How do I assign an error message, if the user does something wrong? Python

Question

I have a directory where csv files are located. The code reads the files and creates histograms based on the data in the files.

However, I am trying to make it so that at the command line, I can enter one of the column headers in the csv file and the code will only make a graph of the specified command. Example: python histogram_maker.py "C:/Folder" Area.

I was able to do that, but I want to add a part that would create an error message in case the user enters a command that is not specified in the csv file. Example: Perimeter does not exist. How can I do that?

for column in df:
    os.chdir(directory)
    if len(sys.argv)>2:
        for x in arguments:
            if x.endswith(column):
                # code to make histogram

Need part that would say if x.endswith(column) not there error message should appear.

When you say "in case the user enters a command that is not specified in the csv file" do you mean "column" instead of "command"? — Brionius
– Brionius, Commented Aug 8, 2013 at 13:36

brianmearns · Accepted Answer · 2013-08-08 13:42:00Z

The standard way of issuing errors in python is with raise Exception("error message"). Or to use a subclass of Exception but in the same way. If you don't want to dump a stack track on the user, you can wrap the entire script in a try...except statement and use something like sys.stderr.write(str(e)) in the except block to just print the error message to STDERR. Then use sys.exit(1) to exit with a non-zero return code, or better use choose an appropriate return code from the errno module to pass to sys.exit.

So I would do something like this (if I understand your question):

import sys, errno

try:
    for column in df:
        os.chdir(directory)
        if len(sys.argv)>2:
            for x in arguments:
                if x.endswith(column):
                    # code to make histogram
                    pass
                else:
                    raise Exception("Perimeter does not exist")
except Exception, e:
    sys.stderr.write("Error: %s" % str(e))
    sys.exit(errno.EINVAL)

See the python documentation on errors and exceptions.

Tiago Martins · Accepted Answer · 2013-08-08 13:36:11Z

0

Are you looking for this?

raise Exception("Perimeter does not exist")

Documentation

answered Aug 8, 2013 at 13:36

Tiago Martins

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Comments

Hesam Qodsi · Accepted Answer · 2013-08-08 13:36:19Z

0

Use exception mechanism. Take a look at http://docs.python.org/2/reference/executionmodel.html#exceptions

answered Aug 8, 2013 at 13:36

Hesam Qodsi

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