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
variable 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
system variables.
To accomplish this you assign the desired string name to class
variable 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 attributes
.
Suppose we want to define a variable called $Hello
and don’t want
lock down the attributes of this variable so that the attributes
cannot be modified.
This is done by setting the Locked
attribute of in the
definition, via the attributes
class variable.
Here is and example of setting some class variables to alter the definition of a function:
from mathics.builtin.base import Predefined
from mathics.core.attributes import locked as A_LOCKED
from mathics.core.evaluation import Evaluation
class Hello(Predefined):
attributes = A_LOCKED
name = "$Hello"
def evaluate(self, evaluation: Evaluation) -> String:
return String("Hello, World!")
Here is a session that demonstrates the above code:
$ mathics
In[1]:= $Hello
Out[1]= Hello, World!
In[2]:= Unprotect[$Hello]
ClearAttributes::locked: Symbol $Hello is locked.
Out[2]= None