Exceptions handlers (except:) are evaluated in the order they are written. Once a match is found, the evaluation stops.
In some contexts an except block is dead code as it will never catch any exception:
- If there is a handler for a base class followed by a handler for class derived from that base class, the second handler will never trigger: The
handler for the base class will match the derived class, and will be the only executed handler.
- When multiple
except statements try to catch the same exception class, only the first one will be executed.
- In python 3,
BaseException is the parent of every exception class. When BaseException is caught and the same
try-except block has a bare except: statement, i.e. an except with no expression, the bare except will never catch
anything.
This rule raises an issue when an except block catches every exception before a later except block could catch it.
Noncompliant Code Example
def foo():
try:
raise FloatingPointError()
except (ArithmeticError, RuntimeError) as e:
print(e)
except FloatingPointError as e: # Noncompliant. FloatingPointError is a subclass of ArithmeticError
print("Never executed")
except OverflowError as e: # Noncompliant. OverflowError is a subclass of ArithmeticError
print("Never executed")
try:
raise TypeError()
except TypeError as e:
print(e)
except TypeError as e: # Noncompliant. Duplicate Except.
print("Never executed")
try:
raise ValueError()
except BaseException as e:
print(e)
except: # Noncompliant. This is equivalent to "except BaseException" block
print("Never executed")
Compliant Solution
def foo():
try:
raise FloatingPointError()
except FloatingPointError as e:
print("Executed")
except OverflowError as e:
print("Executed")
except (ArithmeticError, RuntimeError) as e:
print(e)
try:
raise TypeError()
except TypeError as e:
print(e)
try:
raise ValueError()
except BaseException as e:
print(e)
See
