# 4.4 Defining Exceptions

User defined exceptions are defined by classes.

```python
class NetworkError(Exception):
    pass
```

**Exceptions always inherit from `Exception`.**
Usually they are empty classes. Use `pass` for the body.

You can also make a hierarchy of your exceptions.

```python
class AuthenticationError(NetworkError):
     pass

class ProtocolError(NetworkError):
    pass
```

## Exercises

### Exercise 4.11: Defining a custom exception

It is often good practice for libraries to define their own exceptions.

This makes it easier to distinguish between Python exceptions raised
in response to common programming errors versus exceptions
intentionally raised by a library to a signal a specific usage
problem.

Modify the `create_formatter()` function from the last exercise so
that it raises a custom `FormatError` exception when the user provides
a bad format name.

For example:

```python
>>> from tableformat import create_formatter
>>> formatter = create_formatter('xls')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "tableformat.py", line 71, in create_formatter
    raise FormatError('Unknown table format %s' % name)
FormatError: Unknown table format xls
>>>
```

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