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What's the proper way to declare custom exception classes in modern Python? My primary goal is to follow whatever standard other exception classes have, so that (for instance) any extra string I include in the exception is printed out by whatever tool caught the exception.
By "modern Python" I mean something that will run in Python 2.5 but be 'correct' for the Python 2.6 and Python 3.* way of doing things. And by "custom" I mean an Exception object that can include extra data about the cause of the error: a string, maybe also some other arbitrary object relevant to the exception.
I was tripped up by the following deprecation warning in Python 2.6.2:
>>> class MyError(Exception):
... def __init__(self, message):
... self.message = message
...
>>> MyError("foo")
_sandbox.py:3: DeprecationWarning: BaseException.message has been deprecated as of Python 2.6
It seems crazy that BaseException has a special meaning for attributes named message. I gather from PEP-352 that attribute did have a special meaning in 2.5 they're trying to deprecate away, so I guess that name (and that one alone) is now forbidden? Ugh.
I'm also fuzzily aware that Exception has some magic parameter args, but I've never known how to use it. Nor am I sure it's the right way to do things going forward; a lot of the discussion I found online suggested they were trying to do away with args in Python 3.
Update: two answers have suggested overriding __init__, and __str__/__unicode__/__repr__. That seems like a lot of typing, is it necessary?
Maybe I missed the question, but why not:
class MyException(Exception):
pass
To override something (or pass extra args), do this:
class ValidationError(Exception):
def __init__(self, message, errors):
# Call the base class constructor with the parameters it needs
super().__init__(message)
# Now for your custom code...
self.errors = errors
That way you could pass dict of error messages to the second param, and get to it later with e.errors.
In Python 2, you have to use this slightly more complex form of super():
super(ValidationError, self).__init__(message)
With modern Python Exceptions, you don't need to abuse .message, or override .__str__() or .__repr__() or any of it. If all you want is an informative message when your exception is raised, do this:
class MyException(Exception):
pass
raise MyException("My hovercraft is full of eels")
That will give a traceback ending with MyException: My hovercraft is full of eels.
If you want more flexibility from the exception, you could pass a dictionary as the argument:
raise MyException({"message":"My hovercraft is full of animals", "animal":"eels"})
However, to get at those details in an except block is a bit more complicated. The details are stored in the args attribute, which is a list. You would need to do something like this:
try:
raise MyException({"message":"My hovercraft is full of animals", "animal":"eels"})
except MyException as e:
details = e.args[0]
print(details["animal"])
It is still possible to pass in multiple items to the exception and access them via tuple indexes, but this is highly discouraged (and was even intended for deprecation a while back). If you do need more than a single piece of information and the above method is not sufficient for you, then you should subclass Exception as described in the tutorial.
class MyError(Exception):
def __init__(self, message, animal):
self.message = message
self.animal = animal
def __str__(self):
return self.message
"What is the proper way to declare custom exceptions in modern Python?"
This is fine unless your exception is really a type of a more specific exception:
class MyException(Exception):
pass
Or better (maybe perfect), instead of pass give a docstring:
class MyException(Exception):
"""Raise for my specific kind of exception"""
Subclassing Exception Subclasses
From the docs
Exception
All built-in, non-system-exiting exceptions are derived from this class.
All user-defined exceptions should also be derived from this
class.
That means that if your exception is a type of a more specific exception, subclass that exception instead of the generic Exception (and the result will be that you still derive from Exception as the docs recommend). Also, you can at least provide a docstring (and not be forced to use the pass keyword):
class MyAppValueError(ValueError):
'''Raise when my specific value is wrong'''
Set attributes you create yourself with a custom __init__. Avoid passing a dict as a positional argument, future users of your code will thank you. If you use the deprecated message attribute, assigning it yourself will avoid a DeprecationWarning:
class MyAppValueError(ValueError):
'''Raise when a specific subset of values in context of app is wrong'''
def __init__(self, message, foo, *args):
self.message = message # without this you may get DeprecationWarning
# Special attribute you desire with your Error,
# perhaps the value that caused the error?:
self.foo = foo
# allow users initialize misc. arguments as any other builtin Error
super(MyAppValueError, self).__init__(message, foo, *args)
There's really no need to write your own __str__ or __repr__. The built-in ones are very nice, and your cooperative inheritance ensures that you use them.
Critique of the top answer
Maybe I missed the question, but why not:
class MyException(Exception):
pass
Again, the problem with the above is that in order to catch it, you'll either have to name it specifically (importing it if created elsewhere) or catch Exception, (but you're probably not prepared to handle all types of Exceptions, and you should only catch exceptions you are prepared to handle). Similar criticism to the below, but additionally that's not the way to initialize via super, and you'll get a DeprecationWarning if you access the message attribute:
Edit: to override something (or pass extra args), do this:
class ValidationError(Exception):
def __init__(self, message, errors):
# Call the base class constructor with the parameters it needs
super(ValidationError, self).__init__(message)
# Now for your custom code...
self.errors = errors
That way you could pass dict of error messages to the second param, and get to it later with e.errors
It also requires exactly two arguments to be passed in (aside from the self.) No more, no less. That's an interesting constraint that future users may not appreciate.
To be direct - it violates Liskov substitutability.
I'll demonstrate both errors:
>>> ValidationError('foo', 'bar', 'baz').message
Traceback (most recent call last):
File "<pyshell#10>", line 1, in <module>
ValidationError('foo', 'bar', 'baz').message
TypeError: __init__() takes exactly 3 arguments (4 given)
>>> ValidationError('foo', 'bar').message
__main__:1: DeprecationWarning: BaseException.message has been deprecated as of Python 2.6
'foo'
Compared to:
>>> MyAppValueError('foo', 'FOO', 'bar').message
'foo'
see how exceptions work by default if one vs more attributes are used (tracebacks omitted):
>>> raise Exception('bad thing happened')
Exception: bad thing happened
>>> raise Exception('bad thing happened', 'code is broken')
Exception: ('bad thing happened', 'code is broken')
so you might want to have a sort of "exception template", working as an exception itself, in a compatible way:
>>> nastyerr = NastyError('bad thing happened')
>>> raise nastyerr
NastyError: bad thing happened
>>> raise nastyerr()
NastyError: bad thing happened
>>> raise nastyerr('code is broken')
NastyError: ('bad thing happened', 'code is broken')
this can be done easily with this subclass
class ExceptionTemplate(Exception):
def __call__(self, *args):
return self.__class__(*(self.args + args))
# ...
class NastyError(ExceptionTemplate): pass
and if you don't like that default tuple-like representation, just add __str__ method to the ExceptionTemplate class, like:
# ...
def __str__(self):
return ': '.join(self.args)
and you'll have
>>> raise nastyerr('code is broken')
NastyError: bad thing happened: code is broken
As of Python 3.8 (2018, https://docs.python.org/dev/whatsnew/3.8.html), the recommended method is still:
class CustomExceptionName(Exception):
"""Exception raised when very uncommon things happen"""
pass
Please don't forget to document, why a custom exception is neccessary!
If you need to, this is the way to go for exceptions with more data:
class CustomExceptionName(Exception):
"""Still an exception raised when uncommon things happen"""
def __init__(self, message, payload=None):
self.message = message
self.payload = payload # you could add more args
def __str__(self):
return str(self.message) # __str__() obviously expects a string to be returned, so make sure not to send any other data types
and fetch them like:
try:
raise CustomExceptionName("Very bad mistake.", "Forgot upgrading from Python 1")
except CustomExceptionName as error:
print(str(error)) # Very bad mistake
print("Detail: {}".format(error.payload)) # Detail: Forgot upgrading from Python 1
payload=None is important to make it pickle-able. Before dumping it, you have to call error.__reduce__(). Loading will work as expected.
You maybe should investigate in finding a solution using pythons return statement if you need much data to be transferred to some outer structure. This seems to be clearer/more pythonic to me. Advanced exceptions are heavily used in Java, which can sometimes be annoying, when using a framework and having to catch all possible errors.
To define your own exceptions correctly, there are a few best practices that you should follow:
Define a base class inheriting from Exception. This will allow to easily catch any exceptions related to the project:
class MyProjectError(Exception):
"""A base class for MyProject exceptions."""
Organizing the exception classes in a separate module (e.g. exceptions.py) is generally a good idea.
To create a specific exception, subclass the base exception class.
class CustomError(MyProjectError):
"""A custom exception class for MyProject."""
You can subclass custom exception classes as well to create a hierarchy.
To add support for extra argument(s) to a custom exception, define an __init__() method with a variable number of arguments. Call the base class's __init__(), passing any positional arguments to it (remember that BaseException/Exception expect any number of positional arguments). Store extra keyword arguments to the instance, e.g.:
class CustomError(MyProjectError):
def __init__(self, *args, **kwargs):
super().__init__(*args)
self.custom_kwarg = kwargs.get('custom_kwargs')
Usage example:
try:
raise CustomError('Something bad happened', custom_kwarg='value')
except CustomError as exc:
print(f'Сaught CustomError exception with custom_kwarg={exc.custom_kwarg}')
This design adheres to the Liskov substitution principle, since you can replace an instance of a base exception class with an instance of a derived exception class. Also, it allows you to create an instance of a derived class with the same parameters as the parent.
You should override __repr__ or __unicode__ methods instead of using message, the args you provide when you construct the exception will be in the args attribute of the exception object.
See a very good article "The definitive guide to Python exceptions". The basic principles are:
Always inherit from (at least) Exception.
Always call BaseException.__init__ with only one argument.
When building a library, define a base class inheriting from Exception.
Provide details about the error.
Inherit from builtin exceptions types when it makes sense.
There is also information on organizing (in modules) and wrapping exceptions, I recommend to read the guide.
No, "message" is not forbidden. It's just deprecated. You application will work fine with using message. But you may want to get rid of the deprecation error, of course.
When you create custom Exception classes for your application, many of them do not subclass just from Exception, but from others, like ValueError or similar. Then you have to adapt to their usage of variables.
And if you have many exceptions in your application it's usually a good idea to have a common custom base class for all of them, so that users of your modules can do
try:
...
except NelsonsExceptions:
...
And in that case you can do __init__ and __str__ needed there, so you don't have to repeat it for every exception. But simply calling the message variable something else than message does the trick.
In any case, you only need __init__ or __str__ if you do something different from what Exception itself does. And because if the deprecation, you then need both, or you get an error. That's not a whole lot of extra code you need per class.
For maximum customisation, to define custom errors, you may want to define an intermediate class that inherits from Exception class as:
class BaseCustomException(Exception):
def __init__(self, msg):
self.msg = msg
def __repr__(self):
return self.msg
class MyCustomError(BaseCustomException):
"""raise my custom error"""
Try this Example
class InvalidInputError(Exception):
def __init__(self, msg):
self.msg = msg
def __str__(self):
return repr(self.msg)
inp = int(input("Enter a number between 1 to 10:"))
try:
if type(inp) != int or inp not in list(range(1,11)):
raise InvalidInputError
except InvalidInputError:
print("Invalid input entered")
A really simple approach:
class CustomError(Exception):
pass
raise CustomError("Hmm, seems like this was custom coded...")
Or, have the error raise without printing __main__ (may look cleaner and neater):
class CustomError(Exception):
__module__ = Exception.__module__
raise CustomError("Improved CustomError!")
I had issues with the above methods, as of Python 3.9.5.
However, I found that this works for me:
class MyException(Exception):
"""Port Exception"""
And then it could be used in code like:
try:
raise MyException('Message')
except MyException as err:
print (err)
I came across this thread. This is how I do custom exceptions. While the Fault class is slightly complex, it makes declaring custom expressive exceptions with variable arguments trivial.
FinalViolation, SingletonViolation are both sub classes of TypeError so will be caught code below.
try:
<do something>
except TypeError as ex:
<handler>
That's why Fault doesn't inherit from Exception. To allow derivative exceptions to inherit from the exception of their choice.
class Fault:
"""Generic Exception base class. Note not descendant of Exception
Inheriting exceptions override formats"""
formats = '' # to be overriden in descendant classes
def __init__(self, *args):
"""Just save args for __str__"""
self.args = args
def __str__(self):
"""Use formats declared in descendant classes, and saved args to build exception text"""
return self.formats.format(*self.args)
class TypeFault(Fault, TypeError):
"""Helper class mixing Fault and TypeError"""
class FinalViolation(TypeFault):
"""Custom exception raised if inheriting from 'final' class"""
formats = "type {} is not an acceptable base type. It cannot be inherited from."
class SingletonViolation(TypeFault):
"""Custom exception raised if instancing 'singleton' class a second time"""
formats = "type {} is a singleton. It can only be instanced once."
FinalViolation, SingletonViolation unfortunately only accept 1 argument.
But one could easily create a multi arg error e.g.
class VesselLoadingError(Fault, BufferError):
formats = "My {} is full of {}."
raise VesselLoadingError('hovercraft', 'eels')
__main__.VesselLoadingError: My hovercraft is full of eels.
For me it is just __init__ and variables but making sometimes testing.
My sample:
Error_codes = { 100: "Not enough parameters", 101: "Number of special characters more than limits", 102: "At least 18 alphanumeric characters and list of special chars !##$&*" }
class localbreak( Exception ) :
Message = ""
def __init__(self, Message):
self.Message = Message
return
def __str__(self):
print(self.Message)
return "False"
### When calling ...
raise localbreak(Error_codes[102])
Output:
Traceback (most recent call last): File "ASCII.py", line 150, in <module>
main(OldPassword, Newpassword) File "ASCII.py", line 39, in main
result = read_input("1", "2", Newpassword, "4")
File "ASCII.py", line 69, in read_input
raise localbreak(Error_codes[102]) At least 18 alphanumeric characters and list of special chars !##$&*
__main__.localbreak: False
What's the proper way to declare custom exception classes in modern Python? My primary goal is to follow whatever standard other exception classes have, so that (for instance) any extra string I include in the exception is printed out by whatever tool caught the exception.
By "modern Python" I mean something that will run in Python 2.5 but be 'correct' for the Python 2.6 and Python 3.* way of doing things. And by "custom" I mean an Exception object that can include extra data about the cause of the error: a string, maybe also some other arbitrary object relevant to the exception.
I was tripped up by the following deprecation warning in Python 2.6.2:
>>> class MyError(Exception):
... def __init__(self, message):
... self.message = message
...
>>> MyError("foo")
_sandbox.py:3: DeprecationWarning: BaseException.message has been deprecated as of Python 2.6
It seems crazy that BaseException has a special meaning for attributes named message. I gather from PEP-352 that attribute did have a special meaning in 2.5 they're trying to deprecate away, so I guess that name (and that one alone) is now forbidden? Ugh.
I'm also fuzzily aware that Exception has some magic parameter args, but I've never known how to use it. Nor am I sure it's the right way to do things going forward; a lot of the discussion I found online suggested they were trying to do away with args in Python 3.
Update: two answers have suggested overriding __init__, and __str__/__unicode__/__repr__. That seems like a lot of typing, is it necessary?
Maybe I missed the question, but why not:
class MyException(Exception):
pass
To override something (or pass extra args), do this:
class ValidationError(Exception):
def __init__(self, message, errors):
# Call the base class constructor with the parameters it needs
super().__init__(message)
# Now for your custom code...
self.errors = errors
That way you could pass dict of error messages to the second param, and get to it later with e.errors.
In Python 2, you have to use this slightly more complex form of super():
super(ValidationError, self).__init__(message)
With modern Python Exceptions, you don't need to abuse .message, or override .__str__() or .__repr__() or any of it. If all you want is an informative message when your exception is raised, do this:
class MyException(Exception):
pass
raise MyException("My hovercraft is full of eels")
That will give a traceback ending with MyException: My hovercraft is full of eels.
If you want more flexibility from the exception, you could pass a dictionary as the argument:
raise MyException({"message":"My hovercraft is full of animals", "animal":"eels"})
However, to get at those details in an except block is a bit more complicated. The details are stored in the args attribute, which is a list. You would need to do something like this:
try:
raise MyException({"message":"My hovercraft is full of animals", "animal":"eels"})
except MyException as e:
details = e.args[0]
print(details["animal"])
It is still possible to pass in multiple items to the exception and access them via tuple indexes, but this is highly discouraged (and was even intended for deprecation a while back). If you do need more than a single piece of information and the above method is not sufficient for you, then you should subclass Exception as described in the tutorial.
class MyError(Exception):
def __init__(self, message, animal):
self.message = message
self.animal = animal
def __str__(self):
return self.message
"What is the proper way to declare custom exceptions in modern Python?"
This is fine unless your exception is really a type of a more specific exception:
class MyException(Exception):
pass
Or better (maybe perfect), instead of pass give a docstring:
class MyException(Exception):
"""Raise for my specific kind of exception"""
Subclassing Exception Subclasses
From the docs
Exception
All built-in, non-system-exiting exceptions are derived from this class.
All user-defined exceptions should also be derived from this
class.
That means that if your exception is a type of a more specific exception, subclass that exception instead of the generic Exception (and the result will be that you still derive from Exception as the docs recommend). Also, you can at least provide a docstring (and not be forced to use the pass keyword):
class MyAppValueError(ValueError):
'''Raise when my specific value is wrong'''
Set attributes you create yourself with a custom __init__. Avoid passing a dict as a positional argument, future users of your code will thank you. If you use the deprecated message attribute, assigning it yourself will avoid a DeprecationWarning:
class MyAppValueError(ValueError):
'''Raise when a specific subset of values in context of app is wrong'''
def __init__(self, message, foo, *args):
self.message = message # without this you may get DeprecationWarning
# Special attribute you desire with your Error,
# perhaps the value that caused the error?:
self.foo = foo
# allow users initialize misc. arguments as any other builtin Error
super(MyAppValueError, self).__init__(message, foo, *args)
There's really no need to write your own __str__ or __repr__. The built-in ones are very nice, and your cooperative inheritance ensures that you use them.
Critique of the top answer
Maybe I missed the question, but why not:
class MyException(Exception):
pass
Again, the problem with the above is that in order to catch it, you'll either have to name it specifically (importing it if created elsewhere) or catch Exception, (but you're probably not prepared to handle all types of Exceptions, and you should only catch exceptions you are prepared to handle). Similar criticism to the below, but additionally that's not the way to initialize via super, and you'll get a DeprecationWarning if you access the message attribute:
Edit: to override something (or pass extra args), do this:
class ValidationError(Exception):
def __init__(self, message, errors):
# Call the base class constructor with the parameters it needs
super(ValidationError, self).__init__(message)
# Now for your custom code...
self.errors = errors
That way you could pass dict of error messages to the second param, and get to it later with e.errors
It also requires exactly two arguments to be passed in (aside from the self.) No more, no less. That's an interesting constraint that future users may not appreciate.
To be direct - it violates Liskov substitutability.
I'll demonstrate both errors:
>>> ValidationError('foo', 'bar', 'baz').message
Traceback (most recent call last):
File "<pyshell#10>", line 1, in <module>
ValidationError('foo', 'bar', 'baz').message
TypeError: __init__() takes exactly 3 arguments (4 given)
>>> ValidationError('foo', 'bar').message
__main__:1: DeprecationWarning: BaseException.message has been deprecated as of Python 2.6
'foo'
Compared to:
>>> MyAppValueError('foo', 'FOO', 'bar').message
'foo'
see how exceptions work by default if one vs more attributes are used (tracebacks omitted):
>>> raise Exception('bad thing happened')
Exception: bad thing happened
>>> raise Exception('bad thing happened', 'code is broken')
Exception: ('bad thing happened', 'code is broken')
so you might want to have a sort of "exception template", working as an exception itself, in a compatible way:
>>> nastyerr = NastyError('bad thing happened')
>>> raise nastyerr
NastyError: bad thing happened
>>> raise nastyerr()
NastyError: bad thing happened
>>> raise nastyerr('code is broken')
NastyError: ('bad thing happened', 'code is broken')
this can be done easily with this subclass
class ExceptionTemplate(Exception):
def __call__(self, *args):
return self.__class__(*(self.args + args))
# ...
class NastyError(ExceptionTemplate): pass
and if you don't like that default tuple-like representation, just add __str__ method to the ExceptionTemplate class, like:
# ...
def __str__(self):
return ': '.join(self.args)
and you'll have
>>> raise nastyerr('code is broken')
NastyError: bad thing happened: code is broken
As of Python 3.8 (2018, https://docs.python.org/dev/whatsnew/3.8.html), the recommended method is still:
class CustomExceptionName(Exception):
"""Exception raised when very uncommon things happen"""
pass
Please don't forget to document, why a custom exception is neccessary!
If you need to, this is the way to go for exceptions with more data:
class CustomExceptionName(Exception):
"""Still an exception raised when uncommon things happen"""
def __init__(self, message, payload=None):
self.message = message
self.payload = payload # you could add more args
def __str__(self):
return str(self.message) # __str__() obviously expects a string to be returned, so make sure not to send any other data types
and fetch them like:
try:
raise CustomExceptionName("Very bad mistake.", "Forgot upgrading from Python 1")
except CustomExceptionName as error:
print(str(error)) # Very bad mistake
print("Detail: {}".format(error.payload)) # Detail: Forgot upgrading from Python 1
payload=None is important to make it pickle-able. Before dumping it, you have to call error.__reduce__(). Loading will work as expected.
You maybe should investigate in finding a solution using pythons return statement if you need much data to be transferred to some outer structure. This seems to be clearer/more pythonic to me. Advanced exceptions are heavily used in Java, which can sometimes be annoying, when using a framework and having to catch all possible errors.
To define your own exceptions correctly, there are a few best practices that you should follow:
Define a base class inheriting from Exception. This will allow to easily catch any exceptions related to the project:
class MyProjectError(Exception):
"""A base class for MyProject exceptions."""
Organizing the exception classes in a separate module (e.g. exceptions.py) is generally a good idea.
To create a specific exception, subclass the base exception class.
class CustomError(MyProjectError):
"""A custom exception class for MyProject."""
You can subclass custom exception classes as well to create a hierarchy.
To add support for extra argument(s) to a custom exception, define an __init__() method with a variable number of arguments. Call the base class's __init__(), passing any positional arguments to it (remember that BaseException/Exception expect any number of positional arguments). Store extra keyword arguments to the instance, e.g.:
class CustomError(MyProjectError):
def __init__(self, *args, **kwargs):
super().__init__(*args)
self.custom_kwarg = kwargs.get('custom_kwargs')
Usage example:
try:
raise CustomError('Something bad happened', custom_kwarg='value')
except CustomError as exc:
print(f'Сaught CustomError exception with custom_kwarg={exc.custom_kwarg}')
This design adheres to the Liskov substitution principle, since you can replace an instance of a base exception class with an instance of a derived exception class. Also, it allows you to create an instance of a derived class with the same parameters as the parent.
You should override __repr__ or __unicode__ methods instead of using message, the args you provide when you construct the exception will be in the args attribute of the exception object.
See a very good article "The definitive guide to Python exceptions". The basic principles are:
Always inherit from (at least) Exception.
Always call BaseException.__init__ with only one argument.
When building a library, define a base class inheriting from Exception.
Provide details about the error.
Inherit from builtin exceptions types when it makes sense.
There is also information on organizing (in modules) and wrapping exceptions, I recommend to read the guide.
No, "message" is not forbidden. It's just deprecated. You application will work fine with using message. But you may want to get rid of the deprecation error, of course.
When you create custom Exception classes for your application, many of them do not subclass just from Exception, but from others, like ValueError or similar. Then you have to adapt to their usage of variables.
And if you have many exceptions in your application it's usually a good idea to have a common custom base class for all of them, so that users of your modules can do
try:
...
except NelsonsExceptions:
...
And in that case you can do __init__ and __str__ needed there, so you don't have to repeat it for every exception. But simply calling the message variable something else than message does the trick.
In any case, you only need __init__ or __str__ if you do something different from what Exception itself does. And because if the deprecation, you then need both, or you get an error. That's not a whole lot of extra code you need per class.
For maximum customisation, to define custom errors, you may want to define an intermediate class that inherits from Exception class as:
class BaseCustomException(Exception):
def __init__(self, msg):
self.msg = msg
def __repr__(self):
return self.msg
class MyCustomError(BaseCustomException):
"""raise my custom error"""
Try this Example
class InvalidInputError(Exception):
def __init__(self, msg):
self.msg = msg
def __str__(self):
return repr(self.msg)
inp = int(input("Enter a number between 1 to 10:"))
try:
if type(inp) != int or inp not in list(range(1,11)):
raise InvalidInputError
except InvalidInputError:
print("Invalid input entered")
A really simple approach:
class CustomError(Exception):
pass
raise CustomError("Hmm, seems like this was custom coded...")
Or, have the error raise without printing __main__ (may look cleaner and neater):
class CustomError(Exception):
__module__ = Exception.__module__
raise CustomError("Improved CustomError!")
I had issues with the above methods, as of Python 3.9.5.
However, I found that this works for me:
class MyException(Exception):
"""Port Exception"""
And then it could be used in code like:
try:
raise MyException('Message')
except MyException as err:
print (err)
I came across this thread. This is how I do custom exceptions. While the Fault class is slightly complex, it makes declaring custom expressive exceptions with variable arguments trivial.
FinalViolation, SingletonViolation are both sub classes of TypeError so will be caught code below.
try:
<do something>
except TypeError as ex:
<handler>
That's why Fault doesn't inherit from Exception. To allow derivative exceptions to inherit from the exception of their choice.
class Fault:
"""Generic Exception base class. Note not descendant of Exception
Inheriting exceptions override formats"""
formats = '' # to be overriden in descendant classes
def __init__(self, *args):
"""Just save args for __str__"""
self.args = args
def __str__(self):
"""Use formats declared in descendant classes, and saved args to build exception text"""
return self.formats.format(*self.args)
class TypeFault(Fault, TypeError):
"""Helper class mixing Fault and TypeError"""
class FinalViolation(TypeFault):
"""Custom exception raised if inheriting from 'final' class"""
formats = "type {} is not an acceptable base type. It cannot be inherited from."
class SingletonViolation(TypeFault):
"""Custom exception raised if instancing 'singleton' class a second time"""
formats = "type {} is a singleton. It can only be instanced once."
FinalViolation, SingletonViolation unfortunately only accept 1 argument.
But one could easily create a multi arg error e.g.
class VesselLoadingError(Fault, BufferError):
formats = "My {} is full of {}."
raise VesselLoadingError('hovercraft', 'eels')
__main__.VesselLoadingError: My hovercraft is full of eels.
For me it is just __init__ and variables but making sometimes testing.
My sample:
Error_codes = { 100: "Not enough parameters", 101: "Number of special characters more than limits", 102: "At least 18 alphanumeric characters and list of special chars !##$&*" }
class localbreak( Exception ) :
Message = ""
def __init__(self, Message):
self.Message = Message
return
def __str__(self):
print(self.Message)
return "False"
### When calling ...
raise localbreak(Error_codes[102])
Output:
Traceback (most recent call last): File "ASCII.py", line 150, in <module>
main(OldPassword, Newpassword) File "ASCII.py", line 39, in main
result = read_input("1", "2", Newpassword, "4")
File "ASCII.py", line 69, in read_input
raise localbreak(Error_codes[102]) At least 18 alphanumeric characters and list of special chars !##$&*
__main__.localbreak: False
I currently refactor a class defining a client or a server. The class had a lot of
if client:
XXXXXXXXXXXX
elif server:
YYYYYYYYYYYY
So I decided to create a class A with the similar code and one class C for the client and an other one S for the server which inherit A. (they don't have theses names of course ^^)
So class A is some kind of abstract class. But the problem is there is no abstract classes in Python 2.5, it comes with 2.6 version. So I was wondering if there is a way to forbid instantiations of class A.
One solution would have been to raise a NotImplemented error in the constructor of the class A, but C and S have the same code for it so I put it in the "abstract" class A (bad idea ?).
This may seem stupid but I develop in Python only from time to time and I'm a young programmer.
What are your advices?
In statically-typed languages, you use an abstract base class (ABC) because you need some object with a defined size, interface etc. to pass around. Otherwise, the code trying to call methods on that object can't be compiled.
Python isn't a statically-typed language, and the calling code doesn't need to know the type of the object it's calling at all. So, you can "define" your ABC just by documenting the interface requirements, and implementing that interface directly in two unrelated classes.
Eg,
class Server:
def do_thing(self):
pass #do that server thing here
class Client:
def do_thing(self):
pass #do that client thing here
def do_thing(thingy):
thingy.do_thing() # is it a Client? a Server? something else?
s=Server()
c=Client()
do_thing(s)
do_thing(c)
Here, you could pass in any object with a do_thing method whose arguments match the call.
This approach has the advantage that you do not need to do anything to the subclass to make it non-abstract.
class ABC(object):
abstract = True
def __new__(cls, *args, **kwargs):
if "abstract" in cls.__dict__ and cls.__dict__["abstract"] == True:
raise RuntimeError(cls.__name__ + " is abstract!")
return object.__new__(cls)
class Subclass(ABC):
pass
print Subclass()
print ABC()
Output:
<__main__.Subclass object at 0xb7878a6c>
Traceback (most recent call last):
File "abc.py", line 14, in <module>
print ABC()
File "abc.py", line 6, in __new__
raise RuntimeError(cls.__name__ + " is abstract!")
RuntimeError: ABC is abstract!
If you want to create an abstract subclass, simply do like this:
class AbstractSubclass(ABC):
abstract = True
You can call a method "foo" at the beginning of A constructor. In A, this method raises an exception. In C and in S, you redefine "foo" so there is no more exceptions.
My first question is: why can't you simply avoid to instantiate an object from class A? What I mean is that this is a bit like questions on implementing singletons... As this answerer correctly quoted:
Before the Gang of Four got all academic on us, "singleton" (without the formal name) was just a simple idea that deserved a simple line of code, not a whole religion.
The same - IMO - applies to abstract classes (which in fact have been introduced in Python for other reasons than the one you would intend to use them for.
That said, you could raise an exception in the __init__ method of class A. Something like:
>>> class A():
... def __init__(self):
... raise BaseException()
...
>>> a = A()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 3, in __init__
BaseException
>>> class B(A):
... def __init__(self):
... pass
...
>>> b = B()
>>>
Of course this is just an rough idea: if you have - for example - some useful stuff in the A.__init__ you should check the __class__ attribute, etc...
The real question is: why do you need an abstract class?
if you make 2 classes, and make the second herit from the first, it is an efficient way to clean your code.
There's an abstract base class module for what you want.
Not applicable for 2.5.
The Python documentation for except says:
For an except clause with an
expression, that expression is
evaluated, and the clause matches the
exception if the resulting object is
“compatible” with the exception. An
object is compatible with an exception
if it is the class or a base class of the exception object, [...]
Why doesn't except use isinstance instead of comparing base classes? This is preventing the use of __instancecheck__ to override the instance check.
EDIT:
I can understand that one of the reasons this doesn't exist is that no one considered it. But are there any reasons why this should not be implemented?
EDIT:
Shell session from Python 3.2a showing that trying to use __subclasscheck__ for this doesn't work:
>>> class MyType(type): __subclasscheck__ = lambda cls, other_cls: True
>>> class O(Exception, metaclass=MyType): pass
>>> issubclass(3, O)
0: True
>>> issubclass(int, O)
1: True
>>> try:
... 1/0
... except O:
... print('Success')
Traceback (most recent call last):
File "<pyshell#4>", line 2, in <module>
1/0
ZeroDivisionError: division by zero
>>>
The simple answer is probably that nobody considered it. A more complex answer would be that nobody considered it because it's hard to get this right, because it would mean executing potentially arbitrary Python code while handling an exception, and that it is of dubious value. Exception classes in Python are typically very simple classes, and overloading them with functionality is often a mistake. It's hard to imagine a case for having it consult __instancecheck__. If you have such a case (with or without a patch), file a bug and someone might pick it up.
But are there any reasons why this
should not be implemented?
Can you give an example where this would be useful?
When you do this:
class SomeMeta(type):
def __subclasscheck__(cls, sub):
print (cls, sub)
return True
class Something(Exception):
pass
class SomeType(Exception):
__metaclass__ = SomeMeta
try:
raise Something()
except SomeType, e:
pass
# prints (<class '__main__.SomeType'>, <class '__main__.Something'>)
Python calls __subclasscheck__ on SomeType's metaclass to determine if Something a subclass of SomeType. The Metaclass PEP talks about this in more detail.
What's the proper way to declare custom exception classes in modern Python? My primary goal is to follow whatever standard other exception classes have, so that (for instance) any extra string I include in the exception is printed out by whatever tool caught the exception.
By "modern Python" I mean something that will run in Python 2.5 but be 'correct' for the Python 2.6 and Python 3.* way of doing things. And by "custom" I mean an Exception object that can include extra data about the cause of the error: a string, maybe also some other arbitrary object relevant to the exception.
I was tripped up by the following deprecation warning in Python 2.6.2:
>>> class MyError(Exception):
... def __init__(self, message):
... self.message = message
...
>>> MyError("foo")
_sandbox.py:3: DeprecationWarning: BaseException.message has been deprecated as of Python 2.6
It seems crazy that BaseException has a special meaning for attributes named message. I gather from PEP-352 that attribute did have a special meaning in 2.5 they're trying to deprecate away, so I guess that name (and that one alone) is now forbidden? Ugh.
I'm also fuzzily aware that Exception has some magic parameter args, but I've never known how to use it. Nor am I sure it's the right way to do things going forward; a lot of the discussion I found online suggested they were trying to do away with args in Python 3.
Update: two answers have suggested overriding __init__, and __str__/__unicode__/__repr__. That seems like a lot of typing, is it necessary?
Maybe I missed the question, but why not:
class MyException(Exception):
pass
To override something (or pass extra args), do this:
class ValidationError(Exception):
def __init__(self, message, errors):
# Call the base class constructor with the parameters it needs
super().__init__(message)
# Now for your custom code...
self.errors = errors
That way you could pass dict of error messages to the second param, and get to it later with e.errors.
In Python 2, you have to use this slightly more complex form of super():
super(ValidationError, self).__init__(message)
With modern Python Exceptions, you don't need to abuse .message, or override .__str__() or .__repr__() or any of it. If all you want is an informative message when your exception is raised, do this:
class MyException(Exception):
pass
raise MyException("My hovercraft is full of eels")
That will give a traceback ending with MyException: My hovercraft is full of eels.
If you want more flexibility from the exception, you could pass a dictionary as the argument:
raise MyException({"message":"My hovercraft is full of animals", "animal":"eels"})
However, to get at those details in an except block is a bit more complicated. The details are stored in the args attribute, which is a list. You would need to do something like this:
try:
raise MyException({"message":"My hovercraft is full of animals", "animal":"eels"})
except MyException as e:
details = e.args[0]
print(details["animal"])
It is still possible to pass in multiple items to the exception and access them via tuple indexes, but this is highly discouraged (and was even intended for deprecation a while back). If you do need more than a single piece of information and the above method is not sufficient for you, then you should subclass Exception as described in the tutorial.
class MyError(Exception):
def __init__(self, message, animal):
self.message = message
self.animal = animal
def __str__(self):
return self.message
"What is the proper way to declare custom exceptions in modern Python?"
This is fine unless your exception is really a type of a more specific exception:
class MyException(Exception):
pass
Or better (maybe perfect), instead of pass give a docstring:
class MyException(Exception):
"""Raise for my specific kind of exception"""
Subclassing Exception Subclasses
From the docs
Exception
All built-in, non-system-exiting exceptions are derived from this class.
All user-defined exceptions should also be derived from this
class.
That means that if your exception is a type of a more specific exception, subclass that exception instead of the generic Exception (and the result will be that you still derive from Exception as the docs recommend). Also, you can at least provide a docstring (and not be forced to use the pass keyword):
class MyAppValueError(ValueError):
'''Raise when my specific value is wrong'''
Set attributes you create yourself with a custom __init__. Avoid passing a dict as a positional argument, future users of your code will thank you. If you use the deprecated message attribute, assigning it yourself will avoid a DeprecationWarning:
class MyAppValueError(ValueError):
'''Raise when a specific subset of values in context of app is wrong'''
def __init__(self, message, foo, *args):
self.message = message # without this you may get DeprecationWarning
# Special attribute you desire with your Error,
# perhaps the value that caused the error?:
self.foo = foo
# allow users initialize misc. arguments as any other builtin Error
super(MyAppValueError, self).__init__(message, foo, *args)
There's really no need to write your own __str__ or __repr__. The built-in ones are very nice, and your cooperative inheritance ensures that you use them.
Critique of the top answer
Maybe I missed the question, but why not:
class MyException(Exception):
pass
Again, the problem with the above is that in order to catch it, you'll either have to name it specifically (importing it if created elsewhere) or catch Exception, (but you're probably not prepared to handle all types of Exceptions, and you should only catch exceptions you are prepared to handle). Similar criticism to the below, but additionally that's not the way to initialize via super, and you'll get a DeprecationWarning if you access the message attribute:
Edit: to override something (or pass extra args), do this:
class ValidationError(Exception):
def __init__(self, message, errors):
# Call the base class constructor with the parameters it needs
super(ValidationError, self).__init__(message)
# Now for your custom code...
self.errors = errors
That way you could pass dict of error messages to the second param, and get to it later with e.errors
It also requires exactly two arguments to be passed in (aside from the self.) No more, no less. That's an interesting constraint that future users may not appreciate.
To be direct - it violates Liskov substitutability.
I'll demonstrate both errors:
>>> ValidationError('foo', 'bar', 'baz').message
Traceback (most recent call last):
File "<pyshell#10>", line 1, in <module>
ValidationError('foo', 'bar', 'baz').message
TypeError: __init__() takes exactly 3 arguments (4 given)
>>> ValidationError('foo', 'bar').message
__main__:1: DeprecationWarning: BaseException.message has been deprecated as of Python 2.6
'foo'
Compared to:
>>> MyAppValueError('foo', 'FOO', 'bar').message
'foo'
see how exceptions work by default if one vs more attributes are used (tracebacks omitted):
>>> raise Exception('bad thing happened')
Exception: bad thing happened
>>> raise Exception('bad thing happened', 'code is broken')
Exception: ('bad thing happened', 'code is broken')
so you might want to have a sort of "exception template", working as an exception itself, in a compatible way:
>>> nastyerr = NastyError('bad thing happened')
>>> raise nastyerr
NastyError: bad thing happened
>>> raise nastyerr()
NastyError: bad thing happened
>>> raise nastyerr('code is broken')
NastyError: ('bad thing happened', 'code is broken')
this can be done easily with this subclass
class ExceptionTemplate(Exception):
def __call__(self, *args):
return self.__class__(*(self.args + args))
# ...
class NastyError(ExceptionTemplate): pass
and if you don't like that default tuple-like representation, just add __str__ method to the ExceptionTemplate class, like:
# ...
def __str__(self):
return ': '.join(self.args)
and you'll have
>>> raise nastyerr('code is broken')
NastyError: bad thing happened: code is broken
As of Python 3.8 (2018, https://docs.python.org/dev/whatsnew/3.8.html), the recommended method is still:
class CustomExceptionName(Exception):
"""Exception raised when very uncommon things happen"""
pass
Please don't forget to document, why a custom exception is neccessary!
If you need to, this is the way to go for exceptions with more data:
class CustomExceptionName(Exception):
"""Still an exception raised when uncommon things happen"""
def __init__(self, message, payload=None):
self.message = message
self.payload = payload # you could add more args
def __str__(self):
return str(self.message) # __str__() obviously expects a string to be returned, so make sure not to send any other data types
and fetch them like:
try:
raise CustomExceptionName("Very bad mistake.", "Forgot upgrading from Python 1")
except CustomExceptionName as error:
print(str(error)) # Very bad mistake
print("Detail: {}".format(error.payload)) # Detail: Forgot upgrading from Python 1
payload=None is important to make it pickle-able. Before dumping it, you have to call error.__reduce__(). Loading will work as expected.
You maybe should investigate in finding a solution using pythons return statement if you need much data to be transferred to some outer structure. This seems to be clearer/more pythonic to me. Advanced exceptions are heavily used in Java, which can sometimes be annoying, when using a framework and having to catch all possible errors.
To define your own exceptions correctly, there are a few best practices that you should follow:
Define a base class inheriting from Exception. This will allow to easily catch any exceptions related to the project:
class MyProjectError(Exception):
"""A base class for MyProject exceptions."""
Organizing the exception classes in a separate module (e.g. exceptions.py) is generally a good idea.
To create a specific exception, subclass the base exception class.
class CustomError(MyProjectError):
"""A custom exception class for MyProject."""
You can subclass custom exception classes as well to create a hierarchy.
To add support for extra argument(s) to a custom exception, define an __init__() method with a variable number of arguments. Call the base class's __init__(), passing any positional arguments to it (remember that BaseException/Exception expect any number of positional arguments). Store extra keyword arguments to the instance, e.g.:
class CustomError(MyProjectError):
def __init__(self, *args, **kwargs):
super().__init__(*args)
self.custom_kwarg = kwargs.get('custom_kwargs')
Usage example:
try:
raise CustomError('Something bad happened', custom_kwarg='value')
except CustomError as exc:
print(f'Сaught CustomError exception with custom_kwarg={exc.custom_kwarg}')
This design adheres to the Liskov substitution principle, since you can replace an instance of a base exception class with an instance of a derived exception class. Also, it allows you to create an instance of a derived class with the same parameters as the parent.
You should override __repr__ or __unicode__ methods instead of using message, the args you provide when you construct the exception will be in the args attribute of the exception object.
See a very good article "The definitive guide to Python exceptions". The basic principles are:
Always inherit from (at least) Exception.
Always call BaseException.__init__ with only one argument.
When building a library, define a base class inheriting from Exception.
Provide details about the error.
Inherit from builtin exceptions types when it makes sense.
There is also information on organizing (in modules) and wrapping exceptions, I recommend to read the guide.
No, "message" is not forbidden. It's just deprecated. You application will work fine with using message. But you may want to get rid of the deprecation error, of course.
When you create custom Exception classes for your application, many of them do not subclass just from Exception, but from others, like ValueError or similar. Then you have to adapt to their usage of variables.
And if you have many exceptions in your application it's usually a good idea to have a common custom base class for all of them, so that users of your modules can do
try:
...
except NelsonsExceptions:
...
And in that case you can do __init__ and __str__ needed there, so you don't have to repeat it for every exception. But simply calling the message variable something else than message does the trick.
In any case, you only need __init__ or __str__ if you do something different from what Exception itself does. And because if the deprecation, you then need both, or you get an error. That's not a whole lot of extra code you need per class.
For maximum customisation, to define custom errors, you may want to define an intermediate class that inherits from Exception class as:
class BaseCustomException(Exception):
def __init__(self, msg):
self.msg = msg
def __repr__(self):
return self.msg
class MyCustomError(BaseCustomException):
"""raise my custom error"""
Try this Example
class InvalidInputError(Exception):
def __init__(self, msg):
self.msg = msg
def __str__(self):
return repr(self.msg)
inp = int(input("Enter a number between 1 to 10:"))
try:
if type(inp) != int or inp not in list(range(1,11)):
raise InvalidInputError
except InvalidInputError:
print("Invalid input entered")
A really simple approach:
class CustomError(Exception):
pass
raise CustomError("Hmm, seems like this was custom coded...")
Or, have the error raise without printing __main__ (may look cleaner and neater):
class CustomError(Exception):
__module__ = Exception.__module__
raise CustomError("Improved CustomError!")
I had issues with the above methods, as of Python 3.9.5.
However, I found that this works for me:
class MyException(Exception):
"""Port Exception"""
And then it could be used in code like:
try:
raise MyException('Message')
except MyException as err:
print (err)
I came across this thread. This is how I do custom exceptions. While the Fault class is slightly complex, it makes declaring custom expressive exceptions with variable arguments trivial.
FinalViolation, SingletonViolation are both sub classes of TypeError so will be caught code below.
try:
<do something>
except TypeError as ex:
<handler>
That's why Fault doesn't inherit from Exception. To allow derivative exceptions to inherit from the exception of their choice.
class Fault:
"""Generic Exception base class. Note not descendant of Exception
Inheriting exceptions override formats"""
formats = '' # to be overriden in descendant classes
def __init__(self, *args):
"""Just save args for __str__"""
self.args = args
def __str__(self):
"""Use formats declared in descendant classes, and saved args to build exception text"""
return self.formats.format(*self.args)
class TypeFault(Fault, TypeError):
"""Helper class mixing Fault and TypeError"""
class FinalViolation(TypeFault):
"""Custom exception raised if inheriting from 'final' class"""
formats = "type {} is not an acceptable base type. It cannot be inherited from."
class SingletonViolation(TypeFault):
"""Custom exception raised if instancing 'singleton' class a second time"""
formats = "type {} is a singleton. It can only be instanced once."
FinalViolation, SingletonViolation unfortunately only accept 1 argument.
But one could easily create a multi arg error e.g.
class VesselLoadingError(Fault, BufferError):
formats = "My {} is full of {}."
raise VesselLoadingError('hovercraft', 'eels')
__main__.VesselLoadingError: My hovercraft is full of eels.
For me it is just __init__ and variables but making sometimes testing.
My sample:
Error_codes = { 100: "Not enough parameters", 101: "Number of special characters more than limits", 102: "At least 18 alphanumeric characters and list of special chars !##$&*" }
class localbreak( Exception ) :
Message = ""
def __init__(self, Message):
self.Message = Message
return
def __str__(self):
print(self.Message)
return "False"
### When calling ...
raise localbreak(Error_codes[102])
Output:
Traceback (most recent call last): File "ASCII.py", line 150, in <module>
main(OldPassword, Newpassword) File "ASCII.py", line 39, in main
result = read_input("1", "2", Newpassword, "4")
File "ASCII.py", line 69, in read_input
raise localbreak(Error_codes[102]) At least 18 alphanumeric characters and list of special chars !##$&*
__main__.localbreak: False