{ Snipperize }

The goto decorator Add to Favorite

import dis
import new

class MissingLabelError(Exception):
    """'goto' without matching 'label'."""
    pass


def goto(fn):
    """
    A function decorator to add the goto command for a function.

    Specify labels like so:

    label .foo
    
    Goto labels like so:

    goto .foo
    """
    labels = {}
    gotos = {}
    globalName = None
    index = 0
    end = len(fn.func_code.co_code)
    i = 0

    # scan through the byte codes to find the labels and gotos
    while i < end:
        op = ord(fn.func_code.co_code[i])
        i += 1
        name = dis.opname[op]

        if op > dis.HAVE_ARGUMENT:
            b1 = ord(fn.func_code.co_code[i])
            b2 = ord(fn.func_code.co_code[i+1])
            num = b2 * 256 + b1
            
            if name == 'LOAD_GLOBAL':
                globalName = fn.func_code.co_names[num]
                index = i - 1
                i += 2
                continue
                
            if name == 'LOAD_ATTR':
                if globalName == 'label':
                    labels[fn.func_code.co_names[num]] = index
                elif globalName == 'goto':
                    gotos[fn.func_code.co_names[num]] = index
                    
            name = None
            i += 2

    # no-op the labels
    ilist = list(fn.func_code.co_code)
    for label,index in labels.items():
        ilist[index:index+7] = [chr(dis.opmap['NOP'])]*7

    # change gotos to jumps
    for label,index in gotos.items():
        if label not in labels:
            raise MissingLabelError("Missing label: %s"%label)
        
        target = labels[label] + 7   # skip NOPs
        ilist[index] = chr(dis.opmap['JUMP_ABSOLUTE'])
        ilist[index + 1] = chr(target & 255)
        ilist[index + 2] = chr(target >> 8)

    # create new function from existing function
    c = fn.func_code
    newcode = new.code(c.co_argcount,
                       c.co_nlocals,
                       c.co_stacksize,
                       c.co_flags,
                       ''.join(ilist),
                       c.co_consts,
                       c.co_names,
                       c.co_varnames,
                       c.co_filename,
                       c.co_name,
                       c.co_firstlineno,
                       c.co_lnotab)
    newfn = new.function(newcode,fn.func_globals)
    return newfn


if __name__ == '__main__':
    
    @goto
    def test1(n):

        s = 0

        label .myLoop

        if n <= 0:
            return s
        s += n
        n -= 1

        goto .myLoop

    assert(test1(10) == 55)

This is the recipe for you if you are sick of the slow speed of the existing goto module http://entrian.com/goto/. The goto in this recipe is about 60x faster and is also cleaner (abusing sys.settrace seems hardly pythonic). Because this is a decorator, it alerts the reader which functions use goto. It does not implement the comefrom command, although it is not difficult to extend it to do so (exercise for the reader). Also, computed gotos aren't supported; they're not pythonic.

Use dis.dis(fn) to show the bytecode disassembly of a function. The bytecodes of a function are accessed by fn.func_code.co_code. This is readonly so: The decorated function is created exactly the same as the old one, but with the bytecode updated to obey the goto commands. This is 2.x only; the new module is not in python 3.x (another exercise for the reader!)

Created by ThePeppersStudio (49 days, 8.36 hours ago)