Why do we need the finally clause in Python

Question

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

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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

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Run these Python3 codes to watch the need of finally   CASE1   count   0 while True      count    1     if count  gt  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  gt  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    random integers correct code which is 586 Try this and you will get your answer  random strings      At a very early stage of learning Python

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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

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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    gt  gt  gt  def divide x  y           try              result   x   y         except ZeroDivisionError              print  division by zero            else              print  result is   result          finally              print  executing finally clause        gt  gt  gt  divide 2  1  result is 2 0 executing finally clause  gt  gt  gt  divide 2  0  division by zero  executing finally clause  gt  gt  gt  divide  2    1   executing finally clause Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt    File   lt stdin gt    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

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It makes a difference if you return early   try      run code1   except TypeError      run code2       return None     The finally block is run before the method returns finally      other code     Compare to this   try      run code1   except TypeError      run code2       return None     other code      This doesn t get run if there s an exception    Other situations that can cause differences    If an exception is thrown inside the except block  If an exception is thrown in run code1   but it s not a TypeError  Other control flow statements such as continue and break statements

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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

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

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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

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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

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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

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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

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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

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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

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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 lt type gt  object1 new type          destructors are automagically called here in LIFO order so no finally required

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

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