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+[Contents](../Contents.md) \| [Previous (4.3 Special methods)](03_Special_methods.md) \| [Next (5 Object Model)](../05_Object_model/00_Overview.md)
+
+# 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
+>>>
+```
+
+[Contents](../Contents.md) \| [Previous (4.3 Special methods)](03_Special_methods.md) \| [Next (5 Object Model)](../05_Object_model/00_Overview.md)