chapter 8.3 in python errors-tutorial: update exception order code example for clarification

Documentation

The errors tutorial, chapter 8.3 contains an example that could be improved, as the highlighted subjects (exception ordering and matching) are not that easy to follow: https://docs.python.org/3/tutorial/errors.html

The issue was discussed here:
https://discuss.python.org/t/exception-tutorial-confusion/97456

Original:
A class in an except clause matches exceptions which are instances of the class itself or one of its derived classes (but not the other way around — an except clause listing a derived class does not match instances of its base classes). For example, the following code will print B, C, D in that order:

class B(Exception):
    pass

class C(B):
    pass

class D(C):
    pass

for cls in [B, C, D]:
    try:
        raise cls()
    except D:
        print("D")
    except C:
        print("C")
    except B:
        print("B")

Note that if the except clauses were reversed (with except B first), it would have printed B, B, B — the first matching except clause is triggered.

Suggested Update
Code examples created by Guido van Rossum, see https://discuss.python.org/t/exception-tutorial-confusion/97456/5.
A class in an except clause matches exceptions which are instances of the class itself or one of its derived classes (but not the other way around — an except clause listing a derived class does not match instances of its base classes).

For example, the following code will print B, C in that order:

class B(Exception):
    pass

class C(B):
    pass

for cls in [B, C]:
    try:
        raise cls()
    except C:
        # Matches C but not B.
        print("C")
    except B:
        # Matches B; not reached for C.
        print("B")

Note that if the except clauses were reversed (with except B first), it would have printed B, B — the first matching except clause is triggered, like in the following example:

for cls in [B, C]:
    try:
        raise cls()
    except B:
        # Matches B and C both.
        print("B")
    except C:
        # Not reached (the previous clause ate B and C)
        print("C")

Linked PRs

• gh-136257

閱讀連結文件頁面上的錯誤教學第 8.3 章，並查看關於擬議澄清的討論。比較目前的例外順序範例與建議的範例；當本章能夠清楚地示範匹配和首次匹配順序時，工作就完成了。