll implicitly catch SystemExit and other internal exceptions that you probably shouldn't be dealing interacting with
I am writing a class in Python and part of the code deals with a server. Therefore I need to deal with exceptions like ExpiredSession or ConnectionError.
Instead of writing exception handling code for every try/except block, I have a single function in the class to deal with the exceptions. something like this (inside the class definition)
def job_a(self):
try:
do something
except Exception as e:
#maybe print the error on screen.
self.exception_handling(e)
def job_b(self):
try:
do something else
except Exception as e:
#maybe print the error on screen.
self.exception_handling(e)
def exception_handling(self,e):
if isInstanceOf(e,ExpiredSession):
#deal with expired session.
self.reconnect()
if isInstanceOf(e,ConnectionError):
#deal with connection error
else:
#other exceptions
I am not sure if this kind of code would cause any problem because I haven't seen code do this. like, possible memory leak? (Now I notice the memory usage grows(though slowly) when I have more and more error/exception and eventually I have to restart the process before it eats all my memories). Not sure this is the cause.
Is it a good practice to pass exceptions to a single function?
This is a good use case for a context manager. You can see some examples of using context managers for error handling here. The contextmanager decorator allows you to write context managers concisely in the form of a single function. Here's a simple example:
class Foo(object):
def meth1(self):
with self.errorHandler():
1/0
def meth2(self):
with self.errorHandler():
2 + ""
def meth3(self):
with self.errorHandler():
# an unhandled ("unexpected") kind of exception
""[3]
#contextlib.contextmanager
def errorHandler(self):
try:
yield
except TypeError:
print "A TypeError occurred"
except ZeroDivisionError:
print "Divide by zero occurred"
Then:
>>> x = Foo()
>>> x.meth1()
Divide by zero occurred
>>> x.meth2()
A TypeError occurred
The with statement allows you to "offload" the error handling into a separate function where you catch the exceptions and do what you like with them. In your "real" functions (i.e., the functions that do the work but may raise the exceptions), you just need a with statement instead of an entire block of complicated try/except statements.
An additional advantage of this approach is that if an unforeseen exception is raised, it will propagate up normally with no extra effort:
>>> x.meth3()
Traceback (most recent call last):
File "<pyshell#394>", line 1, in <module>
x.meth3()
File "<pyshell#389>", line 12, in meth3
""[3]
IndexError: string index out of range
In your proposed solution, on the other hand, the exception is already caught in each function and the actual exception object is passed the handler. If the handler gets an unexpected error, it would have to manually reraise it (and can't even use a bare raise to do so). Using a context manager, unexpected exceptions have their ordinary behavior with no extra work required.
I have a class function in Python that either returns a success or a failure, but in case of a failure I want it to send a specific error string back. I have 3 approaches in mind:
Pass in an variable error_msg to the function originally set to None and in case of an error, it gets set to the error string. eg:
if !(foo(self, input, error_msg)):
print "no error"
else:
print error_msg
Return a tuple containing a bool and error_msg from the function.
I raise an exception in case of an error and catch it in the calling code. But since I don't see exceptions being used often in the codebase I am working on, so was not too sure about taking this approach.
What is the Pythonic way of doing this?
Create your own exception and raise that instead:
class MyValidationError(Exception):
pass
def my_function():
if not foo():
raise MyValidationError("Error message")
return 4
You can then call your function as:
try:
result = my_function()
except MyValidationError as exception:
# handle exception here and get error message
print exception.message
This style is called EAFP ("Easier to ask for forgiveness than permission") which means that you write the code as normal, raise exceptions when something goes wrong and handle that later:
This common Python
coding style assumes the existence of valid keys or attributes and
catches exceptions if the assumption proves false. This clean and fast
style is characterized by the presence of many try and except
statements. The technique contrasts with the LBYL style common to many
other languages such as C.
Raise an error:
if foo(self, input, error_msg):
raise SomethingError("You broke it")
And handle it:
try:
something()
except SomethingError as e:
print str(e)
It's the Pythonic approach and the most readable.
Returning a tuple like (12, None) may seem like a good solution, but it's hard to keep track of what each method returns if you're not consistent. Returning two different data types is even worse, as it will probably break code that assumes a constant data type.