Python Code for Scanning and Parsing a String

Inside Python, here is how you parse a string:

from mathics.core.parser import parse, SingleLineFeeder
from mathics.core.definitions import Definitions

definitions = Definitions(add_builtin=True)
str_expression = "1 + 2 / 3"
expr = parse(definitions, SingleLineFeeder(str_expression))
print("type", type(expr))
print("expr: ", expr)

Running the above produces:

type <class 'mathics.core.expression.Expression'>
expr:  System`Plus[1, System`Times[2, System`Power[3, -1]]]

The function SingleLineFeeder should be supplied by the front-end. It reads input a line and a time and returns that back to the parser.