variables - How to get self into a Python method without explicitly accepting it

Question

I'm developing a documentation testing framework -- basically unit tests for PDFs. Tests are (decorated) methods of instances of classes defined by the framework, and these are located and instantiated at runtime and the methods are invoked to execute the tests.

My goal is to cut down on the amount of quirky Python syntax that the people who will write tests need to be concerned about, as these people may or may not be Python programmers, or even very much programmers at all. So I would like them to be able to write "def foo():" instead of "def foo(self):" for methods, but still be able to use "self" to access members.

In an ordinary program I would consider this a horrible idea, but in a domain-specific-languagey kind of program like this one, it seems worth a try.

I have successfully eliminated the self from the method signature by using a decorator (actually, since I am using a decorator already for the test cases, I would just roll it into that), but "self" does not then refer to anything in the test case method.

I have considered using a global for self, and even come up with an implementation that more or less works, but I'd rather pollute the smallest namespace possible, which is why I would prefer to inject the variable directly into the test case method's local namespace. Any thoughts?

Answer 1

My accepted answer to this question was pretty dumb but I was just starting out. Here's a much better way. This is only scantily tested but it's good for a demonstration of the proper way to do this thing which is improper to do. It works on 2.6.5 for sure. I haven't tested any other versions but no opcodes are hardcoded into it so it should be about as portable as most other 2.x code.

add_self can be applied as a decorator but that would defeat the purpose (why not just type 'self'?) It would be easy to adapt the metaclass from my other answer to apply this function instead.

import opcode
import types



def instructions(code):
    """Iterates over a code string yielding integer [op, arg] pairs

    If the opcode does not take an argument, just put None in the second part
    """
    code = map(ord, code)
    i, L = 0, len(code)
    extended_arg = 0
    while i < L:
        op = code[i]
        i+= 1
        if op < opcode.HAVE_ARGUMENT:
            yield [op, None]
            continue
        oparg = code[i] + (code[i+1] << 8) + extended_arg
        extended_arg = 0
        i += 2
        if op == opcode.EXTENDED_ARG:
            extended_arg = oparg << 16
            continue
        yield [op, oparg]


def write_instruction(inst):
    """Takes an integer [op, arg] pair and returns a list of character bytecodes"""
    op, oparg = inst
    if oparg is None:
        return [chr(op)]
    elif oparg <= 65536L:
        return [chr(op), chr(oparg & 255), chr((oparg >> 8) & 255)]
    elif oparg <= 4294967296L:
        # The argument is large enough to need 4 bytes and the EXTENDED_ARG opcode
        return [chr(opcode.EXTENDED_ARG),
                chr((oparg >> 16) & 255),
                chr((oparg >> 24) & 255),
                chr(op),
                chr(oparg & 255),
                chr((oparg >> 8) & 255)]
    else:
        raise ValueError("Invalid oparg: {0} is too large".format(oparg))


def add_self(f):
    """Add self to a method

    Creates a new function by prepending the name 'self' to co_varnames, and      
    incrementing co_argcount and co_nlocals. Increase the index of all other locals
    by 1 to compensate. Also removes 'self' from co_names and decrease the index of 
    all names that occur after it by 1. Finally, replace all occurrences of 
    `LOAD_GLOBAL i,j` that make reference to the old 'self' with 'LOAD_FAST 0,0'.   

    Essentially, just create a code object that is exactly the same but has one more
    argument. 
    """
    code_obj = f.func_code
    try:
        self_index = code_obj.co_names.index('self')
    except ValueError:
        raise NotImplementedError("self is not a global")

    # The arguments are just the first co_argcount co_varnames
    varnames = ('self', ) + code_obj.co_varnames   
    names = tuple(name for name in code_obj.co_names if name != 'self')

    code = []

    for inst in instructions(code_obj.co_code):
        op = inst[0]
        if op in opcode.haslocal:
            # The index is now one greater because we added 'self' at the head of
            # the tuple
            inst[1] += 1
        elif op in opcode.hasname:
            arg = inst[1]
            if arg == self_index:
                # This refers to the old global 'self'
                if op == opcode.opmap['LOAD_GLOBAL']:
                    inst[0] = opcode.opmap['LOAD_FAST']
                    inst[1] = 0
                else:
                    # If `self` is used as an attribute, real global, module
                    # name, module attribute, or gets looked at funny, bail out.
                    raise NotImplementedError("Abnormal use of self")
            elif arg > self_index:
                # This rewrites the index to account for the old global 'self'
                # having been removed.
                inst[1] -= 1

        code += write_instruction(inst)

    code = ''.join(code)

    # type help(types.CodeType) at the interpreter prompt for this one   
    new_code_obj = types.CodeType(code_obj.co_argcount + 1,
                                  code_obj.co_nlocals + 1,
                                  code_obj.co_stacksize,
                                  code_obj.co_flags, 
                                  code,
                                  code_obj.co_consts,
                                  names, 
                                  varnames, 
                                  '<OpcodeCity>',
                                  code_obj.co_name,  
                                  code_obj.co_firstlineno,
                                  code_obj.co_lnotab, 
                                  code_obj.co_freevars,
                                  code_obj.co_cellvars)


    # help(types.FunctionType)
    return types.FunctionType(new_code_obj, f.func_globals)



class Test(object):

    msg = 'Foo'

    @add_self
    def show(msg):
        print self.msg + msg


t = Test()
t.show('Bar')