bpo-37826: Document exception chaining in Python tutorial for errors. (GH-15243)
https://bugs.python.org/issue37826
diff --git a/Doc/tutorial/errors.rst b/Doc/tutorial/errors.rst
index 4e287bb..e9a63e4 100644
--- a/Doc/tutorial/errors.rst
+++ b/Doc/tutorial/errors.rst
@@ -267,6 +267,53 @@
NameError: HiThere
+.. _tut-exception-chaining:
+
+Exception Chaining
+==================
+
+The :keyword:`raise` statement allows an optional :keyword:`from` which enables
+chaining exceptions by setting the ``__cause__`` attribute of the raised
+exception. For example::
+
+ raise RuntimeError from OSError
+
+This can be useful when you are transforming exceptions. For example::
+
+ >>> def func():
+ ... raise IOError
+ ...
+ >>> try:
+ ... func()
+ ... except IOError as exc:
+ ... raise RuntimeError('Failed to open database') from exc
+ ...
+ Traceback (most recent call last):
+ File "<stdin>", line 2, in <module>
+ File "<stdin>", line 2, in func
+ OSError
+ <BLANKLINE>
+ The above exception was the direct cause of the following exception:
+ <BLANKLINE>
+ Traceback (most recent call last):
+ File "<stdin>", line 4, in <module>
+ RuntimeError
+
+The expression following the :keyword:`from` must be either an exception or
+``None``. Exception chaining happens automatically when an exception is raised
+inside an exception handler or :keyword:`finally` section. Exception chaining
+can be disabled by using ``from None`` idiom:
+
+ >>> try:
+ ... open('database.sqlite')
+ ... except IOError:
+ ... raise RuntimeError from None
+ ...
+ Traceback (most recent call last):
+ File "<stdin>", line 4, in <module>
+ RuntimeError
+
+
.. _tut-userexceptions:
User-defined Exceptions