Manually raising (throwing) an exception in Python

Don't do this. Raising a bare Exception is absolutely not the right thing to do; see Aaron Hall's excellent answer instead.

It can't get much more Pythonic than this:

raise Exception("I know Python!")

Replace Exception with the specific type of exception you want to throw.

See the raise statement documentation for Python if you'd like more information.

Just to note: there are times when you do want to handle generic exceptions. If you're processing a bunch of files and logging your errors, you might want to catch any error that occurs for a file, log it, and continue processing the rest of the files. In that case, a

try:
    foo()
except Exception as e:
    print(e) # Print out handled error

block is a good way to do it. You'll still want to raise specific exceptions so you know what they mean, though.

Another way to throw an exception is using assert. You can use assert to verify a condition is being fulfilled. If not, then it will raise AssertionError. For more details have a look here.

def avg(marks):
    assert len(marks) != 0, "List is empty."
    return sum(marks)/len(marks)

mark2 = [55,88,78,90,79]
print("Average of mark2:", avg(mark2))

mark1 = []
print("Average of mark1:", avg(mark1))

You should learn the raise statement of Python for that.

It should be kept inside the try block.

Example -

try:
    raise TypeError            # Replace TypeError by any other error if you want
except TypeError:
    print('TypeError raised')

You might also want to raise custom exceptions. For example, if you're writing a library, it's a very good practice to make a base exception class for your module, and then have custom sub-exceptions to be more specific.

You can achieve that like this:

class MyModuleBaseClass(Exception):
    pass

class MoreSpecificException(MyModuleBaseClass):
    pass


# To raise custom exceptions, you can just
# use the raise keyword
raise MoreSpecificException
raise MoreSpecificException('message')

If you're not interested in having a custom base class, you can just inherit your custom exception classes from an ordinary exception class like Exception, TypeError, ValueError, etc.

If you don't care about which error to raise, you could use assert to raise an AssertionError:

>>> assert False, "Manually raised error"
Traceback (most recent call last):
  File "<pyshell#24>", line 1, in <module>
    assert False, "Manually raised error"
AssertionError: Manually raised error
>>> 

The assert keyword raises an AssertionError if the condition is False. In this case, we specified False directly, so it raises the error, but to have it have a text we want it to raise to, we add a comma and specify the error text we want. In this case, I wrote Manually raised error and this raises it with that text.

If you don’t care about the raised exception, do:

def crash(): return 0/0

The good old division by 0.