[python] Why do we need the "finally" clause in Python?

I am not sure why we need finally in try...except...finally statements. In my opinion, this code block

try:
    run_code1()
except TypeError:
    run_code2()
other_code()

is the same with this one using finally:

try:
    run_code1()
except TypeError:
    run_code2()
finally:
    other_code()

Am I missing something?

As explained in the documentation, the finally clause is intended to define clean-up actions that must be executed under all circumstances.

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.

An example:

>>> def divide(x, y):
...     try:
...         result = x / y
...     except ZeroDivisionError:
...         print("division by zero!")
...     else:
...         print("result is", result)
...     finally:
...         print("executing finally clause")
...
>>> divide(2, 1)
result is 2.0
executing finally clause
>>> divide(2, 0)
division by zero!
executing finally clause
>>> divide("2", "1")
executing finally clause
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in divide
TypeError: unsupported operand type(s) for /: 'str' and 'str'

As you can see, the finally clause is executed in any event. The TypeError raised by dividing two strings is not handled by the except clause and therefore re-raised after the finally clause has been executed.

In real world applications, the finally clause is useful for releasing external resources (such as files or network connections), regardless of whether the use of the resource was successful.

The code blocks are not equivalent. The finally clause will also be run if run_code1() throws an exception other than TypeError, or if run_code2() throws an exception, while other_code() in the first version wouldn't be run in these cases.

To add to the other answers above, the finally clause executes no matter what whereas the else clause executes only if an exception was not raised.

For example, writing to a file with no exceptions will output the following:

file = open('test.txt', 'w')

try:
    file.write("Testing.")
    print("Writing to file.")
except IOError:
    print("Could not write to file.")
else:
    print("Write successful.")
finally:
    file.close()
    print("File closed.")

OUTPUT:

Writing to file.
Write successful.
File closed.

If there is an exception, the code will output the following, (note that a deliberate error is caused by keeping the file read-only.

file = open('test.txt', 'r')

try:
    file.write("Testing.")
    print("Writing to file.")
except IOError:
    print("Could not write to file.")
else:
    print("Write successful.")
finally:
    file.close()
    print("File closed.")

OUTPUT:

Could not write to file.
File closed.

We can see that the finally clause executes regardless of an exception. Hope this helps.

Run these Python3 codes to watch the need of finally:

CASE1:

count = 0
while True:
    count += 1
    if count > 3:
        break
    else:
        try:
            x = int(input("Enter your lock number here: "))

            if x == 586:
                print("Your lock has unlocked :)")

                break
            else:
                print("Try again!!")

                continue

        except:
            print("Invalid entry!!")
        finally:
            print("Your Attempts: {}".format(count))

CASE2:

count = 0

while True:
    count += 1
    if count > 3:
        break
    else:
        try:
            x = int(input("Enter your lock number here: "))

            if x == 586:
                print("Your lock has unlocked :)")

                break
            else:
                print("Try again!!")

                continue

        except:
            print("Invalid entry!!")

        print("Your Attempts: {}".format(count))

Try the following inputs each time:

1. random integers
2. correct code which is 586(Try this and you will get your answer)
3. random strings

** At a very early stage of learning Python.

Using delphi professionally for some years taught me to safeguard my cleanup routines using finally. Delphi pretty much enforces the use of finally to clean up any resources created before the try block, lest you cause a memory leak. This is also how Java, Python and Ruby works.

resource = create_resource
try:
  use resource
finally:
  resource.cleanup

and resource will be cleaned up regardless of what you do between try and finally. Also, it won't be cleaned up if execution never reaches the try block. (i.e. create_resource itself throws an exception) It makes your code "exception safe".

As to why you actually need a finally block, not all languages do. In C++ where you have automatically called destructors which enforce cleanup when an exception unrolls the stack. I think this is a step up in the direction of cleaner code compared to try...finally languages.

{    
  type object1;
  smart_pointer<type> object1(new type());
} // destructors are automagically called here in LIFO order so no finally required.

You can use finally to make sure files or resources are closed or released regardless of whether an exception occurs, even if you don't catch the exception. (Or if you don't catch that specific exception.)

myfile = open("test.txt", "w")

try:
    myfile.write("the Answer is: ")
    myfile.write(42)   # raises TypeError, which will be propagated to caller
finally:
    myfile.close()     # will be executed before TypeError is propagated

In this example you'd be better off using the with statement, but this kind of structure can be used for other kinds of resources.

A few years later, I wrote a blog post about an abuse of finally that readers may find amusing.

finally is for defining "clean up actions". The finally clause is executed in any event before leaving the try statement, whether an exception (even if you do not handle it) has occurred or not.

I second @Byers's example.

I was trying to run a code where i wanted to read excel sheets. Issue was, if there is a file which has no sheet named say : SheetSum I am not able to move it to error location!! Code i wrote was:

def read_file(data_file):
    # data_file = '\rr\ex.xlsx'
    sheets = {}
    try:
        print("Reading file: "+data_file)
        sheets['df_1'] = pd.read_excel(open(data_file,'rb'), 'SheetSum')
    except Exception as excpt:
        print("Exception occurred", exc_info=True)
    return sheets

read_file(file)
shutil.move( file, dirpath +'\\processed_files')

Giving Error :

[WinError 32] The process cannot access the file because it is being used by another process

I had to add full try except with finally block and tell finally i need to close the file in any case like:

def read_file(data_file):
    # data_file = '\rr\ex.xlsx'
    sheets = {}
    try:
        print("Reading file: "+data_file)
        sheets_file = open(data_file,'rb')
        sheets['df_1'] = pd.read_excel(sheets_file, 'SheetSum')
    except Exception as excpt:
        print("Exception occurred", exc_info=True)
    finally:
        sheets_file.close()
    return sheets

read_file(file)
shutil.move( file, dirpath +'\\processed_files')

Otherwise, file still remains open is the background.

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.

..More Here

Perfect example is as below:

try:
    #x = Hello + 20
    x = 10 + 20 
except:
    print 'I am in except block'
    x = 20 + 30
else:
    print 'I am in else block'
    x += 1
finally:
    print 'Finally x = %s' %(x)

A try block has just one mandatory clause: The try statement. The except, else and finally clauses are optional and based on user preference.

finally: Before Python leaves the try statement, it will run the code in the finally block under any conditions, even if it's ending the program. E.g., if Python ran into an error while running code in the except or else block, the finally block will still be executed before stopping the program.

Finally can also be used when you want to run "optional" code before running the code for your main work and that optional code may fail for various reasons.

In the following example, we don't know precisely what kind of exceptions store_some_debug_info might throw.

We could run:

try:
  store_some_debug_info()
except Exception:
  pass
do_something_really_important() 

But, most linters will complain about catching too vague of an exception. Also, since we're choosing to just pass for errors, the except block doesn't really add value.

try:
  store_some_debug_info()
finally:
  do_something_really_important()     

The above code has the same effect as the 1st block of code but is more concise.

In your first example, what happens if run_code1() raises an exception that is not TypeError? ... other_code() will not be executed.

Compare that with the finally: version: other_code() is guaranteed to be executed regardless of any exception being raised.

They are not equivalent. Finally code is run no matter what else happens. It is useful for cleanup code that has to run.