Less Common Features

00:00 In the previous lesson, I covered how to control the number of parameters given to an argument. In this lesson, I’m going to cover a mix of smaller things you might find useful.

00:11 This lesson is going to cover three quick, unrelated topics. First, how to restrict what values are acceptable to the parser. Second, how to make flags mutually exclusive.

00:22 And third, how to read your arguments from a file. Sometimes you want your parameters to be from a given set. Enter the choices parameter. You can use it both for positional arguments and optional flags.

00:38 Here, I’ve set it for a positional argument that can only contain either "box" or "carton". You can also use it with optional flags. choices takes an iterable.

00:49 So instead of writing out 1 through 5, you can use the .range() method, like I have done here. I’ve also introduced another small thing, the required parameter.

00:59 Normally, optional flags are, well, optional. Adding the required flag makes them mandatory. Run, Forrest, run!

01:12 Nothing surprising here …

01:18 or here. If I miss the required flag, it complains …

01:30 and same if I use something besides "box" or "carton". Mmm, truckloads of chocolate. This next miscellaneous bit is mutually exclusive arguments.

01:47 Using the parser’s .add_mutually_exclusive_group() method, you get a group object, which you can then configure. In this case, I want to force the user to give exactly one of the two choices, so I set the group to be required.

02:00 You then add arguments to the group the same way you would add an argument to your parser. The exact same argument features are supported. My first argument is a Boolean named --light, and my second is a Boolean named --dark.

02:14 What droids? I don’t see any droids.

02:21 There’s the light side. There’s the dark side.

02:33 And there’s the angry side. Got to pick one. You’ve got to pick one.

02:43 And there you go, required mutually exclusive arguments. If you’ve got a lot of positional arguments that you use over and over again, argparse has a feature that can shorten the amount of typing you have to do.

02:59 You can specify positional arguments in a file. To do this, you use the fromfile_prefix_chars parameter. What this does is allow you to give an argument that starts with the @ character in this case and contains the name of a file.

03:15 argparse will then read that file and use each line of that file as a positional argument. On line 4, I’m also setting another parameter. This is the allow_abbrev value.

03:28 Setting this to False disallows the use of abbreviations in optional flags. I’ll explain that more in a second. To set up the example, the snowwhite script takes four positional arguments and then a Boolean flag.

03:46 The script is yet another Mad Libs example with the optional feature of turning it into shoutcase. Before I show you snowwhite, let me step back to starwars for a moment.

04:02 Remember this? It was long ago, galaxy far away. argparse supports the use of shortened optional flags. You don’t have to type the whole thing.

04:15 See? --li is good enough. In fact, as long as it doesn’t cause ambiguity, you can take it down to one letter. But what, you might say, if I don’t want this feature?

04:30 Well, then set allow_abbrev to False. Can I just say shortening abbreviation to abbrev just seems like the right thing to do?

04:39 Bravo, Python core team. Bravo. All right, here’s snowwhite.

05:00 This time I attempted to shorten --upper, but because of the allow_abbrev flag, it was disallowed. On to fromfile_prefix.

05:08 In the background, I’ve created a file called madlib_snow. Here it is: four lines for our four arguments. To use this, you specify the name of the file, preceeded by the fromfile_prefix_chars character, which in this case was the @ sign.

05:34 And there you go. All the positional arguments without all the typing. A couple more quick customizations. These are done through additional parameters to the ArgumentParser initializer when you create the object.

05:50 If for some reason you don’t want your program to support the help screen, you can turn it off by using add_help=False. It is completely unclear to me why anyone would want to do this, but you can.

06:04 This one makes a bit more sense to me. You can also modify what character is used for your optional flags. It’s normally the - (dash), but you can change it to something else.

06:13 Windows programs often use / (slash) as their options character, so if you’re writing code for Windows, this is a useful thing. Next up, a couple more advanced topics: callable types and custom actions.