modelverse/wrappers/modelverse.py

import urllib
import urllib2
import json
import random
from urllib2 import URLError
import sys
import time

COMPILER_PATH = "interface/HUTN"

MODE_UNCONNECTED = 0
MODE_UNAUTHORIZED = 1
MODE_MODELLING = 2
MODE_MODIFY = 3
MODE_DIALOG = 4
MODE_MANUAL = 5

# Bind to the compiler (might have to update path manually!)
sys.path.append(COMPILER_PATH)
from hutn_compiler.compiler import main as do_compile

# Exceptions
class ModelverseException(Exception):
    pass

class UnknownError(ModelverseException):
    pass

class UnknownIdentifier(ModelverseException):
    pass

class UnknownType(ModelverseException):
    pass

class NotAnAssociation(ModelverseException):
    pass

class UnsupportedValue(ModelverseException):
    pass

class CompilationError(ModelverseException):
    pass

class NoSuchAttribute(ModelverseException):
    pass

class UnknownModel(ModelverseException):
    pass

class ConnectionError(ModelverseException):
    pass

class ModelExists(ModelverseException):
    pass

class PermissionDenied(ModelverseException):
    pass

class InvalidMode(ModelverseException):
    pass

class InterfaceMismatch(ModelverseException):
    pass

# Helper functions and configuration: do not use yourself!
taskname = None
address = None
last_output = None
mode = MODE_UNCONNECTED
prev_mode = None
current_model = None

def _goto_mode(new_mode, model_name=None):
    global mode

    if mode == MODE_MANUAL and new_mode == MODE_MODIFY:
        if model_name != None and current_model != model_name:
            raise InvalidMode("Mode error: cannot modify other models!")
        else:
            return
    elif mode == MODE_MODELLING and new_mode == MODE_MODIFY:
        # Are in root view, but want to modify a model
        model_modify(model_name)
    elif mode == MODE_MODIFY and new_mode == MODE_MODIFY and model_name != None and current_model != model_name:
        # Are in modify mode, but want to modify a different model
        model_exit()
        model_modify(model_name)
    elif mode == MODE_MODIFY and new_mode == MODE_MODELLING:
        model_exit()
    elif mode == new_mode:
        return
    else:
        # Go to a mode that we have no automatic transfer to: raise exception
        raise InvalidMode("Required mode: %s, current mode: %s" % (new_mode, mode))

def _input(value):
    # Ugly json encoding of primitives
    #print("[IN] %s" % value)
    if isinstance(value, type([])):
        value = json.dumps(value)
        urllib2.urlopen(urllib2.Request(address, urllib.urlencode({"op": "set_input", "data": value, "taskname": taskname}))).read()
    else:
        value = json.dumps(value)
        #print("Set input: " + str(value))
        urllib2.urlopen(urllib2.Request(address, urllib.urlencode({"op": "set_input", "value": value, "taskname": taskname}))).read()

def _input_raw(value, taskname):
    # Ugly json encoding of primitives
    urllib2.urlopen(urllib2.Request(address, urllib.urlencode({"op": "set_input", "value": value, "taskname": taskname}))).read()

def _compile_AL(code):
    # Compile an action language file and send the compiled code
    code_fragments = code.split("\n")
    code_fragments = [i for i in code_fragments if i.strip() != ""]
    code_fragments = [i.replace("    ", "\t") for i in code_fragments]
    initial_tabs = min([len(i) - len(i.lstrip("\t")) for i in code_fragments])
    code_fragments = [i[initial_tabs:] for i in code_fragments]
    code_fragments.append("")
    code = "\n".join(code_fragments)

    with open("__constraint.alc", "w") as f:
        f.write(code)
        f.flush()

    return do_compile("__constraint.alc", COMPILER_PATH + "/grammars/actionlanguage.g", "CS")

def _compile_model(code):
    # Compile a model and send the compiled graph
    # First change multiple spaces to a tab
    code_fragments = code.split("\n")
    code_fragments = [i for i in code_fragments if i.strip() != ""]
    code_fragments = [i.replace("    ", "\t") for i in code_fragments]
    initial_tabs = min([len(i) - len(i.lstrip("\t")) for i in code_fragments])
    code_fragments = [i[initial_tabs:] for i in code_fragments]
    code_fragments.append("")
    code = "\n".join(code_fragments)

    with open("__model.mvc", "w") as f:
        f.write(code)
        f.flush()

    return do_compile("__model.mvc", COMPILER_PATH + "/grammars/modelling.g", "M") + ["exit"]

def _output(expected=None):
    try:
        global last_output
        last_output = json.loads(urllib2.urlopen(urllib2.Request(address, urllib.urlencode({"op": "get_output", "taskname": taskname}))).read())
        #print("[OUT] %s" % last_output)
    except:
        raise UnknownError()
    if expected is not None and last_output != expected:
        raise InterfaceMismatch(_last_output(), expected)
    return last_output

def _last_output():
    return last_output

# Raise common exceptions
def _handle_output(requested=None, split=None):
    value = _output()
    if value.startswith("Model exists: "):
        raise ModelExists(value.split(": ", 1)[1])
    elif value.startswith("Permission denied"):
        raise PermissionDenied(value.split(": ", 1)[1])
    elif value.startswith("Model not found: "):
        raise UnknownModel(value.split(": ", 1)[1])
    elif value.startswith("Element not found: "):
        raise UnknownIdentifier(value.split(": ", 1)[1])
    elif value.startswith("Element exists: "):
        raise ElementExists(value.split(": ", 1)[1])
    elif value.startswith("Attribute not found: "):
        raise NoSuchAttribute(value.split(": ", 1)[1])
    elif requested is not None and value.startswith(requested):
        if split is None:
            return value
        else:
            splitted = value.strip().split(split, 1)
            if len(splitted) == 1:
                return ""
            else:
                return splitted[1].rstrip()
    else:
        raise InterfaceMismatch(value)
    
# Main MvC operations
def init(address_param="http://127.0.0.1:8001", timeout=20.0):
    """Starts up the connection to the Modelverse."""
    # return None
    # raises ConnectionError
    # raises UnknownError
    # raises InvalidMode
    global mode
    global address
    global taskname
    address = address_param
    start_time = time.time()
    taskname = random.random()
    while 1:
        try:
            _input_raw('"%s"' % taskname, "task_manager")
            mode = MODE_UNAUTHORIZED
            break
        except URLError as e:
            if time.time() - start_time > timeout:
                raise ConnectionError(e.reason)
            else:
                time.sleep(0.1)

def login(username, password):
    """Log in a user, if user doesn't exist, it is created."""
    # return None
    # raises UnknownError
    # raises PermissionDenied
    # raises InterfaceMismatch
    global mode
    _goto_mode(MODE_UNAUTHORIZED)

    _output("Log on as which user?")
    _input(username)
    if _output() == "Password for existing user?":
        _input(password)
        if _output() == "Welcome to the Model Management Interface v2.0!":
            _output("Use the 'help' command for a list of possible commands")
            _input("quiet")
            mode = MODE_MODELLING
        elif _last_output() == "Wrong password!":
            raise PermissionDenied()
        else:
            raise InterfaceMismatch(_last_output())
    elif _last_output() == "This is a new user: please give password!":
        _input(password)
        _output("Please repeat the password")
        _input(password)
        if _output() == "Passwords match!":
            _output("Welcome to the Model Management Interface v2.0!")
            _output("Use the 'help' command for a list of possible commands")
            _input("quiet")
            mode = MODE_MODELLING
        elif _last_output() == "Not the same password!":
            # We just sent the same password, so it should be identical, unless the interface changed
            raise InterfaceMismatch(_last_output())
        else:
            raise InterfaceMismatch(_last_output())
    else:
        raise InterfaceMismatch(_last_output())

def model_add(model_name, metamodel_name, model_code=None):
    """Instantiate a new model."""
    # return None
    # raises UnknownModel
    # raises ModelExists
    # raises UnknownError
    # raises PermissionDenied
    # raises CompilationError
    _goto_mode(MODE_MODELLING)

    # Do this before creating the model, as otherwise compilation errors would make us inconsistent
    if model_code is not None:
        try:
            compiled = _compile_model(model_code)
        except:
            raise CompilationError()
    else:
        compiled = ["exit"]

    _input(["model_add", metamodel_name, model_name])
    _handle_output("Waiting for model constructors...")
    _input(compiled)
    _output("Success")

def model_delete(model_name):
    """Delete an existing model."""
    _goto_mode(MODE_MODELLING)

    _input(["model_delete", model_name])
    _handle_output("Success")

def model_modify(model_name):
    """Modify an existing model."""
    # raises UnknownModel
    # raises PermissionDenied
    # raises UnknownError
    global mode
    global prev_mode

    if mode == MODE_MANUAL:
        prev_mode = MODE_MANUAL
        mode = MODE_MODIFY
        return None

    _goto_mode(MODE_MODELLING)
    prev_mode = MODE_MODELLING

    _input(["model_modify", model_name])
    _handle_output("Success")
    global current_model
    current_model = model_name
    # Mode has changed
    mode = MODE_MODIFY
    _output("Model loaded, ready for commands!")

def model_list():
    """List all models."""
    # return [(model1, metamodel1), (model2, metamodel2), ...]
    # raises UnknownError
    _goto_mode(MODE_MODELLING)
    _input("model_list")
    output = _handle_output("Success: ", split=" ")
    if output == "":
        return set([])

    lst = set([])
    value = output.strip().split("\n")
    for v in value:
        m, mm = v.split(":")
        m = m.strip()
        mm = mm.strip()
        lst.add((m, mm))

    return lst

def model_list_full():
    """List full information on all models."""
    # return [(model1, metamodel1, owner1, group1, permissions1), (model2, metamodel2, owner2, group2, permissions2), ...]
    # raises UnknownError
    _goto_mode(MODE_MODELLING)
    _input("model_list_full")
    output = _handle_output("Success: ", split=" ")
    if output == "":
        return set([])

    lst = set([])
    value = output.strip().split("\n")
    for v in value:
        m, mm = v.split(":")
        m = m.strip()
        mm = mm.strip()
        perm, own, grp, m = m.split(" ")
        lst.add((m, mm, own, grp, perm))

    return lst

def verify(model):
    """Verify if a model conforms to its metamodel."""
    # return "verification_result"
    # raises UnknownError
    # raises UnknownModel
    _goto_mode(MODE_MODELLING)
    _input(["verify", model])
    return _handle_output("Success: ", split=" ")

def model_overwrite(model_name, new_model=None):
    """Upload a new model and overwrite an existing model."""
    # return None
    # raises UnknownModel
    # raises PermissionDenied
    # raises CompilationError
    # raises UnknownError
    if mode not in [MODE_MODELLING, MODE_MODIFY, MODE_MANUAL]:
        raise InvalidMode()

    if new_model is not None:
        try:
            compiled = _compile_model(new_model)
        except Exception as e:
            raise CompilationError(e)
    else:
        compiled = ["exit"]

    if mode == MODE_MODIFY:
        _goto_mode(MODE_MODELLING)

    if mode == MODE_MODELLING:
        _input(["model_overwrite", model_name])
    elif mode == MODE_MANUAL:
        _input("upload")
    else:
        raise InvalidMode()

    _handle_output("Waiting for model constructors...")
    _input(compiled)
    _output("Success")

def user_logout():
    """Log out the current user and break the connection."""
    # return None
    # raises UnknownError
    global mode
    _goto_mode(MODE_MODELLING)
    _input("exit")
    mode = MODE_UNCONNECTED

def user_delete():
    """Removes the current user and break the connection."""
    # return None
    # raises UnknownError
    global mode
    _goto_mode(MODE_MODELLING)
    _input("self-destruct")
    mode = MODE_UNCONNECTED

def model_render(model, mapper):
    """Fetch a rendered verion of a model."""
    # return JSON_representation
    # raises UnknownError
    # raises UnknownIdentifier
    # raises InterfaceMismatch
    _goto_mode(MODE_MODELLING)

    _input(["model_render", model, mapper])
    return _handle_output("Success: ", split=" ")

def transformation_between(source, target):
    _goto_mode(MODE_MODELLING)

    _input(["transformation_between", source, target])
    output = _handle_output("Success: ", split=" ")
    if output == "":
        return set([])
    lst = set([v for v in output.split("\n")])

def transformation_add_MT_language(metamodels, RAMified_name):
    """Create a new Model Transformation language out of a set of metamodels."""
    _goto_mode(MODE_MODELLING)

    _input(["transformation_add_MT_language"] + metamodels + ["", RAMified_name])
    _handle_output("Success")

def transformation_add_MT(RAMified_metamodel, source_metamodels, target_metamodels, operation_name, code):
    """Create a new model transformation."""
    _goto_mode(MODE_MODELLING)

    try:
        compiled = _compile_model(code)
    except Exception as e:
        raise CompilationError(e)

    _input(["transformation_add_MT", RAMified_metamodel] + source_metamodels + [""] + target_metamodels + ["", operation_name])
    _handle_output("Waiting for model constructors...")
    _input(compiled)
    _handle_output("Success")

def transformation_add_AL(source_metamodels, target_metamodels, operation_name, code):
    """Create a new action language model, which can be executed."""
    _goto_mode(MODE_MODELLING)

    try:
        compiled = _compile_AL(code)
    except Exception as e:
        raise CompilationError(e)

    _input(["transformation_add_AL"] + source_metamodels + [""] + target_metamodels + [""] + [operation_name])
    _handle_output("Waiting for code constructors...")
    _input(compiled)
    _output("Success")

def transformation_add_MANUAL(source_metamodels, target_metamodels, operation_name):
    """Create a new manual model operation."""
    _goto_mode(MODE_MODELLING)

    _input(["transformation_add_MANUAL"] + source_metamodels + [""] + target_metamodels + [""] + [operation_name])
    _handle_output("Success")

def transformation_execute_AL(operation_name, input_models_dict, output_models_dict, callback=lambda i: None):
    """Execute an existing model operation."""
    global mode
    _goto_mode(MODE_MODELLING)

    mv_dict_rep = []
    for key, value in input_models_dict.items():
        mv_dict_rep += [key, value]
    mv_dict_rep += [""]
    for key, value in output_models_dict.items():
        mv_dict_rep += [key, value]
    mv_dict_rep += [""]

    _input(["transformation_execute", operation_name] + mv_dict_rep)
    _handle_output("Success: ready for AL execution")

    # We are now executing, so everything we get is part of the dialog, except if it is the string for transformation termination
    while _output() not in ["Success", "Failure"]:
        mode = MODE_DIALOG
        reply = callback(_last_output())
        mode = MODE_MODELLING
        if reply is not None:
            _input(reply)

    # Got termination message, so we are done!
    if _last_output() == "Success":
        return True
    else:
        return False

def transformation_execute_MANUAL(operation_name, input_models_dict, output_models_dict, callback=lambda i: None):
    """Execute an existing model operation."""
    global mode
    _goto_mode(MODE_MODELLING)

    mv_dict_rep = []
    for key, value in input_models_dict.items():
        mv_dict_rep += [key, value]
    mv_dict_rep += [""]
    for key, value in output_models_dict.items():
        mv_dict_rep += [key, value]
    mv_dict_rep += [""]

    _input(["transformation_execute", operation_name] + mv_dict_rep)
    _handle_output("Success: ready for MANUAL execution")

    # Skip over the begin of mini_modify
    _output() # Please perform manual operation X
    _output() # Model loaded, ready for commands

    # We are now executing, so everything we get is part of the dialog, except if it is the string for transformation termination
    mode = MODE_MANUAL
    callback()
    # Finished, so leave
    _input("exit")
    mode = MODE_MODELLING

    # Got termination message, so we are done!
    if _output() == "Success":
        return True
    else:
        return False

def transformation_execute_MT(operation_name, input_models_dict, output_models_dict, callback=lambda i: None):
    """Execute an existing model operation."""
    global mode
    _goto_mode(MODE_MODELLING)

    mv_dict_rep = []
    for key, value in input_models_dict.items():
        mv_dict_rep += [key, value]
    mv_dict_rep += [""]
    for key, value in output_models_dict.items():
        mv_dict_rep += [key, value]
    mv_dict_rep += [""]

    _input(["transformation_execute", operation_name] + mv_dict_rep)
    _handle_output("Success: ready for MT execution")

    # We are now executing, so everything we get is part of the dialog, except if it is the string for transformation termination
    while _output() not in ["Success", "Failure"]:
        mode = MODE_DIALOG
        reply = callback(_last_output())
        mode = MODE_MODELLING
        if reply is not None:
            _input(reply)

    # Got termination message, so we are done!
    if _last_output() == "Success":
        return True
    else:
        return False

def transformation_list():
    """List existing model operations."""
    _goto_mode(MODE_MODELLING)

    _input("transformation_list")
    output = _handle_output("Success: ", split=" ")
    if output == "":
        return set([])

    lst = set([])
    value = output.strip().split("\n")
    for v in value:
        t, m = v.strip().split(" ", 1)
        t = t[1:-1].strip()
        m = m.strip().split(":")[0].strip()
        lst.add((t, m))

    return lst

def transformation_RAMify(metamodel_name, RAMified_metamodel_name):
    """Ramify an existing metamodel."""
    _goto_mode(MODE_MODELLING)

    _input(["transformation_RAMify", metamodel_name, RAMified_metamodel_name])
    _handle_output("Success")

def process_execute(process_name, prefix, callbacks):
    """Execute a process model."""
    global mode
    _goto_mode(MODE_MODELLING)

    _input(["process_execute", process_name, prefix])
    _handle_output("Success")

    while _output() != "Success":
        output = _last_output()
        if output.startswith("Enacting "):
            # Next activity!
            t = output.split(" ", 1)[1].split(":", 1)[0]
            name = output.split(": ", 1)[1]
            if name in callbacks:
                callback = callbacks[name]
                if t == "ModelTransformation" or t == "ActionLanguage":
                    while not (_output().startswith("Enacting ") or _last_output() == "Success"):
                        mode = MODE_DIALOG
                        reply = callback(_last_output())
                        mode = MODE_MODELLING
                        if reply is not None:
                            _input(reply)
                elif t == "ManualOperation":
                    _output() # Please perform manual operation X
                    _output() # Model loaded, ready for commands
                    mode = MODE_MANUAL
                    callback()
                    _input("exit")
                    mode = MODE_MODELLING

def permission_modify():
    """Modify permissions of a model."""
    raise NotImplementedError()

def permission_owner():
    """Modify the owning user of a model."""
    raise NotImplementedError()

def permission_group():
    """Modify the owning group of a model."""
    raise NotImplementedError()

def group_create():
    """Create a new group."""
    raise NotImplementedError()

def group_delete():
    """Delete a group of which you are an owner."""
    raise NotImplementedError()

def group_owner_add():
    """Add a new owning user to a group you own."""
    raise NotImplementedError()

def group_owner_delete():
    """Delete an owning user to a group you own."""
    raise NotImplementedError()

def group_join():
    """Add a new user to a group you own."""
    raise NotImplementedError()

def group_kick():
    """Delete a user from a group you own."""
    raise NotImplementedError()

def group_list():
    """List existing groups."""
    raise NotImplementedError()

def admin_promote():
    """Promote a user to admin status."""
    raise NotImplementedError()

def admin_demote():
    """Demote a user from admin status."""
    raise NotImplementedError()

# Actual operations on the model
def element_list(model_name):
    """Return a list of all IDs and the type of the element"""
    # return [(name1, type1), (name2, type2), ...]
    # raises UnknownError
    _goto_mode(MODE_MODIFY, model_name)

    _input("list_full")
    lst = set([])
    output = _handle_output("Success: ", split=" ")
    if output == "":
        return set([])
    for v in output.split("\n"):
        m, mm = v.split(":")
        m = m.strip()
        mm = mm.strip()
        lst.add((m, mm))
    return lst

def types(model_name):
    """Return a list of all types usable in the model"""
    # return [type1, type2, ...]
    # raises UnknownError
    _goto_mode(MODE_MODIFY, model_name)

    _input("types")
    lst = set([])
    output = _handle_output("Success: ", split=" ")
    if output == "":
        return set([])
    for v in output.split("\n"):
        m, mm = v.split(":")
        m = m.strip()
        lst.add(m)
    return lst

def types_full(model_name):
    """Return a list of full types usable in the model"""
    # return [(type1, typetype1), (type2, typetype2), ...]
    # raises UnknownError
    _goto_mode(MODE_MODIFY, model_name)

    _input("types")
    lst = set([])
    output = _handle_output("Success: ", split=" ")
    if output == "":
        return set([])
    for v in output.split("\n"):
        m, mm = v.split(":")
        m = m.strip()
        mm = mm.strip()
        lst.add((m, mm))
    return lst

def read(model_name, ID):
    """Return a tuple of information on the element: its type and source/target (None if not an edge)"""
    # return (type, (source, target))
    # raises UnknownError
    # raises UnknownIdentifier
    _goto_mode(MODE_MODIFY, model_name)

    _input(["read", ID])
    output = _handle_output("Success: ", split=" ")
    v = output.split("\n")
    t = v[1].split(":")[1].strip()
    if (not v[0].startswith("Source:")):
        rval = (t, None)
    else:
        src = v[0].split(":")[1].strip()
        trg = v[1].split(":")[1].strip()
        rval = (t, (src, trg))
    return rval

def read_attrs(model_name, ID):
    """Return a dictionary of attribute value pairs"""
    # return {attr1: value1, attr2: value2, ...}
    # raises UnknownError
    # raises UnknownIdentifier
    _goto_mode(MODE_MODIFY, model_name)

    _input(["read", ID])
    output = _handle_output("Success: ", split=" ")
    v = output.split("\n")
    searching = True
    rval = {}
    for r in v:
        if searching:
            if r == "Attributes:":
                # Start working on attributes
                searching = False
        else:
            key, value = r.split(":", 1)
            _, value = value.split("=", 1)
            key = json.loads(key.strip())
            value = value.strip()
            if value == "None":
                value = None
            elif value == "True":
                value = True
            elif value == "False":
                value = False
            else:
                value = json.loads(value)
            rval[key] = value
    return rval

def instantiate(model_name, typename, edge=None, ID=""):
    """Create a new instance of the specified typename, between the selected elements (if not None), and with the provided ID (if any)"""
    # return instantiated_ID
    # raises UnknownError
    # raises UnknownType
    # raises UnknownIdentifier
    # raises NotAnEdge
    _goto_mode(MODE_MODIFY, model_name)

    if edge is None:
        _input(["instantiate_node", typename, ID])
    else:
        _input(["instantiate_edge", typename, ID, edge[0], edge[1]])
    return _handle_output("Success: ", split=" ")

def delete_element(model_name, ID):
    """Delete the element with the given ID"""
    # return None
    # raises UnknownError
    # raises UnknownIdentifier
    _goto_mode(MODE_MODIFY, model_name)

    _input(["delete", ID])
    _handle_output("Success")

def attr_assign(model_name, ID, attr, value):
    """Assign a value to an attribute"""
    # return None
    # raises UnknownError
    # raises UnknownIdentifier
    # raises NoSuchAttribute
    # raises UnsupportedValue
    _goto_mode(MODE_MODIFY, model_name)

    _input(["attr_add", ID, attr, value])
    _handle_output("Success")

def attr_assign_code(model_name, ID, attr, code):
    """Assign a piece of Action Language code to the attribute"""
    # return None
    # raises UnknownError
    # raises UnknownIdentifier
    # raises NoSuchAttribute
    # raises UnsupportedValue
    try:
        compiled = _compile_AL(code)
    except Exception as e:
        raise CompilationError(e)

    _goto_mode(MODE_MODIFY, model_name)

    _input(["attr_add", ID, attr])
    _handle_output("Waiting for code constructors...")
    _input(compiled)
    _output("Success")

def attr_delete(model_name, ID, attr):
    """Remove an attribute."""
    _goto_mode(MODE_MODIFY, model_name)

    _input(["attr_del", ID, attr])
    _handle_output("Success")

def read_outgoing(model_name, ID, typename):
    """Returns a list of all outgoing associations of a specific type ("" = all)"""
    # return [name1, name2, ...]
    # raises UnknownError
    # raises UnknownIdentifier
    _goto_mode(MODE_MODIFY, model_name)

    _input(["read_outgoing", ID, typename])
    output = _handle_output("Success: ", split=" ")
    if output == "":
        return set([])
    else:
        return set(output.split("\n"))

def read_incoming(model_name, ID, typename):
    """Returns a list of all incoming associations of a specific type ("" = all)"""
    # return [name1, name2, ...]
    # raises UnknownError
    # raises UnknownIdentifier
    # raises UnknownType
    _goto_mode(MODE_MODIFY, model_name)

    _input(["read_incoming", ID, typename])
    output = _handle_output("Success: ", split=" ")
    if output == "":
        return set([])
    else:
        return set(output.split("\n"))

def read_association_source(model_name, ID):
    """Returns the source of an association."""
    # returns name
    # raises UnknownError
    # raises UnknownIdentifier
    # raises NotAnAssociation
    _goto_mode(MODE_MODIFY, model_name)

    _input(["read_association_source", ID])
    return _handle_output("Success: ", split=" ")

def read_association_destination(model_name, ID):
    """Returns the destination of an association."""
    # returns name
    # raises UnknownError
    # raises UnknownIdentifier
    # raises NotAnAssociation
    _goto_mode(MODE_MODIFY, model_name)

    _input(["read_association_destination", ID])
    return _handle_output("Success: ", split=" ")

def model_exit():
    """Leave model modify mode."""
    # return None
    # raises UnknownError
    global mode
    global prev_mode

    if prev_mode == MODE_MANUAL:
        mode = MODE_MANUAL
        return

    if mode != MODE_MODIFY:
        raise InvalidMode()

    _input("exit")
    _output("Success")
    mode = MODE_MODELLING