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8.4 Raising Exceptions

The raise statement allows the programmer to force a specified exception to occur. For example:

    >>> raise NameError, 'HiThere'
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
    NameError: HiThere

The first argument to raise names the exception to be raised. The optional second argument specifies the exception's argument. Alternatively, the above could be written as raise NameError('HiThere'). Either form works fine, but there seems to be a growing stylistic preference for the latter.

If you need to determine whether an exception was raised but don't intend to handle it, a simpler form of the raise statement allows you to re-raise the exception:

    >>> try:
    ...     raise NameError, 'HiThere'
    ... except NameError:
    ...     print 'An exception flew by!'
    ...     raise
    ...
    An exception flew by!
    Traceback (most recent call last):
      File "<stdin>", line 2, in ?
    NameError: HiThere