Description
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
Metadata
Metadata
Assignees
Projects
Status