add solution for 4.2

_episodes/04-argparse.md

@@ -93,7 +93,7 @@ END
 > > but in a standalone script or a module
 > > it can be helpful, e.g. for including in logging messages created by
 > > functions that could be imported into other programs.
-> > 
+> >
 > > Whether the full path to the program is included or not depends on the
 > > operating system.
 > > If you need to rely on the full path being available,
@@ -170,6 +170,70 @@ TypeError: can only concatenate str (not "int") to str
 > Do you think this is a good way to construct an interface for a program like this?
 > What potential problems can you identify with this approach?
 > Pair up and discuss with a partner.
+>
+> > ## Solution
+> >
+> > ~~~
+> > from sys import argv, stdout
+> > from numpy import prod # use math.prod here if you're running Python 3.8
+> > from statistics import mean
+> >
+> > input_files = argv[-3:]
+> >
+> > modes = {'--sum': sum,
+> >          '--product': prod,
+> >          '--mean': mean}
+> > output_file = stdout
+> >
+> > for option in argv[1:-3]:
+> >     if option in modes:
+> >         output_function=modes[option]
+> >     elif option.startswith('--output='):
+> >         output_file = option.split('=')[1]
+> > ~~~
+> > {: .language-python}
+> >
+> > Changing the code to take an arbirary number of input files
+> > is possible,
+> > but you will have to assume that your user(s) will remember the required
+> > syntax for running your program:
+> > ~~~
+> > from sys import argv, stdout, exit
+> > from numpy import prod # use math.prod here if you're running Python 3.8
+> > from statistics import mean
+> >
+> > input_files = []
+> >
+> > modes = {'--sum': sum,
+> >          '--product': prod,
+> >          '--mean': mean}
+> > output_file = stdout
+> >
+> > for argument in argv[1:]:
+> >     if argument in modes:
+> >         output_function=modes[argument]
+> >     elif argument.startswith('--output='):
+> >         output_file = argument.split('=')[1]
+> >     elif not argument.startswith('--'):
+> >         input_files.append(argument)
+> >     else:
+> >         print('unexpected option,', argument, 'given! exiting.')
+> >         exit(1)
+> > ~~~
+> > {: .language-python}
+> >
+> > The example solution above makes the assumption that anything
+> > passed on the command line that doesn't begin with `--` must be
+> > the name of an input file.
+> > This is certainly not the most robust interface ever designed!
+> >
+> > Note also that, to encourage good practices,
+> > we use the `sys.exit` function,
+> > which you may not have been introduced to before.
+> > This function call will cause the script to quit with error code 1
+> > (after printing a semi-helpful error message)
+> > when an invalid option is provided.
+> {: .solution }
 {: .challenge}
 
 `sys.argv` is a great choice when the interface of our program is