Class attributes are used a lot in classes that create Mathics definitions, such as in the Builtin class that we have mostly been using.
In fact we already saw one example in the last section where the class
rules was used to create some Mathics Rules in the
definition of the Hello function.
Suppose you want to create a Mathics variable that starts with a
dollar sign (
$) in accordance to the naming convention for many
To accomplish this you assign the desired string name to class
name, and that will override the default name.
Similarly, setting a variable’s or function’s attributes can be done
by setting the class variable name
Suppose we want to define a variable called
$Hello and don’t want
lock down the attributes of this variable so that the attributes
cannot be modified.
This is done by setting the
Locked attribute of in the
definition, via the
attributes class variable.
Here is and example of setting some class variables to alter the definition of a function:
from mathics.builtin.base import Predefined class Hello(Predefined): name = "$Hello" attributes = ('Locked',) def evaluate(self evaluation) -> String: return String("Hello, World!")
Here is a session that demonstrates the above code:
$ mathics In:= $Hello Out= Hello, World! In:= Unprotect[$Hello] ClearAttributes::locked: Symbol $Hello is locked. Out= None