Class Attributes

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 variable 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 system variables.

To accomplish this you assign the desired string name to class variable 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 attributes.

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[1]:= $Hello
Out[1]= Hello, World!

In[2]:= Unprotect[$Hello]
ClearAttributes::locked: Symbol $Hello is locked.
Out[2]= None