Class variables and Function Attributes¶
Class variables on the class that implements a Mathics3 Built-in Function are used to set various characteristics of the built-in function.
In fact we already saw one example in the last section where the class
rules was used to create some Mathics3 Rules in the
definition of the Hello function.
Suppose you want to create a Mathics3 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.
All Functions in Mathics3 have Attributes which specify certain properties in evaluation such as whether a nested invocations of a function are automatically flattened. (This is the Flat attribute).
Setting a built-in function’s attribute is done
by setting the class variable name
Suppose we want to define a variable called
$Hello and don’t want
lock down the attributes of this variable so that the attributes
cannot be modified.
This is done by setting the
Locked attribute of in the
definition, via the
attributes class variable.
Here is and example of setting some class variables to alter the definition of a function:
from mathics.builtin.base import Predefined from mathics.core.attributes import locked as A_LOCKED from mathics.core.evaluation import Evaluation class Hello(Predefined): attributes = A_LOCKED name = "$Hello" def evaluate(self, evaluation: 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