How do I catch only a specific error in Python3? - python-3.x

Question

Say I have a function that can produce a vast variety of errors.
I have a ValueError that I need to catch, a specific AttributeError, and then I also need to handle any other type of error.
try:
func()
except AttributeError as e:
if "specific error text" in str(e):
print("The specific AttributeError occurred")
else:
raise
except ValueError:
print("A value error occurred")
except Exception as e:
print("Another error occurred: {}".format(str(e)))
Problem: If func() bubbles an AttributeError that's not the specific one I'm looking for, in this case, it'll be re-raised and not handled how I want it to be handled (via the general Exception handler).
How do I force non-specific errors to be handled further down in the chain, without duplicating code from the Exception section into the AttributeError section?

Answer 1

As an option you can process AttributeError and ValueError in one try-except block and all other Exceptions on the top level like
try:
try:
func()
except AttributeError as e:
if "specific error text" in str(e):
print("The specific AttributeError occurred")
else:
raise
except ValueError:
print("A value error occurred")
except Exception as e:
print("Another error occurred: {}".format(str(e)))
this may look a bit ugly though, so we can extract inner try-except block in a separate function like
def func_with_expected_exceptions_handling():
try:
func()
except AttributeError as e:
if "specific error text" in str(e):
print("The specific AttributeError occurred")
else:
raise
except ValueError:
print("A value error occurred")
and after that
try:
func_with_expected_exceptions_handling()
except Exception as e:
print("Another error occurred: {}".format(str(e)))
this doesn't save us from an actual nested structure, but it may come in handy if this kind of func processing arises in other places.
BTW, I don't think checking for a specific error message in exception is a good idea, we need a little bit more context to see if it can be done easier.
EDIT
If I understood correctly your func looks like
def func(...):
getattr(COMMANDS, cmd.command).command(irc_c, msg, cmd)
and you want to handle error from getattr call.
I can see next options here:
Wrap getattr call in try-except and process AttributeError in-place
def func(...):
try:
commander = getattr(COMMANDS, cmd.command)
except AttributeError:
print('Command {} not found'.format(cmd.command))
else:
commander.command(irc_c, msg, cmd)
Wrap getattr call in try-except, re-raise a custom exception (or ValueError) and process it afterwards in OP try-except
class CommandNotFound(Exception): pass
def func(...):
try:
commander = getattr(COMMANDS, cmd.command)
except AttributeError:
raise CommandNotFound() # or we can use `ValueError` instead
else:
commander.command(irc_c, msg, cmd)
Use default parameter of getattr function and make some kind of logging there like
class DefaultCommand:
def command(self, irc_c, msg, cmd):
print("Command {} is not found".format(cmd.command))
and after that used like
getattr(COMMANDS, cmd.command, DefaultCommand()).command(irc_c, msg, cmd)

Answer 2

Basically you need to handle the specific error first. From more general to more specific, i.e Exception => AttributeError => YourError
>>> try:
... raise MyCustomAttrErr("Hey, this failed!")
... except MyCustomAttrErr as e:
... print(e)
... except AttributteError as e:
... print("Attribute error raised")
... except Exception as e:
... print("Base exception raised")
...
Hey, this failed!
Python handled the except blocks in order from top to bottom and stops in the first block that captures it.