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.

For the common case where you need to throw an exception in response to some unexpected conditions, and that you never intend to catch, but simply to fail fast to enable you to debug from there if it ever happens — the most logical one seems to be AssertionError:

if 0 < distance <= RADIUS:
    #Do something.
elif RADIUS < distance:
    #Do something.
else:
    raise AssertionError("Unexpected value of 'distance'!", distance)

In Python 3 there are four different syntaxes for raising exceptions:

1. raise exception
2. raise exception (args)
3. raise
4. raise exception (args) from original_exception

1. Raise exception vs. 2. raise exception (args)

If you use raise exception (args) to raise an exception then the args will be printed when you print the exception object – as shown in the example below.

# Raise exception (args)
try:
    raise ValueError("I have raised an Exception")
except ValueError as exp:
    print("Error", exp)     # Output -> Error I have raised an Exception


# Raise exception
try:
    raise ValueError
except ValueError as exp:
    print("Error", exp)     # Output -> Error

3. Statement raise

The raise statement without any arguments re-raises the last exception.

This is useful if you need to perform some actions after catching the exception and then want to re-raise it. But if there wasn’t any exception before, the raise statement raises a TypeError Exception.

def somefunction():
    print("some cleaning")

a = 10
b = 0
result = None

try:
    result = a / b
    print(result)

except Exception:            # Output ->
    somefunction()           # Some cleaning
    raise                    # Traceback (most recent call last):
                             # File "python", line 9, in <module>
                             # ZeroDivisionError: division by zero

4. Raise exception (args) from original_exception

This statement is used to create exception chaining in which an exception that is raised in response to another exception can contain the details of the original exception – as shown in the example below.

class MyCustomException(Exception):
    pass

a = 10
b = 0
reuslt = None
try:
    try:
        result = a / b

    except ZeroDivisionError as exp:
        print("ZeroDivisionError -- ",exp)
        raise MyCustomException("Zero Division ") from exp

except MyCustomException as exp:
    print("MyException",exp)
    print(exp.__cause__)

Output:

ZeroDivisionError --  division by zero
MyException Zero Division
division by zero

Read the existing answers first, this is just an addendum.

Notice that you can raise exceptions with or without arguments.

Example:

raise SystemExit

exits the program, but you might want to know what happened. So you can use this.

raise SystemExit("program exited")

This will print "program exited" to standard error before closing the program.

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

It does not allow you to throw a specific message to your user but will crash the python interpriter.

For the more advanced python users who have used python may think that the expression will be evaluated at compile time (python is compiled) but the python compiler wont evaluate the expression at run time.
If we look at what tthe dis dissasembely module for python shows, we can see the bytecode.

              2 LOAD_CONST               1 (0)
              4 LOAD_CONST               1 (0)
              6 BINARY_OP               11 (/)

So essentialy python will push the constant 0 onto the stack and another 0 then run a binary operator, to divide.

Although this function is quite useless, and should never be used in production code, it can still crash the python interpriter.