python and sys.argv

Question

if len(sys.argv) < 2:
    sys.stderr.write('Usage: sys.argv[0] ')
    sys.exit(1)


if not os.path.exists(sys.argv[1]):
    sys.stderr.write('ERROR: Database sys.argv[1] was not found!')
    sys.exit(1)

This is a portion of code I'm working on. The first part I'm trying to say if the user doesn't type python programname something then it will exit.

The second part I'm trying to see if the database exists. On both places I'm unsure if I have the correct way to write out the sys.argv's by stderr or not.

This should be called "python and sys.stderr", tho it actually helped me on the `argv` :P — cregox, Apr 13 '10 at 21:49
In your first if-block doing print `'Usage:' + sys.argv[0]` does not tell the user that they need to type "python [program\_name] [something]". It only prints "Usage: [program\_name]". You might want to come up with a better error message. — ntownsend, Jun 11 '09 at 19:57

score 99 · Accepted Answer · answered Jun 11 '09 at 22:41

99

BTW you can pass the error message directly to sys.exit:

if len(sys.argv) < 2:
    sys.exit('Usage: %s database-name' % sys.argv[0])

if not os.path.exists(sys.argv[1]):
    sys.exit('ERROR: Database %s was not found!' % sys.argv[1])

answered Jun 11 '09 at 22:41

Marius Gedminas

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Greg Hewgill · Answer 2 · 2009-06-11T19:52:48.723

34

In Python, you can't just embed arbitrary Python expressions into literal strings and have it substitute the value of the string. You need to either:

sys.stderr.write("Usage: " + sys.argv[0])

or

sys.stderr.write("Usage: %s" % sys.argv[0])

Also, you may want to consider using the following syntax of print (for Python earlier than 3.x):

print >>sys.stderr, "Usage:", sys.argv[0]

Using print arguably makes the code easier to read. Python automatically adds a space between arguments to the print statement, so there will be one space after the colon in the above example.

In Python 3.x, you would use the print function:

print("Usage:", sys.argv[0], file=sys.stderr)

Finally, in Python 2.6 and later you can use .format:

print >>sys.stderr, "Usage: {0}".format(sys.argv[0])

edited Jun 11 '09 at 19:52

answered Jun 11 '09 at 19:47

Greg Hewgill

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Note that 'Usage: {0}'.format(sys.argv[0]) is now the recommended way of formatting strings. – Bastien Léonard Jun 11 '09 at 19:50
4

Also note that sys.stderr.write() will not add a trailing newline, so you should include one explicitly. – Marius Gedminas Jun 11 '09 at 22:39

score 32 · Answer 3 · answered Jun 11 '09 at 20:09

32

I would do it this way:

import sys

def main(argv):
    if len(argv) < 2:
        sys.stderr.write("Usage: %s <database>" % (argv[0],))
        return 1

    if not os.path.exists(argv[1]):
        sys.stderr.write("ERROR: Database %r was not found!" % (argv[1],))
        return 1

if __name__ == "__main__":
    sys.exit(main(sys.argv))

This allows main() to be imported into other modules if desired, and simplifies debugging because you can choose what argv should be.

answered Jun 11 '09 at 20:09

John Millikin

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This is a nice explanation - I was looking for more information on using sys.exit( main(sys.argv) ) and why it was encouraged. Thanks for the explanation. – Helen Neely Feb 28 '10 at 22:46

python and sys.argv

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