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This rule raises an issue when a bare except:, an except BaseException or an except SystemExit block does
not re-raise the exception caught.
Why is this an issue?
A SystemExit exception is raised when sys.exit() is called. This exception is used to signal the interpreter to
exit. The exception is expected to propagate up until the program stops. It is possible to catch this exception in order to perform, for example,
clean-up tasks. It should, however, be raised again to allow the interpreter to exit as expected. Not re-raising such exception could lead to
undesired behaviour.
A bare except: statement, i.e. an
except block without any exception class, is equivalent to except BaseException. Both statements will catch every
exceptions, including SystemExit. It is recommended to catch instead a more specific exception. If it is not possible, the exception
should be raised again.
It is also a good idea to re-raise the KeyboardInterrupt exception. Similarly to
SystemExit,KeyboardInterrupt is used to signal the interpreter to exit. Not re-raising such exception could also lead to
undesired behaviour.
How to fix it
Re-raise SystemExit, BaseException and any exceptions caught in a bare except clause.
Code examples
Noncompliant code example
try:
    ...
except SystemExit:  # Noncompliant: the SystemExit exception is not re-raised.
    pass

try:
    ...
except BaseException:  # Noncompliant: BaseExceptions encompass SystemExit exceptions and should be re-raised.
    pass

try:
    ...
except:  # Noncompliant: exceptions caught by this statement should be re-raised or a more specific exception should be caught.
    pass

Compliant solution
try:
    ...
except SystemExit as e:
    ...
    raise e

try:
    ...
except BaseException as e:
    ...
    raise e

try:
    ...
except FileNotFoundError:
    ... # Handle a more specific exception

Resources
Documentation

   PEP 352 - Required Superclass for Exceptions 
   Python Documentation - Built-in exceptions 
   Python Documentation - The try statement
  
   CWE - CWE-391, Unchecked Error Condition