def extract_constant(code, symbol, default=-1):
    """Extract the constant value of 'symbol' from 'code'

    If the name 'symbol' is bound to a constant value by the Python code
    object 'code', return that value.  If 'symbol' is bound to an expression,
    return 'default'.  Otherwise, return 'None'.

    Return value is based on the first assignment to 'symbol'.  'symbol' must
    be a global, or at least a non-"fast" local in the code block.  That is,
    only 'STORE_NAME' and 'STORE_GLOBAL' opcodes are checked, and 'symbol'
    must be present in 'code.co_names'.
    """
    if symbol not in code.co_names:
        # name's not there, can't possibly be an assignment
        return None

    name_idx = list(code.co_names).index(symbol)

    STORE_NAME = 90
    STORE_GLOBAL = 97
    LOAD_CONST = 100

    const = default

    for byte_code in dis.Bytecode(code):
        op = byte_code.opcode
        arg = byte_code.arg

        if op == LOAD_CONST:
            const = code.co_consts[arg]
        elif arg == name_idx and (op == STORE_NAME or op == STORE_GLOBAL):
            return const
        else:
            const = default
Extract the constant value of 'symbol' from 'code'

    If the name 'symbol' is bound to a constant value by the Python code
    object 'code', return that value.  If 'symbol' is bound to an expression,
    return 'default'.  Otherwise, return 'None'.

    Return value is based on the first assignment to 'symbol'.  'symbol' must
    be a global, or at least a non-"fast" local in the code block.  That is,
    only 'STORE_NAME' and 'STORE_GLOBAL' opcodes are checked, and 'symbol'
    must be present in 'code.co_names'.
    
product/gradle-plugin/src/main/python/setuptools/depends.py