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Except block handles ‘BaseException’

ID: py/catch-base-exception
Kind: problem
Security severity: 
Severity: recommendation
Precision: very-high
Tags:
   - reliability
   - readability
   - convention
   - external/cwe/cwe-396
Query suites:
   - python-security-and-quality.qls

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All exception classes in Python derive from BaseException. BaseException has three important subclasses, Exception from which all errors and normal exceptions derive, KeyboardInterrupt which is raised when the user interrupts the program from the keyboard and SystemExit which is raised by the sys.exit() function to terminate the program.

Since KeyboardInterrupt and SystemExit are special they should not be grouped together with other Exception classes.

Catching BaseException, rather than its subclasses may prevent proper handling of KeyboardInterrupt or SystemExit. It is easy to catch BaseException accidentally as it is caught implicitly by an empty except: statement.

Recommendation

Handle Exception, KeyboardInterrupt and SystemExit separately. Do not use the plain except: form.

Example

In these examples, a function application.main() is called that might raise SystemExit. In the first two functions, BaseException is caught, but this will discard KeyboardInterrupt. In the third function, call_main_program_fixed only SystemExit is caught, leaving KeyboardInterrupt to propagate.

In these examples KeyboardInterrupt is accidentally ignored.


def call_main_program_implicit_handle_base_exception():
    try:
        #application.main calls sys.exit() when done.
        application.main()
    except Exception as ex:
        log(ex)
    except:
        pass

def call_main_program_explicit_handle_base_exception():
    try:
        #application.main calls sys.exit() when done.
        application.main()
    except Exception as ex:
        log(ex)
    except BaseException:
        pass

def call_main_program_fixed():
    try:
        #application.main calls sys.exit() when done.
        application.main()
    except Exception as ex:
        log(ex)
    except SystemExit:
        pass

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