Use Try, Except, and Open

Now we’re expanding our user input program for selecting specific file.

fname = raw_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('Python') :
        count = count + 1
print 'There were', count, 'subject lines in', fname

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:

There were 3 subject lines in P-files.txt

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.