The Inline Function Parser

The FuncParser extracts and executes ‘inline functions’ embedded in a string on the form $funcname(args, kwargs). Under the hood, this will lead to a call to a Python function you control. The inline function call will be replaced by the return from the function.

from evennia.utils.funcparser import FuncParser

def _power_callable(*args, **kwargs):
    """This will be callable as $pow(number, power=<num>) in string"""
    pow = int(kwargs.get('power', 2))
    return float(args[0]) ** pow

# create a parser and tell it that '$pow' means using _power_callable
parser = FuncParser({"pow": _power_callable})

Next, just pass a string into the parser, containing $func(...) markers:

parser.parse("We have that 4 x 4 x 4 is $pow(4, power=3).")
"We have that 4 x 4 x 4 is 64."

Normally the return is always converted to a string but you can also get the actual data type from the call:

parser.parse_to_any("$pow(4)")
16

To show a $func() verbatim in your code without parsing it, escape it as either $$func() or \$func():

parser.parse("This is an escaped $$pow(4) and so is this \$pow(3)")
"This is an escaped $pow(4) and so is this $pow(3)"

Uses in default Evennia

The FuncParser can be applied to any string. Out of the box it’s applied in a few situations:

  • Outgoing messages. All messages sent from the server is processed through FuncParser and every callable is provided the Session of the object receiving the message. This potentially allows a message to be modified on the fly to look different for different recipients.

  • Prototype values. A Prototype dict’s values are run through the parser such that every callable gets a reference to the rest of the prototype. In the Prototype ORM, this would allow builders to safely call functions to set non-string values to prototype values, get random values, reference other fields of the prototype, and more.

  • Actor-stance in messages to others. In the Object.msg_contents method, the outgoing string is parsed for special $You() and $conj() callables to decide if a given recipient should see “You” or the character’s name.

Important

The inline-function parser is not intended as a ‘softcode’ programming language. It does not have things like loops and conditionals, for example. While you could in principle extend it to do very advanced things and allow builders a lot of power, all-out coding is something Evennia expects you to do in a proper text editor, outside of the game, not from inside it.

Using the FuncParser

You can apply inline function parsing to any string. The FuncParser is imported as evennia.utils.funcparser.

from evennia.utils import funcparser

parser = FuncParser(callables, **default_kwargs)
parsed_string = parser.parse(input_string, raise_errors=False,
                              escape=False, strip=False,
                              return_str=True, **reserved_kwargs)

# callables can also be passed as paths to modules
parser = FuncParser(["game.myfuncparser_callables", "game.more_funcparser_callables"])

Here, callables points to a collection of normal Python functions (see next section) for you to make available to the parser as you parse strings with it. It can either be

  • A dict of {"functionname": callable, ...}. This allows you do pick and choose exactly which callables to include and how they should be named. Do you want a callable to be available under more than one name? Just add it multiple times to the dict, with a different key.

  • A module or (more commonly) a python-path to a module. This module can define a dict FUNCPARSER_CALLABLES = {"funcname": callable, ...} - this will be imported and used like the dict above. If no such variable is defined, every top-level function in the module (whose name doesn’t start with an underscore _) will be considered a suitable callable. The name of the function will be the $funcname by which it can be called.

  • A list of modules/paths. This allows you to pull in modules from many sources for your parsing.

  • The **default kwargs are optional kwargs that will be passed to all callables every time this parser is used - unless the user overrides it explicitly in their call. This is great for providing sensible standards that the user can tweak as needed.

FuncParser.parse takes further arguments, and can vary for every string parsed.

  • raise_errors - By default, any errors from a callable will be quietly ignored and the result will be that the failing function call will show verbatim. If raise_errors is set, then parsing will stop and whatever exception happened will be raised. It’d be up to you to handle this properly.

  • escape - Returns a string where every $func(...) has been escaped as \$func().

  • strip - Remove all $func(...) calls from string (as if each returned '').

  • return_str - When True (default), parser always returns a string. If False, it may return the return value of a single function call in the string. This is the same as using the .parse_to_any method.

  • The **reserved_keywords are always passed to every callable in the string. They override any **defaults given when instantiating the parser and cannot be overridden by the user - if they enter the same kwarg it will be ignored. This is great for providing the current session, settings etc.

  • The funcparser and raise_errors are always added as reserved keywords - the first is a back-reference to the FuncParser instance and the second is the raise_errors boolean given to FuncParser.parse.

Here’s an example of using the default/reserved keywords:

def _test(*args, **kwargs):
    # do stuff
    return something

parser = funcparser.FuncParser({"test": _test}, mydefault=2)
result = parser.parse("$test(foo, bar=4)", myreserved=[1, 2, 3])

Here the callable will be called as

_test('foo', bar='4', mydefault=2, myreserved=[1, 2, 3],
      funcparser=<FuncParser>, raise_errors=False)

The mydefault=2 kwarg could be overwritten if we made the call as $test(mydefault=...) but myreserved=[1, 2, 3] will always be sent as-is and will override a call $test(myreserved=...). The funcparser/raise_errors kwargs are also always included as reserved kwargs.

Defining custom callables

All callables made available to the parser must have the following signature:

def funcname(*args, **kwargs):
    # ...
    return something

The *args and **kwargs must always be included. If you are unsure how *args and **kwargs work in Python, read about them here.

The input from the innermost $funcname(...) call in your callable will always be a str. Here’s an example of an $toint function; it converts numbers to integers.

"There's a $toint(22.0)% chance of survival."

What will enter the $toint callable (as args[0]) is the string "22.0". The function is responsible for converting this to a number so that we can convert it to an integer. We must also properly handle invalid inputs (like non-numbers).

If you want to mark an error, raise evennia.utils.funcparser.ParsingError. This stops the entire parsing of the string and may or may not raise the exception depending on what you set raise_errors to when you created the parser.

However, if you nest functions, the return of the innermost function may be something other than a string. Let’s introduce the $eval function, which evaluates simple expressions using Python’s literal_eval and/or simple_eval. It returns whatever data type it evaluates to.

"There's a $toint($eval(10 * 2.2))% chance of survival."

Since the $eval is the innermost call, it will get a string as input - the string "10 * 2.2". It evaluates this and returns the float 22.0. This time the outermost $toint will be called with this float instead of with a string.

It’s important to safely validate your inputs since users may end up nesting your callables in any order. See the next section for useful tools to help with this.

In these examples, the result will be embedded in the larger string, so the result of the entire parsing will be a string:

  parser.parse(above_string)
  "There's a 22% chance of survival."

However, if you use the parse_to_any (or parse(..., return_str=False)) and don’t add any extra string around the outermost function call, you’ll get the return type of the outermost callable back:

parser.parse_to_any("$toint($eval(10 * 2.2)")
22
parser.parse_to_any("the number $toint($eval(10 * 2.2).")
"the number 22"
parser.parse_to_any("$toint($eval(10 * 2.2)%")
"22%"

Escaping special character

When entering funcparser callables in strings, it looks like a regular function call inside a string:

"This is a $myfunc(arg1, arg2, kwarg=foo)."

Commas (,) and equal-signs (=) are considered to separate the arguments and kwargs. In the same way, the right parenthesis ()) closes the argument list. Sometimes you want to include commas in the argument without it breaking the argument list.

"There is a $format(beautiful meadow, with dandelions) to the west."

You can escape in various ways.

  • Prepending with the escape character \

      ```python
  "There is a $format(beautiful meadow\, with dandelions) to the west."
  ```

  • Wrapping your strings in quotes. This works like Python, and you can nest double and single quotes inside each other if so needed. The result will be a verbatim string that contains everything but the outermost quotes.

      ```python
  "There is a $format('beautiful meadow, with dandelions') to the west."
  ```

  • If you want verbatim quotes in your string, you can escape them too.

      ```python
  "There is a $format('beautiful meadow, with \'dandelions\'') to the west."
  ```

Safe convertion of inputs

Since you don’t know in which order users may use your callables, they should always check the types of its inputs and convert to the type the callable needs. Note also that when converting from strings, there are limits what inputs you can support. This is because FunctionParser strings can be used by non-developer players/builders and some things (such as complex classes/callables etc) are just not safe/possible to convert from string representation.

In evennia.utils.utils is a helper called safe_convert_to_types. This function automates the conversion of simple data types in a safe way:

from evennia.utils.utils import safe_convert_to_types

def _process_callable(*args, **kwargs):
    """
    $process(expression, local, extra1=34, extra2=foo)

    """
    args, kwargs = safe_convert_to_type(
      (('py', str), {'extra1': int, 'extra2': str}),
      *args, **kwargs)

    # args/kwargs should be correct types now

In other words, in the callable $process(expression, local, extra1=.., extra2=...), the first argument will be handled by the ‘py’ converter (described below), the second will passed through regular Python str, kwargs will be handled by int and str respectively. You can supply your own converter function as long as it takes one argument and returns the converted result.

In other words,

args, kwargs = safe_convert_to_type(
        (tuple_of_arg_converters, dict_of_kwarg_converters), *args, **kwargs)

The special converter "py" will try to convert a string argument to a Python structure with the help of the following tools (which you may also find useful to experiment with on your own):

  • ast.literal_eval is an in-built Python function. It only supports strings, bytes, numbers, tuples, lists, dicts, sets, booleans and None. That’s it - no arithmetic or modifications of data is allowed. This is good for converting individual values and lists/dicts from the input line to real Python objects.

  • simpleeval is a third-party tool included with Evennia. This allows for evaluation of simple (and thus safe) expressions. One can operate on numbers and strings with +-/* as well as do simple comparisons like 4 > 3 and more. It does not accept more complex containers like lists/dicts etc, so this and literal_eval are complementary to each other.

Warning

It may be tempting to run use Python’s in-built eval() or exec() functions as converters since these are able to convert any valid Python source code to Python. NEVER DO THIS unless you really, really know that ONLY developers will ever modify the string going into the callable. The parser is intended for untrusted users (if you were trusted you’d have access to Python already). Letting untrusted users pass strings to eval/exec is a MAJOR security risk. It allows the caller to run arbitrary Python code on your server. This is the path to maliciously deleted hard drives. Just don’t do it and sleep better at night.

Default callables

These are some example callables you can import and add your parser. They are divided into global-level dicts in evennia.utils.funcparser. Just import the dict(s) and merge/add one or more to them when you create your FuncParser instance to have those callables be available.

evennia.utils.funcparser.FUNCPARSER_CALLABLES

These are the ‘base’ callables.

  • $eval(expression) (code) - this uses literal_eval and simple_eval (see previous section) attemt to convert a string expression to a python object. This handles e.g. lists of literals [1, 2, 3] and simple expressions like "1 + 2".

  • $toint(number) (code) - always converts an output to an integer, if possible.

  • $add/sub/mult/div(obj1, obj2) (code) - this adds/subtracts/multiplies and divides to elements together. While simple addition could be done with $eval, this could for example be used also to add two lists together, which is not possible with eval; for example $add($eval([1,2,3]), $eval([4,5,6])) -> [1, 2, 3, 4, 5, 6].

  • $round(float, significant) (code) - rounds an input float into the number of provided significant digits. For example $round(3.54343, 3) -> 3.543.

  • $random([start, [end]]) (code) - this works like the Python random() function, but will randomize to an integer value if both start/end are integers. Without argument, will return a float between 0 and 1.

  • $randint([start, [end]]) (code) - works like the randint() python function and always returns an integer.

  • $choice(list) (code) - the input will automatically be parsed the same way as $eval and is expected to be an iterable. A random element of this list will be returned.

  • $pad(text[, width, align, fillchar]) (code) - this will pad content. $pad("Hello", 30, c, -) will lead to a text centered in a 30-wide block surrounded by - characters.

  • $crop(text, width=78, suffix='[...]') (code) - this will crop a text longer than the width, by default ending it with a [...]-suffix that also fits within the width. If no width is given, the client width or settings.DEFAULT_CLIENT_WIDTH will be used.

  • $space(num) (code) - this will insert num spaces.

  • $just(string, width=40, align=c, indent=2) (code) - justifies the text to a given width, aligning it left/right/center or ‘f’ for full (spread text across width).

  • $ljust - shortcut to justify-left. Takes all other kwarg of $just.

  • $rjust - shortcut to right justify.

  • $cjust - shortcut to center justify.

  • $clr(startcolor, text[, endcolor]) (code) - color text. The color is given with one or two characters without the preceeding |. If no endcolor is given, the string will go back to neutral, so $clr(r, Hello) is equivalent to |rHello|n.

evennia.utils.funcparser.SEARCHING_CALLABLES

These are callables that requires access-checks in order to search for objects. So they require some extra reserved kwargs to be passed when running the parser:


parser.parse_to_any(string, caller=<object or account>, access="control", ...)

The caller is required, it’s the the object to do the access-check for. The access kwarg is the lock type to check, default being "control".

  • $search(query,type=account|script,return_list=False) (code) - this will look up and try to match an object by key or alias. Use the type kwarg to search for account or script instead. By default this will return nothing if there are more than one match; if return_list is True a list of 0, 1 or more matches will be returned instead.

  • $obj(query), $dbref(query) - legacy aliases for $search.

  • $objlist(query) - legacy alias for $search, always returning a list.

evennia.utils.funcparser.ACTOR_STANCE_CALLABLES

These are used to implement actor-stance emoting. They are used by the DefaultObject.msg_contents method by default. You can read a lot more about this on the page Change messages per receiver.

On the parser side, all these inline functions require extra kwargs be passed into the parser (done by msg_contents by default):

parser.parse(string, caller=<obj>, receiver=<obj>, mapping={'key': <obj>, ...})

Here the caller is the one sending the message and receiver the one to see it. The mapping contains references to other objects accessible via these callables.

  • $you([key]) (code) - if no key is given, this represents the caller, otherwise an object from mapping will be used. As this message is sent to different recipients, the receiver will change and this will be replaced either with the string you (if you and the receiver is the same entity) or with the result of you_obj.get_display_name(looker=receiver). This allows for a single string to echo differently depending on who sees it, and also to reference other people in the same way.

  • $You([key]) - same as $you but always capitalized.

  • $conj(verb) (code) - conjugates a verb between 2nd person presens to 3rd person presence depending on who sees the string. For example "$You() $conj(smiles)". will show as “You smile.” and “Tom smiles.” depending on who sees it. This makes use of the tools in evennia.utils.verb_conjugation to do this, and only works for English verbs.

  • $pron(pronoun [,options]) (code) - Dynamically map pronouns (like his, herself, you, its etc) between 1st/2nd person to 3rd person.

Example

Here’s an example of including the default callables together with two custom ones.

from evennia.utils import funcparser
from evennia.utils import gametime

def _dashline(*args, **kwargs):
    if args:
        return f"\n-------- {args[0]} --------"
    return ''

def _uptime(*args, **kwargs):
    return gametime.uptime()

callables = {
    "dashline": _dashline,
    "uptime": _uptime,
    **funcparser.FUNCPARSER_CALLABLES,
    **funcparser.ACTOR_STANCE_CALLABLES,
    **funcparser.SEARCHING_CALLABLES
}

parser = funcparser.FuncParser(callables)

string = "This is the current uptime:$dashline($toint($uptime()) seconds)"
result = parser.parse(string)

Above we define two callables _dashline and _uptime and map them to names "dashline" and "uptime", which is what we then can call as $header and $uptime in the string. We also have access to all the defaults (like $toint()).

The parsed result of the above would be something like this:

This is the current uptime:
------- 343 seconds -------