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New options and mappings in the core.add_slack_variables Transformation #3869

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94050a4
Adding an option to not touch the objective in the add slacks transfo…
emma58 Mar 6, 2026
41880c6
Fixing a few typo bugs and adding tests
emma58 Mar 6, 2026
155815b
black
emma58 Mar 6, 2026
db1946c
Adding API for getting sum of slack expression
emma58 Mar 27, 2026
53b2f14
Merge branch 'main' into add_slacks_additions
emma58 Mar 27, 2026
4c62d85
Making the add slacks transformation picky about active components--i…
emma58 Mar 27, 2026
40252d8
Black
emma58 Mar 27, 2026
ba337df
Merge branch 'main' into add_slacks_additions
jsiirola Apr 2, 2026
31a8486
Merge branch 'main' into add_slacks_additions
jsiirola Apr 3, 2026
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110 changes: 102 additions & 8 deletions pyomo/core/plugins/transform/add_slack_vars.py
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,8 @@
# software. This software is distributed under the 3-clause BSD License.
# ____________________________________________________________________________________

from collections import defaultdict

from pyomo.core import (
TransformationFactory,
Var,
Expand All @@ -17,12 +19,18 @@
value,
)

from pyomo.common.autoslots import AutoSlots
from pyomo.common.collections import ComponentMap
from pyomo.common.modeling import unique_component_name
from pyomo.core.plugins.transform.hierarchy import NonIsomorphicTransformation
from pyomo.common.config import ConfigBlock, ConfigValue
from pyomo.core.base import ComponentUID
from pyomo.common.deprecation import deprecation_warning

import logging

logger = logging.getLogger('pyomo.core')


def target_list(x):
deprecation_msg = (
Expand Down Expand Up @@ -64,9 +72,15 @@ def target_list(x):
)


import logging
class _AddSlackVariablesData(AutoSlots.Mixin):
__slots__ = ('slack_variables', 'relaxed_constraint')

logger = logging.getLogger('pyomo.core')
def __init__(self):
self.slack_variables = defaultdict(list)
self.relaxed_constraint = ComponentMap()


Block.register_private_data_initializer(_AddSlackVariablesData)


@TransformationFactory.register(
Expand All @@ -90,6 +104,22 @@ class AddSlackVariables(NonIsomorphicTransformation):
doc="This specifies the list of Constraints to add slack variables to.",
),
)
CONFIG.declare(
'add_slack_objective',
ConfigValue(
default=True,
domain=bool,
description="Whether or not to change the model objective to minimizing "
"the added slack variables.",
doc="""
Whether or not to change the problem objective to minimize the added slack
variables. If True (the default), the original objective is deactivated
and the transformation adds an objective to minimize the sum of the added
(non-negative) slack variables. If False, the transformation does not
change the model objective.
""",
),
)

def __init__(self, **kwds):
kwds['name'] = "add_slack_vars"
Expand All @@ -103,6 +133,8 @@ def _apply_to_impl(self, instance, **kwds):
config.set_value(kwds)
targets = config.targets

trans_info = instance.private_data()

if targets is None:
constraintDatas = instance.component_data_objects(
Constraint, descend_into=True
Expand All @@ -126,10 +158,6 @@ def _apply_to_impl(self, instance, **kwds):
else:
constraintDatas.append(t)

# deactivate the objective
for o in instance.component_data_objects(Objective):
o.deactivate()

# create block where we can add slack variables safely
xblockname = unique_component_name(instance, "_core_add_slack_variables")
instance.add_component(xblockname, Block())
Expand Down Expand Up @@ -161,6 +189,8 @@ def _apply_to_impl(self, instance, **kwds):
body += posSlack
# penalize slack in objective
obj_expr += posSlack
trans_info.slack_variables[cons].append(posSlack)
trans_info.relaxed_constraint[posSlack] = cons
Comment on lines +203 to +204
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@jsiirola jsiirola Mar 12, 2026

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Philosophical question: now that we are providing a useful API for mapping slacks to constraints and back, does it make sense to avoid all the name munging and just make a single indexed xblock.slacks = Var(NonNegativeIntegers, bounds=(0, None))?

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@emma58 emma58 Mar 27, 2026

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Maaaaybe. Though the way it is now, pprint is enough for debugging... I am certainly guilty of this method.

if upper is not None:
# we subtract a positive slack variable from the body:
# declare slack
Expand All @@ -171,6 +201,70 @@ def _apply_to_impl(self, instance, **kwds):
body -= negSlack
# add slack to objective
obj_expr += negSlack
trans_info.slack_variables[cons].append(negSlack)
trans_info.relaxed_constraint[negSlack] = cons

cons.set_value((lower, body, upper))
# make a new objective that minimizes sum of slack variables
xblock._slack_objective = Objective(expr=obj_expr)

if config.add_slack_objective:
# deactivate the objective
for o in instance.component_data_objects(Objective):
o.deactivate()

# make a new objective that minimizes sum of slack variables
xblock._slack_objective = Objective(expr=obj_expr)
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def get_slack_variables(self, model, constraint):
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@blnicho blnicho Apr 8, 2026

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Do you need to pass the model as an argument? Why not get the model from the constraint object directly, constraint.model()?

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@jsiirola jsiirola Apr 8, 2026

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model is probably a misnomer. It is really the "root block" that was passed to the transformation, which may or may not be the the model() (which is where the transformation hides the mapping between constraints and slack variables.

@emma58, question: would it make sense to put a private_data on every block holding a transformation target that has a pointer to the "root block" from the transformation that holds the mappings? If we do that, then we wouldn't need the model attribute (as @blnicho was suggesting).

"""Return the list of slack variables used to relax 'constraint.' Note
that if 'constraint' is one-sided, there will be a single variable in
the list, but if it is a ranged constraint (l <= expr <= u) or an
equality, there will be two variables.

Returns
-------
List of slack variables

Parameters
----------
model: ConcreteModel
A model, having had the 'core.add_slack_variables' transformation
applied to it
constraint: Constraint
A constraint that was relaxed by the transformation (either
because no targets were specified or because it was a target)
"""
slack_variables = model.private_data().slack_variables
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@jsiirola jsiirola Mar 12, 2026

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Do you get a meaningful error if get_slack_variables is called on a model that was not transformed by this Transformation?

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@emma58 emma58 Mar 27, 2026

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Wow, significantly more meaningful than I was expecting, actually:

>>> from pyomo.environ import *
>>> m = ConcreteModel()
>>> m.x = Var()
>>> m.y = Var()
>>> m.c = Constraint(expr=(3, m.x + m.y, 4))
>>> m.obj = Objective(expr= m.x - m.y)
>>> TransformationFactory('core.add_slack_variables').get_slack_variables(m, m.c)
Traceback (most recent call last):
  File "<python-input-6>", line 1, in <module>
    TransformationFactory('core.add_slack_variables').get_slack_variables(m, m.c)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/Users/esjohn/src/pyomo/pyomo/core/plugins/transform/add_slack_vars.py", line 240, in get_slack_variables
    raise ValueError(
    ...<3 lines>...
    )
ValueError: It does not appear that c is a constraint on model unknown that was relaxed by the 'core.add_slack_variables' transformation.

We were having a very smart day when we implemented the private data Blocks... This is working because, in the scope of that file, slack_variables gets created when we get the private_data, so then it's just empty and we get a good error.

if constraint in slack_variables:
return slack_variables[constraint]
else:
raise ValueError(
f"It does not appear that {constraint.name} is a constraint "
f"on model {model.name} that was relaxed by the "
f"'core.add_slack_variables' transformation."
)

def get_relaxed_constraint(self, model, slack_var):
"""Return the constraint that 'slack_var' is used to relax.

Returns
-------
Constraint

Parameters
-----------
model: ConcreteModel
A model, having had the 'core.add_slack_variables' transformation
applied to it
slack_var: Var
A variable created by the 'core.add_slack_variables' transformation to
relax a constraint.
"""
relaxed_constraints = model.private_data().relaxed_constraint
if slack_var in relaxed_constraints:
return relaxed_constraints[slack_var]
else:
raise ValueError(
f"It does not appear that {slack_var.name} is a slack variable "
f"created by applying the 'core.add_slack_variables' transformation "
f"to model {model.name}."
)
70 changes: 70 additions & 0 deletions pyomo/core/tests/transform/test_add_slacks.py
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Original file line number Diff line number Diff line change
Expand Up @@ -87,6 +87,26 @@ def test_slack_vars_added(self):
self.assertEqual(xblock._slack_plus_rule2.bounds, (0, None))
self.assertEqual(xblock._slack_plus_rule3.bounds, (0, None))

def test_mapping_api(self):
m = self.makeModel()
trans = TransformationFactory('core.add_slack_variables')
trans.apply_to(m)

slacks = trans.get_slack_variables(m, m.rule1)
self.assertEqual(len(slacks), 1)
slack = slacks[0]
self.assertIs(m.rule1, trans.get_relaxed_constraint(m, slack))

slacks = trans.get_slack_variables(m, m.rule2)
self.assertEqual(len(slacks), 2)
self.assertIs(m.rule2, trans.get_relaxed_constraint(m, slacks[0]))
self.assertIs(m.rule2, trans.get_relaxed_constraint(m, slacks[1]))

slacks = trans.get_slack_variables(m, m.rule3)
self.assertEqual(len(slacks), 1)
slack = slacks[0]
self.assertIs(m.rule3, trans.get_relaxed_constraint(m, slack))

# wrapping this as a method because I use it again later when I test
# targets
def checkRule1(self, m):
Expand Down Expand Up @@ -180,6 +200,17 @@ def test_new_obj_created(self):
),
)

def test_no_objective_change(self):
m = self.makeModel()
TransformationFactory('core.add_slack_variables').apply_to(
m, add_slack_objective=False
)

self.assertTrue(m.obj.active)
transBlock = m.component("_core_add_slack_variables")
obj = transBlock.component("_slack_objective")
self.assertIsNone(obj)

def test_badModel_err(self):
model = ConcreteModel()
model.x = Var(within=NonNegativeReals)
Expand Down Expand Up @@ -345,6 +376,30 @@ def test_error_for_non_constraint_target_in_list(self):
targets=[m.rule1, m.x],
)

def test_error_for_mapping_untransformed_constraint(self):
m = self.makeModel()
trans = TransformationFactory('core.add_slack_variables')
trans.apply_to(m, targets=[m.rule1])
with self.assertRaisesRegex(
ValueError,
f"It does not appear that {m.rule2.name} is a constraint "
f"on model {m.name} that was relaxed by the "
f"'core.add_slack_variables' transformation.",
):
cons = trans.get_slack_variables(m, m.rule2)

def test_error_for_mapping_non_slack_variables(self):
m = self.makeModel()
trans = TransformationFactory('core.add_slack_variables')
trans.apply_to(m)
with self.assertRaisesRegex(
ValueError,
f"It does not appear that {m.x} is a slack variable "
f"created by applying the 'core.add_slack_variables' transformation "
f"to model {m.name}.",
):
cons = trans.get_relaxed_constraint(m, m.x)

def test_deprecation_warning_for_cuid_target(self):
m = self.makeModel()
out = StringIO()
Expand Down Expand Up @@ -449,6 +504,21 @@ def rule1_rule(m, s):
m.obj = Objective(expr=sum(m.x[s] for s in m.S) - m.y)
return m

def test_mapping_api(self):
m = self.makeModel()
trans = TransformationFactory('core.add_slack_variables')
trans.apply_to(m)

for i in m.S:
slacks = trans.get_slack_variables(m, m.rule1[i])
self.assertEqual(len(slacks), 1)
slack = slacks[0]
self.assertIs(m.rule1[i], trans.get_relaxed_constraint(m, slack))

slacks = trans.get_slack_variables(m, m.rule2)
self.assertEqual(len(slacks), 1)
self.assertIs(m.rule2, trans.get_relaxed_constraint(m, slacks[0]))

def checkSlackVars_indexedtarget(self, transBlock):
self.assertIsInstance(transBlock.component("_slack_plus_rule1[1]"), Var)
self.assertIsInstance(transBlock.component("_slack_plus_rule1[2]"), Var)
Expand Down