Python - The try-finally Block

You can use a finally: block along with a try: block. The finally: block is a place to put any code that must execute, whether the try-block raised an exception or not.

The syntax of the try-finally statement is this −

try:
   You do your operations here;
   ......................
   Due to any exception, this may be skipped.
finally:
   This would always be executed.
   ......................

Note − You can provide except clause(s), or a finally clause, but not both. You cannot use else clause as well along with a finally clause.

Example

try:
   fh = open("testfile", "w")
   fh.write("This is my test file for exception handling!!")
finally:
   print ("Error: can\'t find file or read data")
   fh.close()

If you do not have permission to open the file in writing mode, then it will produce the following output −

Error: can't find file or read data

The same example can be written more cleanly as follows −

try:
   fh = open("testfile", "w")
   try:
      fh.write("This is my test file for exception handling!!")
   finally:
      print ("Going to close the file")
      fh.close()
except IOError:
   print ("Error: can\'t find file or read data")

When an exception is thrown in the try block, the execution immediately passes to the finally block. After all the statements in the finally block are executed, the exception is raised again and is handled in the except statements if present in the next higher layer of the try-except statement.

Exception with Arguments

An exception can have an argument, which is a value that gives additional information about the problem. The contents of the argument vary by exception. You capture an exception's argument by supplying a variable in the except clause as follows −

try:
   You do your operations here
   ......................
except ExceptionType as Argument:
   You can print value of Argument here...

If you write the code to handle a single exception, you can have a variable follow the name of the exception in the except statement. If you are trapping multiple exceptions, you can have a variable follow the tuple of the exception.

This variable receives the value of the exception mostly containing the cause of the exception. The variable can receive a single value or multiple values in the form of a tuple. This tuple usually contains the error string, the error number, and an error location.

Example

Following is an example for a single exception −

# Define a function here.
def temp_convert(var):
   try:
      return int(var)
   except ValueError as Argument:
      print("The argument does not contain numbers\n",Argument)
# Call above function here.
temp_convert("xyz")

It will produce the following output −

The argument does not contain numbers
invalid literal for int() with base 10: 'xyz'
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