Skip to main content
Engineering LibreTexts

7.7: Using try, except, and open

[ "article:topic", "authorname:severancec", "python (language)", "license:ccbyncsa" ]
  • Page ID
    3117
  • I told you not to peek. This is your last chance.

    What if our user types something that is not a file name?

    python search6.py
    Enter the file name: missing.txt
    Traceback (most recent call last):
      File "search6.py", line 2, in <module>
        fhand = open(fname)
    FileNotFoundError: [Errno 2] No such file or directory: 'missing.txt'
    
    python search6.py
    Enter the file name: na na boo boo
    Traceback (most recent call last):
      File "search6.py", line 2, in <module>
        fhand = open(fname)
    FileNotFoundError: [Errno 2] No such file or directory: 'na na boo boo'

    Do not laugh. Users will eventually do every possible thing they can do to break your programs, either on purpose or with malicious intent. As a matter of fact, an important part of any software development team is a person or group called Quality Assurance (or QA for short) whose very job it is to do the craziest things possible in an attempt to break the software that the programmer has created.

     

    The QA team is responsible for finding the flaws in programs before we have delivered the program to the end users who may be purchasing the software or paying our salary to write the software. So the QA team is the programmer's best friend.

     

    So now that we see the flaw in the program, we can elegantly fix it using the try/except structure. We need to assume that the open call might fail and add recovery code when the open fails as follows:

    fname = input('Enter the file name: ')
    try:
        fhand = open(fname)
    except:
        print('File cannot be opened:', fname)
        exit()
    count = 0
    for line in fhand:
        if line.startswith('Subject:'):
            count = count + 1
    print('There were', count, 'subject lines in', fname)
    
    # Code: http://www.py4e.com/code3/search7.py

    The exit function terminates the program. It is a function that we call that never returns. Now when our user (or QA team) types in silliness or bad file names, we "catch" them and recover gracefully:

    python search7.py
    Enter the file name: mbox.txt
    There were 1797 subject lines in mbox.txt
    
    python search7.py
    Enter the file name: na na boo boo
    File cannot be opened: na na boo boo

     

    Protecting the open call is a good example of the proper use of try and except in a Python program. We use the term "Pythonic" when we are doing something the "Python way". We might say that the above example is the Pythonic way to open a file.

    Once you become more skilled in Python, you can engage in repartee with other Python programmers to decide which of two equivalent solutions to a problem is "more Pythonic". The goal to be "more Pythonic" captures the notion that programming is part engineering and part art. We are not always interested in just making something work, we also want our solution to be elegant and to be appreciated as elegant by our peers.