9.9 finally Clause

The finally Clause of the try Statement

• try statement may have a finally clause after any except clauses or the else clause
• finally clause is guaranteed to execute
  • In other languages, this makes the finally suite ideal for resource-deallocation code
  • In Python, we prefer the with statement for this purpose

Example

try:
    print('try suite with no exceptions raised')
except:
    print('this will not execute')
else:
    print('else executes because no exceptions in the try suite')
finally:  
    print('finally always executes')
    

try suite with no exceptions raised
else executes because no exceptions in the try suite
finally always executes

try:
    print('try suite that raises an exception')
    int('hello')
    print('this will not execute')
except ValueError:
    print('a ValueError occurred')
else:
    print('else will not execute because an exception occurred')
finally:  
    print('finally always executes')
    

try suite that raises an exception
a ValueError occurred
finally always executes

Combining with Statements and try…except Statements

• Most resources that require explicit release, such as files, network connections and database connections, have potential exceptions associated with processing those resources
• Robust file-processing code normally appears in a try suite containing a with statement to guarantee that the resource gets released

open('gradez.txt')  # non-existent file

------------------------------------------------------------------------
FileNotFoundError                      Traceback (most recent call last)
<ipython-input-3-5b552683292b> in <module>
----> 1 open('gradez.txt')  # non-existent file

FileNotFoundError: [Errno 2] No such file or directory: 'gradez.txt'

try:
    with open('gradez.txt', 'r') as accounts:
        print(f'{"ID":<3}{"Name":<7}{"Grade"}')
        for record in accounts:  
            student_id, name, grade = record.split()
            print(f'{student_id:<3}{name:<7}{grade}')
except FileNotFoundError:
    print('The file name you specified does not exist')
    

The file name you specified does not exist

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