Defining Variable $MachineName

To start our list of case studies, we’ll start off with a very simple example: defining a read-only variable.

The Mathics3 Built-in Symbol $MachineName holds a Mathics3 string value containing the computer machine name.

Since it is a variable, not a function we use the simpler Predefined class to define this.

Here is the code for this.

import platform
from mathics.core.builtin import Predefined
from mathics.core.atoms import String

class MachineName(Predefined):
    """
    <url>:WMA link:https://reference.wolfram.com/language/ref/MachineName.html</url>

    <dl>
      <dt>'\\$MachineName'
      <dd>is a string that gives the assigned name of the computer on which Mathics3 \
          is being run, if such a name is defined.
    </dl>

    S> $MachineName
     = ...
    """

    name = "$MachineName"
    summary_text = "get the name of computer that Mathics3 is running"

    def evaluate(self, evaluation: Evaluation) -> String:
        return String(platform.uname().node)

The docstring for this class lists:

• The variable name : $MachineName.

• A link to the WMA reference.

• A HTML “definition list” to enclose the define how to use this and what the variable does.

• An example of how this variable might be used. The S> indicates that this example can’t be run in a “sandboxed” environment.

The Python code makes heavy use of the Predefined class properties to do most of the work that to set up this variable.

The class variable name is what defines the name seen inside Mathics3. Note that $MachineName is not a valid Python class name, even though it is a valid Symbol name in Mathics3. So we specify the Mathics3 name using the class variable name of the Predefined class.

If we wanted to define a variable that matches the class name, e.g. MachineName, we would not need to set this class variable. The class would use the class name as its corresponding Mathics3 Symbol name.

The evaluate() method is used to produce the read-only value of this variable. When the value is a simple literal value the class variable value can be used instead of defining function evaluate.

For more detail on defining a Builtin Variable, see Adding Builtin Undefined.

Next:

Selecting a Builtin to Add