CriticalFile-scopePython Plug & play

Only Catch Exceptions That Derive from `BaseException`

Detect exception handling blocks that catch objects not derived from `BaseException`. This causes a `TypeError` and should be avoided.

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Why this matters

Catching objects that don’t inherit from `BaseException` causes a `TypeError`. Only valid exception classes should be caught.

Bad vs. good

Side-by-side examples engineers can pattern-match during review.

Avoid

class CustomException(object):
    """An Invalid exception class."""
    pass

try:
    ...
except CustomException:  # Noncompliant: this custom exception does not derive from BaseException or Exception.
    print("exception")

try:
    ...
except [TypeError, ValueError]:  # Noncompliant: list of exceptions, only tuples are valid.
    print("exception")

Prefer

class CustomException(Exception):
    pass

try:
    ...
except CustomException:
    print("exception")

try:
    ...
except (TypeError, ValueError):
    print("exception")

More snippets

Bad

class CustomException(object):
    """An Invalid exception class."""
    pass

try:
    ...
except CustomException:  # Noncompliant: this custom exception does not derive from BaseException or Exception.
    print("exception")

try:
    ...
except [TypeError, ValueError]:  # Noncompliant: list of exceptions, only tuples are valid.
    print("exception")

Good

class CustomException(Exception):
    pass

try:
    ...
except CustomException:
    print("exception")

try:
    ...
except (TypeError, ValueError):
    print("exception")

Related rules

From the same buckets as this rule.

Critical
Avoid equality operators in loop termination conditions
Check if loops use equality operators (== or !=) in termination conditions. These can lead to infinite loops if the condition is never met exactly. Instead, use relational operators like < or > for safer loop termination.
error-handlingreadability-refactor
Critical
Avoid Returning Null in Non-Async Task Methods
Returning null from a Task or Task<T> method causes a NullReferenceException if awaited. Use Task.CompletedTask or Task.FromResult<T>(null) instead.
error-handlingreadability-refactor+1
Critical
Avoid Using Boolean Expressions in `except` Statements
Detect `except` clauses that use `or` or `and` to catch multiple exceptions. This does not work as intended. Recommend using a tuple of exceptions instead.
error-handlingstack-python

Bad vs. good

Side-by-side examples engineers can pattern-match during review.

Avoid

class CustomException(object):
    """An Invalid exception class."""
    pass

try:
    ...
except CustomException:  # Noncompliant: this custom exception does not derive from BaseException or Exception.
    print("exception")

try:
    ...
except [TypeError, ValueError]:  # Noncompliant: list of exceptions, only tuples are valid.
    print("exception")

Prefer

class CustomException(Exception):
    pass

try:
    ...
except CustomException:
    print("exception")

try:
    ...
except (TypeError, ValueError):
    print("exception")

More snippets

Bad

class CustomException(object):
    """An Invalid exception class."""
    pass

try:
    ...
except CustomException:  # Noncompliant: this custom exception does not derive from BaseException or Exception.
    print("exception")

try:
    ...
except [TypeError, ValueError]:  # Noncompliant: list of exceptions, only tuples are valid.
    print("exception")

Good

class CustomException(Exception):
    pass

try:
    ...
except CustomException:
    print("exception")

try:
    ...
except (TypeError, ValueError):
    print("exception")

Related rules

From the same buckets as this rule.

Critical

Avoid equality operators in loop termination conditions

Check if loops use equality operators (== or !=) in termination conditions. These can lead to infinite loops if the condition is never met exactly. Instead, use relational operators like < or > for safer loop termination.

error-handlingreadability-refactor

Critical

Avoid Returning Null in Non-Async Task Methods

Returning null from a Task or Task<T> method causes a NullReferenceException if awaited. Use Task.CompletedTask or Task.FromResult<T>(null) instead.

error-handlingreadability-refactor+1

Critical

Avoid Using Boolean Expressions in `except` Statements

Detect `except` clauses that use `or` or `and` to catch multiple exceptions. This does not work as intended. Recommend using a tuple of exceptions instead.

error-handlingstack-python