February 2, 2024

Introduction

Suppose you have the following code:

def foo():
    print("writing data to a temp file")
    # write data here
    # ...

    print("doing some other stuff that raises an exception")
    # ...
    raise ValueError

def remove_temp_file():
    print("removing temp file")
    # do removal here
    # ...

try:
    foo()
    remove_temp_file()
except ValueError:
    print("doing some logging")
    # do some logging here
    # ...
    remove_temp_file()
    raise

Note that regardless of succeeding or not, you want the temp file to be cleaned up (removed).

finally

We can remove some of this duplication by using finally

def foo():
    print("writing data to a temp file")
    # write data here
    # ...

    print("doing some other stuff that raises an exception")
    # ...
    raise ValueError

def remove_temp_file():
    print("removing temp file")
    # do removal here
    # ...

try:
    foo()
except ValueError:
    print("doing some logging")
    # do some logging here
    # ...
    raise
finally:
    remove_temp_file()

The finally block will run no matter what. However, it has some special properties. According to the documentation:

If finally is present, it specifies a ‘cleanup’ handler. The try clause is executed, including any except and else clauses. If an exception occurs in any of the clauses and is not handled, the exception is temporarily saved. The finally clause is executed. If there is a saved exception it is re-raised at the end of the finally clause. If the finally clause raises another exception, the saved exception is set as the context of the new exception. If the finally clause executes a return, break or continue statement, the saved exception is discarded.

Docs

The catch here is that the code block under finally, will run before the raise exception in the except block runs, but after the code inside the except block.

If we run the code above, we have the following print output:

writing data to a temp file
doing some other stuff that raises an exception
doing some logging
removing temp file

Note that if the finally block raised an exception, that exception would take precedence over ValueError being re-raised:

try:
    foo()
except ValueError:
    print("doing some logging")
    # do some logging here
    # ...
    raise
finally:
    remove_temp_file()
    raise TypeError("error in finally block")

> TypeError: error in finally block

Also note that using break or return will cause the ValueError exception to not be re-raised. This can come as a shock, but it is helpful if you are looping through a lot of try/catch/finally and you don't want any exception to affect your loop.

def bar():
    try:
        foo()
    except ValueError:
        print("doing some logging")
        # do some logging here
        # ...
        raise
    finally:
        remove_temp_file()
        return

# No exceptions raised
bar()