interp.lark

?start: (statement | _NL) *

?statement: simple_statement _NL
		  | print_statement _NL

?simple_statement: assign | expr

assign: NAME "=" sum
?expr: sum
print_statement: "print" expr

?sum: product
    | sum "+" product   -> add
    | sum "-" product   -> sub

?product: atom
    | product "*" atom  -> mul
    | product "/" atom  -> div

?atom: NUMBER           -> number
     | "-" atom         -> neg
     | NAME             -> var
     | "(" sum ")"

%import common.CNAME -> NAME
%import common.NUMBER
%import common.WS_INLINE
%import common.NEWLINE -> _NL
%ignore WS_INLINE

main.py

from lark import Lark, v_args, Transformer
from lark.visitors import Interpreter, visit_children_decor


p = Lark.open('interp.lark', parser='lalr', rel_to=__file__)

code = """
b = 4
a = b*2
print a+1
"""

tree = p.parse(code)



@v_args(inline=True)
class MyEval(Transformer):
    from operator import add, sub, mul, truediv as div, neg
    number = float

    def __init__(self, ns):
        self.ns = ns

    def var(self, name):
        return self.ns[name]


def eval_expr(tree, ns):
    return MyEval(ns).transform(tree)

@v_args(inline=True)
class MyInterp(Interpreter):
    def __init__(self):
        self.namespace = {}

    def assign(self, var, expr):
        self.namespace[var] = eval_expr(expr, self.namespace)

    def print_statement(self, expr):
        res = eval_expr(expr, self.namespace)
        print(res)

# print(tree.pretty())
MyInterp().visit(tree)