Python "Raise From" Usage

Python raise from usage

The difference is that when you use from, the __cause__ attribute is set and the message states that the exception was directly caused by. If you omit the from then no __cause__ is set, but the __context__ attribute may be set as well, and the traceback then shows the context as during handling something else happened.

Setting the __context__ happens if you used raise in an exception handler; if you used raise anywhere else no __context__ is set either.

If a __cause__ is set, a __suppress_context__ = True flag is also set on the exception; when __suppress_context__ is set to True, the __context__ is ignored when printing a traceback.

When raising from a exception handler where you don't want to show the context (don't want a during handling another exception happened message), then use raise ... from None to set __suppress_context__ to True.

In other words, Python sets a context on exceptions so you can introspect where an exception was raised, letting you see if another exception was replaced by it. You can also add a cause to an exception, making the traceback explicit about the other exception (use different wording), and the context is ignored (but can still be introspected when debugging). Using raise ... from None lets you suppress the context being printed.

See the raise statement documenation:

The from clause is used for exception chaining: if given, the second expression must be another exception class or instance, which will then be attached to the raised exception as the __cause__ attribute (which is writable). If the raised exception is not handled, both exceptions will be printed:
>>> try:
...     print(1 / 0)
... except Exception as exc:
...     raise RuntimeError("Something bad happened") from exc
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: int division or modulo by zero

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
RuntimeError: Something bad happened
A similar mechanism works implicitly if an exception is raised inside an exception handler or a finally clause: the previous exception is then attached as the new exception’s __context__ attribute:
>>> try:
...     print(1 / 0)
... except:
...     raise RuntimeError("Something bad happened")
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: int division or modulo by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
RuntimeError: Something bad happened

Also see the Built-in Exceptions documentation for details on the context and cause information attached to exceptions.

How to use raise keyword in Python

It has two purposes.

jackcogdill has given the first one:

It's used for raising your own errors.
if something:
   raise Exception('My error!')

The second is to reraise the current exception in an exception handler, so that it can be handled further up the call stack.

try:
  generate_exception()
except SomeException as e:
  if not can_handle(e):
    raise
  handle_exception(e)

Manually raising (throwing) an exception in Python

How do I manually throw/raise an exception in Python?

Use the most specific Exception constructor that semantically fits your issue.

Be specific in your message, e.g.:

raise ValueError('A very specific bad thing happened.')

Don't raise generic exceptions

Avoid raising a generic Exception. To catch it, you'll have to catch all other more specific exceptions that subclass it.

Problem 1: Hiding bugs

raise Exception('I know Python!') # Don't! If you catch, likely to hide bugs.

For example:

def demo_bad_catch():
    try:
        raise ValueError('Represents a hidden bug, do not catch this')
        raise Exception('This is the exception you expect to handle')
    except Exception as error:
        print('Caught this error: ' + repr(error))

>>> demo_bad_catch()
Caught this error: ValueError('Represents a hidden bug, do not catch this',)

Problem 2: Won't catch

And more specific catches won't catch the general exception:

def demo_no_catch():
    try:
        raise Exception('general exceptions not caught by specific handling')
    except ValueError as e:
        print('we will not catch exception: Exception')
 

>>> demo_no_catch()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in demo_no_catch
Exception: general exceptions not caught by specific handling

Best Practices: `raise` statement

Instead, use the most specific Exception constructor that semantically fits your issue.

raise ValueError('A very specific bad thing happened')

which also handily allows an arbitrary number of arguments to be passed to the constructor:

raise ValueError('A very specific bad thing happened', 'foo', 'bar', 'baz')

These arguments are accessed by the args attribute on the Exception object. For example:

try:
    some_code_that_may_raise_our_value_error()
except ValueError as err:
    print(err.args)

prints

('message', 'foo', 'bar', 'baz')

In Python 2.5, an actual message attribute was added to BaseException in favor of encouraging users to subclass Exceptions and stop using args, but the introduction of message and the original deprecation of args has been retracted.

Best Practices: `except` clause

When inside an except clause, you might want to, for example, log that a specific type of error happened, and then re-raise. The best way to do this while preserving the stack trace is to use a bare raise statement. For example:

logger = logging.getLogger(__name__)

try:
    do_something_in_app_that_breaks_easily()
except AppError as error:
    logger.error(error)
    raise                 # just this!
    # raise AppError      # Don't do this, you'll lose the stack trace!

Don't modify your errors... but if you insist.

You can preserve the stacktrace (and error value) with sys.exc_info(), but this is way more error prone and has compatibility problems between Python 2 and 3, prefer to use a bare raise to re-raise.

To explain - the sys.exc_info() returns the type, value, and traceback.

type, value, traceback = sys.exc_info()

This is the syntax in Python 2 - note this is not compatible with Python 3:

raise AppError, error, sys.exc_info()[2] # avoid this.
# Equivalently, as error *is* the second object:
raise sys.exc_info()[0], sys.exc_info()[1], sys.exc_info()[2]

If you want to, you can modify what happens with your new raise - e.g. setting new args for the instance:

def error():
    raise ValueError('oops!')

def catch_error_modify_message():
    try:
        error()
    except ValueError:
        error_type, error_instance, traceback = sys.exc_info()
        error_instance.args = (error_instance.args[0] + ' <modification>',)
        raise error_type, error_instance, traceback

And we have preserved the whole traceback while modifying the args. Note that this is not a best practice and it is invalid syntax in Python 3 (making keeping compatibility much harder to work around).

>>> catch_error_modify_message()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in catch_error_modify_message
  File "<stdin>", line 2, in error
ValueError: oops! <modification>

In Python 3:

raise error.with_traceback(sys.exc_info()[2])

Again: avoid manually manipulating tracebacks. It's less efficient and more error prone. And if you're using threading and sys.exc_info you may even get the wrong traceback (especially if you're using exception handling for control flow - which I'd personally tend to avoid.)

Python 3, Exception chaining

In Python 3, you can chain Exceptions, which preserve tracebacks:

raise RuntimeError('specific message') from error

Be aware:

this does allow changing the error type raised, and
this is not compatible with Python 2.

Deprecated Methods:

These can easily hide and even get into production code. You want to raise an exception, and doing them will raise an exception, but not the one intended!

Valid in Python 2, but not in Python 3 is the following:

raise ValueError, 'message' # Don't do this, it's deprecated!

Only valid in much older versions of Python (2.4 and lower), you may still see people raising strings:

raise 'message' # really really wrong. don't do this.

In all modern versions, this will actually raise a TypeError, because you're not raising a BaseException type. If you're not checking for the right exception and don't have a reviewer that's aware of the issue, it could get into production.

Example Usage

I raise Exceptions to warn consumers of my API if they're using it incorrectly:

def api_func(foo):
    '''foo should be either 'baz' or 'bar'. returns something very useful.'''
    if foo not in _ALLOWED_ARGS:
        raise ValueError('{foo} wrong, use "baz" or "bar"'.format(foo=repr(foo)))

Create your own error types when apropos

"I want to make an error on purpose, so that it would go into the except"

You can create your own error types, if you want to indicate something specific is wrong with your application, just subclass the appropriate point in the exception hierarchy:

class MyAppLookupError(LookupError):
    '''raise this when there's a lookup error for my app'''

and usage:

if important_key not in resource_dict and not ok_to_be_missing:
    raise MyAppLookupError('resource is missing, and that is not ok.')

How to raise exception in except block without original traceback in python 3.5+

Use raise from None to suppress earlier exceptions:

try:
    can_raise_custom_lib_exception()
except custom_lib_exception as e:
    cleanup()
    raise myOwnException("my own extra text") from None

7.8. The raise statement
[...]

Exception chaining can be explicitly suppressed by specifying None in the from clause:

Get only last traceback without using raise from None?

Use the __suppress_context__ attribute to disable context printing.

According to the docs, using raise MyCustomException(foo) from bar sets __cause__ to bar, and __context__ to the original exception (the implicitly chained exception).

An implicitly chained exception in __context__ is shown only if __cause__ is None and __suppress_context__ is false.

Here's an example:

# Declare an exception that never shows context.

class MyCustomException(Exception):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.__suppress_context__ = True

try:
    1/0
except ZeroDivisionError as e:
    raise MyCustomException(str(e))

Here's the output I get:

Traceback (most recent call last):
  File "/home/don/workspace/scratch/scratch.py", line 12, in <module>
    raise MyCustomException(str(e))
MyCustomException: division by zero

Here's the output if I set __suppress_context__ to False:

Traceback (most recent call last):
  File "/home/don/workspace/scratch/scratch.py", line 10, in <module>
    1/0
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/don/workspace/scratch/scratch.py", line 12, in <module>
    raise MyCustomException(str(e))
MyCustomException: division by zero

Use of except Exception vs. except ... raise in Python

No, your code is not equivalent, for several reasons:

A blank except: catches all exceptions, including those derived from BaseException (SystemExit, KeyboardInterrupt and GeneratorExit); catching Exception filters out those exceptions you generally want to avoid catching without a re-raise. In older Python releases, it would also catch string exceptions (no longer permitted).
The except Exception as e catches subclasses, but then raises a new Exception() instance; the specific type information can't be used anymore in downstream try...except statements.
In Python 3, raising a new exception from an exception handler creates an exception chain (where the original exception is added as the Exception.__context__ attribute, see Python "raise from" usage)
The message is updated; that's probably the whole point here, is to give the exception a different message.

The code you found is.. rather bad practice. The top-level exception handler should just catch and print a message and perhaps a traceback, rather than re-raise the exception with a new message (and in Python 2 lose all information on the original exception, in Python 3 make it inaccessible to exception matching in later handlers).

Pylint raise-missing-from

The link in the comment on your question above outlines the issue and provides a solution, but for clarity of those landing straight on this page like myself, without having to go off to another thread, read and gain context, here is the answer to your specific problem:

TL;DR;

This is simply solved by aliasing the Exception you are 'excepting' and refering to it in your second raise.

Taking your code snippet above, see the bottom two lines, I've added 'under-carets' to denote what I've added.

class SnippetDetail(APIView):
    """
    Retrieve, update or delete a snippet instance.
    """
    def get_object(self, pk):
        try:
            return Snippet.objects.get(pk=pk)
        except Snippet.DoesNotExist as snip_no_exist:
#                                   ^^^^^^^^^^^^^^^^
            raise Http404 from snip_no_exist
#                         ^^^^^^^^^^^^^^^^^^

Note: The alias can be any well formed string.

Python "Raise From" Usage