Compile Python to Bytecode

A toy example that compiles Python directly to bytecode, without generating an AST. It currently only works for very very simple Python code.

It requires the ‘bytecode’ library. You can get it using

$ pip install bytecode

from lark import Lark, Transformer, v_args
from lark.indenter import Indenter

from bytecode import Instr, Bytecode

class PythonIndenter(Indenter):
    NL_type = '_NEWLINE'
    OPEN_PAREN_types = ['LPAR', 'LSQB', 'LBRACE']
    CLOSE_PAREN_types = ['RPAR', 'RSQB', 'RBRACE']
    INDENT_type = '_INDENT'
    DEDENT_type = '_DEDENT'
    tab_len = 8


@v_args(inline=True)
class Compile(Transformer):
    def number(self, n):
        return [Instr('LOAD_CONST', int(n))]
    def string(self, s):
        return [Instr('LOAD_CONST', s[1:-1])]
    def var(self, n):
        return [Instr('LOAD_NAME', n)]

    def arith_expr(self, a, op, b):
        # TODO support chain arithmetic
        assert op == '+'
        return a + b + [Instr('BINARY_ADD')]

    def arguments(self, args):
        return args

    def funccall(self, name, args):
        return name + args + [Instr('CALL_FUNCTION', 1)]

    @v_args(inline=False)
    def file_input(self, stmts):
        return sum(stmts, []) + [Instr("RETURN_VALUE")]

    def expr_stmt(self, lval, rval):
        # TODO more complicated than that
        name ,= lval
        assert name.name == 'LOAD_NAME' # XXX avoid with another layer of abstraction
        return rval + [Instr("STORE_NAME", name.arg)]

    def __default__(self, *args):
        assert False, args


python_parser3 = Lark.open('python3.lark', rel_to=__file__, start='file_input',
                           parser='lalr', postlex=PythonIndenter(),
                           transformer=Compile(), propagate_positions=False)

def compile_python(s):
    insts = python_parser3.parse(s+"\n")
    return Bytecode(insts).to_code()

code = compile_python("""
a = 3
b = 5
print("Hello World!")
print(a+(b+2))
print((a+b)+2)
""")
exec(code)
# -- Output --
# Hello World!
# 10
# 10