list_concat.py

import array
import functools
import types

# reuse the bytecode magic from
# https://github.com/snoack/python-goto/blob/master/goto.py
# Install with `pip3 install goto-statement`
import goto


def _make_code(code, codestring, co_names):
    return code.replace(co_code=codestring)

def _patch_code(code):
    concatenator_name_index = len(code.co_names)
    co_names = code.co_names + ("concatenator",)
    instructions = list(goto._parse_instructions(code.co_code))

    buf = array.array('B', code.co_code)
    i = -1
    while i+2 < len(instructions):
        i += 1
        inst = instructions[i]
        if inst[0] not in ("BUILD_LIST", "LIST_EXTEND"):
            # BUILD_LIST for py < 3.9, LIST_EXTEND for py >= 3.9
            continue
        if instructions[i+1][0] != "LOAD_CONST":
            continue
        if instructions[i+2][0] != "BINARY_SUBSCR":
            continue
        const_idx = instructions[i+1][1]
        const = code.co_consts[const_idx]
        if isinstance(const, tuple):
            # py >= 3.9 only
            goto._write_instruction(buf, instructions[i+2][2], "LIST_EXTEND", 1)
        else:
            goto._write_instruction(buf, instructions[i+2][2], "LIST_APPEND", 1)
        i += 2

    return _make_code(code, buf.tobytes(), co_names)

def with_list_concat(func_or_code):
    if isinstance(func_or_code, types.CodeType):
        return _patch_code(func_or_code)

    return functools.update_wrapper(
        types.FunctionType(
            _patch_code(func_or_code.__code__),
            func_or_code.__globals__,
            func_or_code.__name__,
            func_or_code.__defaults__,
            func_or_code.__closure__,
        ),
        func_or_code
    )

def seven_factory():
    return 7

@with_list_concat
def foo():
    a = 1
    print(a)                                 # 1
    b = [1, 2, 3] [4]
    print(b)                                 # [1, 2, 3, 4]
    c = [1, 2, 3] [5, 6]
    print(c)                                 # [1, 2, 3, 5, 6]
    d = [1, 2, 3] [(5, 6), 7]
    print(d)                                 # [1, 2, 3, (5, 6), 7]
    e = [1, 2, 3] [(5, 6), seven_factory()]  # actually, this one crashes
    print(e)

foo()