Understanding Control flow in Exception Handling in Python

Question

a=0
try:
    print("In Try Block !")
    a = 10/0
except Exception as e:
    print("In Exception Block !")
    raise e
finally:
    print("In Finally Block !")
    print(a)
    pass

When I run the above code I get output as below. I don't get the flow of this script.

In Try Block !
Traceback (most recent call last):
In Exception Block !
In Finally Block !
0
  File "C:\Users\reccer\Documents\Practicals\test.py", line 7, in <module>
    raise e
  File "C:\Users\reccer\Documents\Practicals\test.py", line 4, in <module>
    a = 10/0
ZeroDivisionError: division by zero
[Finished in 0.3s with exit code 1]

Enters try block.
founds error
Enters except block.
prints "In Exception Block !"
Enters finally block.
prints "In Finally Block !" & prints a.
then performs formalities of raise e from except block.

I can't get any concrete reference for flow of control.

@mkrieger1 I was expecting that `except` block will execute once at all, then it will go in `finally` block. But as we can see it executes `print` statement and then goes to `final` block, then comes back to `except` block and `raise e` is executed. — Prashant Mahure, Oct 13 '20 at 06:12
It's explained here: https://stackoverflow.com/questions/34249269/finally-and-rethowing-of-exception-in-except-raise-in-python — mkrieger1, Oct 13 '20 at 07:12

score 1 · Accepted Answer · answered Oct 11 '20 at 13:55

The flow is correct. The finally block is executed before leaving the try\except\finally structure. Since you re-raised the exception, the exception is being passed out of the structure, so finally must run first.

a=0
try:
    print("In Try Block !")
    a = 10/0
except Exception as e:
    print("In Exception Block !")
    raise e   # send exception up the call stack, but must run finally first 
finally:
    print("In Finally Block !")
    print(a)
    pass   # not needed

The error trace you see is generated by the code that called your code - probably the core python engine.

Understanding Control flow in Exception Handling in Python

1 Answers1