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to subclass Exceptions and stop using args, but the introduction of message and the original deprecation of args has been retracted.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.')

Okey, so there a few things that need to be explained where.

What is try-except used for?

It is used for catching errors raised by the program. Any code susceptible of raising an exception is inserted inside a try statement, and below that statement, any number of except statements with any single error that you want to catch.

try:
    user_input = int(input('Give me a number: '))
except ValueError:
    print('That is not a number!')

When should i use try-except?

It is not a good practice to use a try-except on every single line of code that could raise an error, because that may be half of it, or more. So when shall you use it? Simple, ask this question: Do I want to do any custom action with that error being raised? If the answer is yes, you are good to go.

Catching Exception or empty except

As I see in your example, you are using an empty except. Using an empty except statement will catch every single error raised that the surrounded code, which is similar (but not the same) as catching Exception. The Exception class is the superclass of every single built-in exception in the Python environment that are non-system-exiting (read here) and its generally a bad practice to catch either all exceptions with except: or Exception with except Exception:. Why? Because you are not letting the user (or even you, the programmer) know what error you are handling. For example:

fruits = ['apple', 'pear', 'banana']
try: 
    selection = fruits[int(input('Select a fruit number (0-2): '))]  
except Exception:
    print('Error!')
    # But wait, are you catching ValueError because the user did not input a number, 
    # or are you catching IndexError because he selected an out of bound array index? 
    # You don't know  

Catching multiple exceptions

Based on the previous example, you can use multiple try-except statements to difference which errors are being raised.

fruits = ['apple', 'pear', 'banana']
try: 
    selection = fruits[int(input('Select a fruit number (0-2): '))]  
except ValueError:
    print('That is not a number')
except IndexError:
    print('That fruit number does not exist!')  

Grouping exceptions

If there are two particular exceptions that you want to use for a same purpose, you can group them in a tuple:

fruits = ['apple', 'pear', 'banana']
try: 
    selection = fruits[int(input('Select a fruit number (0-2): '))]  
except (ValueError, IndexError):
    print('Invalid selection!')  

Your case

Based on this information, add those try-except blocks to your code, and see what possible errors that could be raised during its execution, asking the previously recommended question Do I want to execute some custom action with this error?

Additionally

• There are try-except-else statements. See here
• There are try-except-finally statements. See here
• You can combine them all in a try-except1-except2...exceptN-else-finally statement.
• I recommend you get familiar with built-in errors why practicing this!

The answer to your question is you want meaningful exceptions.

In most contexts, this involves an exception that adds actionable information in one of two ways:

• Useful typing to the developer who might be catching the exception
• Useful error information to the user (and/or to the developer who can translate to that the user)

In your example, you aren't really adding anything to the normal exception. Catching and then effectively reraising the exception isn't useful. You could log this information, in which case you now added some value to the exception, but otherwise it's pointless.

Imagine this scenario:

try:
   my_web_function()
except my_custom_web_exception:
   do_something_with_this_exception()

It's entirely reasonable to imagine this scenario. You might make an http request. Most libraries have defined exception types, so you could catch those exceptions, and if it happens, do something.

The "something" is dependent on the specific application. In a http exception, maybe it's just retry the request. Etc.

But the point is catching the exception adds value and information and is actionable.

You should never use except: because that catches ALL exceptions, including SystemExit, you should probably do:

try:
    if not check():
        raise MyNewException()
except ExceptionRaisedByCheck:
    raise MyNewException()

If the exception coming from check() is the same as the one you want to raise, you should change that.

Edit: If the exception is the same, you can also simply do this (Mark Byers had this in his answer, but it's gone now):

if not check():
    raise TheSameException()

This will propagate the exception or raise if False.