Parsing command arguments, theory and best practices

This tutorial will elaborate on the many ways one can parse command arguments. The first step after adding a command usually is to parse its arguments. There are lots of ways to do it, but some are indeed better than others and this tutorial will try to present them.

If you’re a Python beginner, this tutorial might help you a lot. If you’re already familiar with Python syntax, this tutorial might still contain useful information. There are still a lot of things I find in the standard library that come as a surprise, though they were there all along. This might be true for others.

In this tutorial we will:

  • Parse arguments with numbers.

  • Parse arguments with delimiters.

  • Take a look at optional arguments.

  • Parse argument containing object names.

What are command arguments?

I’m going to talk about command arguments and parsing a lot in this tutorial. So let’s be sure we talk about the same thing before going any further:

A command is an Evennia object that handles specific user input.

For instance, the default look is a command. After having created your Evennia game, and connected to it, you should be able to type look to see what’s around. In this context, look is a command.

Command arguments are additional text passed after the command.

Following the same example, you can type look self to look at yourself. In this context, self is the text specified after look. " self" is the argument to the look command.

Part of our task as a game developer is to connect user inputs (mostly commands) with actions in the game. And most of the time, entering commands is not enough, we have to rely on arguments for specifying actions with more accuracy.

Take the say command. If you couldn’t specify what to say as a command argument (say hello!), you would have trouble communicating with others in the game. One would need to create a different command for every kind of word or sentence, which is, of course, not practical.

Last thing: what is parsing?

In our case, parsing is the process by which we convert command arguments into something we can work with.

We don’t usually use the command argument as is (which is just text, of type str in Python). We need to extract useful information. We might want to ask the user for a number, or the name of another character present in the same room. We’re going to see how to do all that now.

Working with strings

In object terms, when you write a command in Evennia (when you write the Python class), the arguments are stored in the args attribute. Which is to say, inside your func method, you can access the command arguments in self.args.

self.args

To begin with, look at this example:

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class CmdTest(Command):

    """
    Test command.

    Syntax:
      test [argument]

    Enter any argument after test.

    """

    key = "test"

    def func(self):
        self.msg(f"You have entered: {self.args}.")

If you add this command and test it, you will receive exactly what you have entered without any parsing:

> test Whatever
You have entered:  Whatever.
> test
You have entered: .

The lines starting with > indicate what you enter into your client. The other lines are what you receive from the game server.

Notice two things here:

  1. The left space between our command key (“test”, here) and our command argument is not removed. That’s why there are two spaces in our output at line 2. Try entering something like “testok”.

  2. Even if you don’t enter command arguments, the command will still be called with an empty string in self.args.

Perhaps a slight modification to our code would be appropriate to see what’s happening. We will force Python to display the command arguments as a debug string using a little shortcut.

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class CmdTest(Command):

    """
    Test command.

    Syntax:
      test [argument]

    Enter any argument after test.

    """

    key = "test"

    def func(self):
        self.msg(f"You have entered: {self.args!r}.")

The only line we have changed is the last one, and we have added !r between our braces to tell Python to print the debug version of the argument (the repr-ed version). Let’s see the result:

> test Whatever
You have entered: ' Whatever'.
> test
You have entered: ''.
> test And something with '?
You have entered: " And something with '?".

This displays the string in a way you could see in the Python interpreter. It might be easier to read… to debug, anyway.

I insist so much on that point because it’s crucial: the command argument is just a string (of type str) and we will use this to parse it. What you will see is mostly not Evennia-specific, it’s Python-specific and could be used in any other project where you have the same need.

Stripping

As you’ve seen, our command arguments are stored with the space. And the space between the command and the arguments is often of no importance.

Why is it ever there?

Evennia will try its best to find a matching command. If the user enters your command key with arguments (but omits the space), Evennia will still be able to find and call the command. You might have seen what happened if the user entered testok. In this case, testok could very well be a command (Evennia checks for that) but seeing none, and because there’s a test command, Evennia calls it with the arguments "ok".

But most of the time, we don’t really care about this left space, so you will often see code to remove it. There are different ways to do it in Python, but a command use case is the strip method on str and its cousins, lstrip and rstrip.

  • strip: removes one or more characters (either spaces or other characters) from both ends of the string.

  • lstrip: same thing but only removes from the left end (left strip) of the string.

  • rstrip: same thing but only removes from the right end (right strip) of the string.

Some Python examples might help:

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>>> '   this is '.strip() # remove spaces by default
'this is'
>>> "   What if I'm right?   ".lstrip() # strip spaces from the left
"What if I'm right?   "
>>> 'Looks good to me...'.strip('.') # removes '.'
'Looks good to me'
>>> '"Now, what is it?"'.strip('"?') # removes '"' and '?' from both ends
'Now, what is it'

Usually, since we don’t need the space separator, but still want our command to work if there’s no separator, we call lstrip on the command arguments:

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class CmdTest(Command):

    """
    Test command.

    Syntax:
      test [argument]

    Enter any argument after test.

    """

    key = "test"

    def parse(self):
        """Parse arguments, just strip them."""
        self.args = self.args.lstrip()

    def func(self):
        self.msg(f"You have entered: {self.args!r}.")

We are now beginning to override the command’s parse method, which is typically useful just for argument parsing. This method is executed before func and so self.args in func() will contain our self.args.lstrip().

Let’s try it:

> test Whatever
You have entered: 'Whatever'.
> test
You have entered: ''.
> test And something with '?
You have entered: "And something with '?".
> test     And something with lots of spaces
You have entered: 'And something with lots of spaces'.

Spaces at the end of the string are kept, but all spaces at the beginning are removed:

strip, lstrip and rstrip without arguments will strip spaces, line breaks and other common separators. You can specify one or more characters as a parameter. If you specify more than one character, all of them will be stripped from your original string.

Convert arguments to numbers

As pointed out, self.args is a string (of type str). What if we want the user to enter a number?

Let’s take a very simple example: creating a command, roll, that allows to roll a six-sided die. The player has to guess the number, specifying the number as argument. To win, the player has to match the number with the die. Let’s see an example:

> roll 3
You roll a die.  It lands on the number 4.
You played 3, you have lost.
> dice 1
You roll a die.  It lands on the number 2.
You played 1, you have lost.
> dice 1
You roll a die.  It lands on the number 1.
You played 1, you have won!

If that’s your first command, it’s a good opportunity to try to write it. A command with a simple and finite role always is a good starting choice. Here’s how we could (first) write it… but it won’t work as is, I warn you:

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from random import randint

from evennia import Command

class CmdRoll(Command):

    """
    Play random, enter a number and try your luck.

    Usage:
      roll <number>

    Enter a valid number as argument.  A random die will be rolled and you
    will win if you have specified the correct number.

    Example:
      roll 3

    """

    key = "roll"

    def parse(self):
        """Convert the argument to a number."""
        self.args = self.args.lstrip()

    def func(self):
        # Roll a random die
        figure = randint(1, 6) # return a pseudo-random number between 1 and 6, including both
        self.msg(f"You roll a die.  It lands on the number {figure}.")

        if self.args == figure: # THAT WILL BREAK!
            self.msg(f"You played {self.args}, you have won!")
        else:
            self.msg(f"You played {self.args}, you have lost.")

If you try this code, Python will complain that you try to compare a number with a string: figure is a number and self.args is a string and can’t be compared as-is in Python. Python doesn’t do “implicit converting” as some languages do. By the way, this might be annoying sometimes, and other times you will be glad it tries to encourage you to be explicit rather than implicit about what to do. This is an ongoing debate between programmers. Let’s move on!

So we need to convert the command argument from a str into an int. There are a few ways to do it. But the proper way is to try to convert and deal with the ValueError Python exception.

Converting a str into an int in Python is extremely simple: just use the int function, give it the string and it returns an integer, if it could. If it can’t, it will raise ValueError. So we’ll need to catch that. However, we also have to indicate to Evennia that, should the number be invalid, no further parsing should be done. Here’s a new attempt at our command with this converting:

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from random import randint

from evennia import Command, InterruptCommand

class CmdRoll(Command):

    """
    Play random, enter a number and try your luck.

    Usage:
      roll <number>

    Enter a valid number as argument.  A random die will be rolled and you
    will win if you have specified the correct number.

    Example:
      roll 3

    """

    key = "roll"

    def parse(self):
        """Convert the argument to number if possible."""
        args = self.args.lstrip()

        # Convert to int if possible
        # If not, raise InterruptCommand.  Evennia will catch this
        # exception and not call the 'func' method.
        try:
            self.entered = int(args)
        except ValueError:
            self.msg(f"{args} is not a valid number.")
            raise InterruptCommand

    def func(self):
        # Roll a random die
        figure = randint(1, 6) # return a pseudo-random number between 1 and 6, including both
        self.msg(f"You roll a die.  It lands on the number {figure}.")

        if self.entered == figure:
            self.msg(f"You played {self.entered}, you have won!")
        else:
            self.msg(f"You played {self.entered}, you have lost.")

Before enjoying the result, let’s examine the parse method a little more: what it does is try to convert the entered argument from a str to an int. This might fail (if a user enters roll something). In such a case, Python raises a ValueError exception. We catch it in our try/except block, send a message to the user and raise the InterruptCommand exception in response to tell Evennia to not run func(), since we have no valid number to give it.

In the func method, instead of using self.args, we use self.entered which we have defined in our parse method. You can expect that, if func() is run, then self.entered contains a valid number.

If you try this command, it will work as expected this time: the number is converted as it should and compared to the die roll. You might spend some minutes playing this game. Time out!

Something else we could want to address: in our small example, we only want the user to enter a positive number between 1 and 6. And the user can enter roll 0 or roll -8 or roll 208 for that matter, the game still works. It might be worth addressing. Again, you could write a condition to do that, but since we’re catching an exception, we might end up with something cleaner by grouping:

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from random import randint

from evennia import Command, InterruptCommand

class CmdRoll(Command):

    """
    Play random, enter a number and try your luck.

    Usage:
      roll <number>

    Enter a valid number as argument.  A random die will be rolled and you
    will win if you have specified the correct number.

    Example:
      roll 3

    """

    key = "roll"

    def parse(self):
        """Convert the argument to number if possible."""
        args = self.args.lstrip()

        # Convert to int if possible
        try:
            self.entered = int(args)
            if not 1 <= self.entered <= 6:
                # self.entered is not between 1 and 6 (including both)
                raise ValueError
        except ValueError:
            self.msg(f"{args} is not a valid number.")
            raise InterruptCommand

    def func(self):
        # Roll a random die
        figure = randint(1, 6) # return a pseudo-random number between 1 and 6, including both
        self.msg(f"You roll a die.  It lands on the number {figure}.")

        if self.entered == figure:
            self.msg(f"You played {self.entered}, you have won!")
        else:
            self.msg(f"You played {self.entered}, you have lost.")

Using grouped exceptions like that makes our code easier to read, but if you feel more comfortable checking, afterward, that the number the user entered is in the right range, you can do so in a latter condition.

Notice that we have updated our parse method only in this last attempt, not our func() method which remains the same. This is one goal of separating argument parsing from command processing, these two actions are best kept isolated.

Working with several arguments

Often a command expects several arguments. So far, in our example with the “roll” command, we only expect one argument: a number and just a number. What if we want the user to specify several numbers? First the number of dice to roll, then the guess?

You won’t win often if you roll 5 dice but that’s for the example.

So we would like to interpret a command like this:

> roll 3 12

(To be understood: roll 3 dice, my guess is the total number will be 12.)

What we need is to cut our command argument, which is a str, break it at the space (we use the space as a delimiter). Python provides the str.split method which we’ll use. Again, here are some examples from the Python interpreter:

>>> args = "3 12"
>>> args.split(" ")
['3', '12']
>>> args = "a command with several arguments"
>>> args.split(" ")
['a', 'command', 'with', 'several', 'arguments']
>>>

As you can see, str.split will “convert” our strings into a list of strings. The specified argument (" " in our case) is used as delimiter. So Python browses our original string. When it sees a delimiter, it takes whatever is before this delimiter and append it to a list.

The point here is that str.split will be used to split our argument. But, as you can see from the above output, we can never be sure of the length of the list at this point:

>>> args = "something"
>>> args.split(" ")
['something']
>>> args = ""
>>> args.split(" ")
['']
>>>

Again we could use a condition to check the number of split arguments, but Python offers a better approach, making use of its exception mechanism. We’ll give a second argument to str.split, the maximum number of splits to do. Let’s see an example, this feature might be confusing at first glance:

>>> args = "that is something great"
>>> args.split(" ", 1) # one split, that is a list with two elements (before, after)

[‘that’, ‘is something great’]

Read this example as many times as needed to understand it. The second argument we give to str.split is not the length of the list that should be returned, but the number of times we have to split. Therefore, we specify 1 here, but we get a list of two elements (before the separator, after the separator).

What will happen if Python can’t split the number of times we ask?

It won’t:

>>> args = "whatever"
>>> args.split(" ", 1) # there isn't even a space here...
['whatever']
>>>

This is one moment I would have hoped for an exception and didn’t get one. But there’s another way which will raise an exception if there is an error: variable unpacking.

We won’t talk about this feature in details here. It would be complicated. But the code is really straightforward to use. Let’s take our example of the roll command but let’s add a first argument: the number of dice to roll.

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from random import randint

from evennia import Command, InterruptCommand

class CmdRoll(Command):

    """
    Play random, enter a number and try your luck.

    Specify two numbers separated by a space.  The first number is the
    number of dice to roll (1, 2, 3) and the second is the expected sum
    of the roll.

    Usage:
      roll <dice> <number>

    For instance, to roll two 6-figure dice, enter 2 as first argument.
    If you think the sum of these two dice roll will be 10, you could enter:

        roll 2 10

    """

    key = "roll"

    def parse(self):
        """Split the arguments and convert them."""
        args = self.args.lstrip()

        # Split: we expect two arguments separated by a space
        try:
            number, guess = args.split(" ", 1)
        except ValueError:
            self.msg("Invalid usage.  Enter two numbers separated by a space.")
            raise InterruptCommand

        # Convert the entered number (first argument)
        try:
            self.number = int(number)
            if self.number <= 0:
                raise ValueError
        except ValueError:
            self.msg(f"{number} is not a valid number of dice.")
            raise InterruptCommand

        # Convert the entered guess (second argument)
        try:
            self.guess = int(guess)
            if not 1 <= self.guess <= self.number * 6:
                raise ValueError
        except ValueError:
            self.msg(f"{self.guess} is not a valid guess.")
            raise InterruptCommand

    def func(self):
        # Roll a random die X times (X being self.number)
        figure = 0
        for _ in range(self.number):
            figure += randint(1, 6)

        self.msg(f"You roll {self.number} dice and obtain the sum {figure}.")

        if self.guess == figure:
            self.msg(f"You played {self.guess}, you have won!")
        else:
            self.msg(f"You played {self.guess}, you have lost.")

The beginning of the parse() method is what interests us most:

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try:
    number, guess = args.split(" ", 1)
except ValueError:
    self.msg("Invalid usage.  Enter two numbers separated by a space.")
    raise InterruptCommand

We split the argument using str.split but we capture the result in two variables. Python is smart enough to know that we want what’s left of the space in the first variable, what’s right of the space in the second variable. If there is not even a space in the string, Python will raise a ValueError exception.

This code is much easier to read than browsing through the returned strings of str.split. We can convert both variables the way we did previously. Actually there are not so many changes in this version and the previous one, most of it is due to name changes for clarity.

Splitting a string with a maximum of splits is a common occurrence while parsing command arguments. You can also see the str.rspli8t method that does the same thing but from the right of the string. Therefore, it will attempt to find delimiters at the end of the string and work toward the beginning of it.

We have used a space as a delimiter. This is absolutely not necessary. You might remember that most default Evennia commands can take an = sign as a delimiter. Now you know how to parse them as well:

>>> cmd_key = "tel"
>>> cmd_args = "book = chest"
>>> left, right = cmd_args.split("=") # mighht raise ValueError!
>>> left
'book '
>>> right
' chest'
>>>

Optional arguments

Sometimes, you’ll come across commands that have optional arguments. These arguments are not necessary but they can be set if more information is needed. I will not provide the entire command code here but just enough code to show the mechanism in Python:

Again, we’ll use str.split, knowing that we might not have any delimiter at all. For instance, the player could enter the “tel” command like this:

> tel book
> tell book = chest

The equal sign is optional along with whatever is specified after it. A possible solution in our parse method would be:

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    def parse(self):
        args = self.args.lstrip()

        # = is optional
        try:
            obj, destination = args.split("=", 1)
        except ValueError:
            obj = args
            destination = None

This code would place everything the user entered in obj if she didn’t specify any equal sign. Otherwise, what’s before the equal sign will go in obj, what’s after the equal sign will go in destination. This makes for quick testing after that, more robust code with less conditions that might too easily break your code if you’re not careful.

Again, here we specified a maximum numbers of splits. If the users enters:

> tel book = chest = chair

Then destination will contain: " chest = chair". This is often desired, but it’s up to you to set parsing however you like.

Evennia searches

After this quick tour of some str methods, we’ll take a look at some Evennia-specific features that you won’t find in standard Python.

One very common task is to convert a str into an Evennia object. Take the previous example: having "book" in a variable is great, but we would prefer to know what the user is talking about… what is this "book"?

To get an object from a string, we perform an Evennia search. Evennia provides a search method on all typeclassed objects (you will most likely use the one on characters or accounts). This method supports a very wide array of arguments and has its own tutorial. Some examples of useful cases follow:

Local searches

When an account or a character enters a command, the account or character is found in the caller attribute. Therefore, self.caller will contain an account or a character (or a session if that’s a session command, though that’s not as frequent). The search method will be available on this caller.

Let’s take the same example of our little “tel” command. The user can specify an object as argument:

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    def parse(self):
        name = self.args.lstrip()

We then need to “convert” this string into an Evennia object. The Evennia object will be searched in the caller’s location and its contents by default (that is to say, if the command has been entered by a character, it will search the object in the character’s room and the character’s inventory).

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    def parse(self):
        name = self.args.lstrip()

        self.obj = self.caller.search(name)

We specify only one argument to the search method here: the string to search. If Evennia finds a match, it will return it and we keep it in the obj attribute. If it can’t find anything, it will return None so we need to check for that:

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    def parse(self):
        name = self.args.lstrip()

        self.obj = self.caller.search(name)
        if self.obj is None:
            # A proper error message has already been sent to the caller
            raise InterruptCommand

That’s it. After this condition, you know that whatever is in self.obj is a valid Evennia object (another character, an object, an exit…).

Quiet searches

By default, Evennia will handle the case when more than one match is found in the search. The user will be asked to narrow down and re-enter the command. You can, however, ask to be returned the list of matches and handle this list yourself:

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    def parse(self):
        name = self.args.lstrip()

        objs = self.caller.search(name, quiet=True)
        if not objs:
            # This is an empty list, so no match
            self.msg(f"No {name!r} was found.")
            raise InterruptCommand
        
        self.obj = objs[0] # Take the first match even if there are several

All we have changed to obtain a list is a keyword argument in the search method: quiet. If set to True, then errors are ignored and a list is always returned, so we need to handle it as such. Notice in this example, self.obj will contain a valid object too, but if several matches are found, self.obj will contain the first one, even if more matches are available.

Global searches

By default, Evennia will perform a local search, that is, a search limited by the location in which the caller is. If you want to perform a global search (search in the entire database), just set the global_search keyword argument to True:

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    def parse(self):
        name = self.args.lstrip()
        self.obj = self.caller.search(name, global_search=True)

Conclusion

Parsing command arguments is vital for most game designers. If you design “intelligent” commands, users should be able to guess how to use them without reading the help, or with a very quick peek at said help. Good commands are intuitive to users. Better commands do what they’re told to do. For game designers working on MUDs, commands are the main entry point for users into your game. This is no trivial. If commands execute correctly (if their argument is parsed, if they don’t behave in unexpected ways and report back the right errors), you will have happier players that might stay longer on your game. I hope this tutorial gave you some pointers on ways to improve your command parsing. There are, of course, other ways you will discover, or ways you are already using in your code.