Make python-constraint optional

qiskit/transpiler/passes/layout/_csp_custom_solver.py

@@ -0,0 +1,63 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2022.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+"""A custom python-constraint solver used by the :class:`~.CSPLayout` pass"""
+from time import time
+
+from qiskit.utils import optionals as _optionals
+
+# This isn't ideal usage because we will import constraint at import time
+# but to ensure the CustomSolver class is defined we need to do this.
+# If constraint is not installed this will not raise a missing library
+# exception until CSPLayout is initialized
+if _optionals.HAS_CONSTRAINT:
+    from constraint import RecursiveBacktrackingSolver
+
+    class CustomSolver(RecursiveBacktrackingSolver):
+        """A wrap to RecursiveBacktrackingSolver to support ``call_limit``"""
+
+        def __init__(self, call_limit=None, time_limit=None):
+            self.call_limit = call_limit
+            self.time_limit = time_limit
+            self.call_current = None
+            self.time_start = None
+            self.time_current = None
+            super().__init__()
+
+        def limit_reached(self):
+            """Checks if a limit is reached."""
+            if self.call_current is not None:
+                self.call_current += 1
+                if self.call_current > self.call_limit:
+                    return True
+            if self.time_start is not None:
+                self.time_current = time() - self.time_start
+                if self.time_current > self.time_limit:
+                    return True
+            return False
+
+        def getSolution(self, domains, constraints, vconstraints):
+            """Wrap RecursiveBacktrackingSolver.getSolution to add the limits."""
+            if self.call_limit is not None:
+                self.call_current = 0
+            if self.time_limit is not None:
+                self.time_start = time()
+            return super().getSolution(domains, constraints, vconstraints)
+
+        def recursiveBacktracking(self, solutions, domains, vconstraints, assignments, single):
+            """Like ``constraint.RecursiveBacktrackingSolver.recursiveBacktracking`` but
+            limited in the amount of calls by ``self.call_limit``"""
+            if self.limit_reached():
+                return None
+            return super().recursiveBacktracking(
+                solutions, domains, vconstraints, assignments, single
+            )

qiskit/transpiler/passes/layout/csp_layout.py

@@ -16,52 +16,13 @@
 found, no ``property_set['layout']`` is set.
 """
 import random
-from time import time
-from constraint import Problem, RecursiveBacktrackingSolver, AllDifferentConstraint
 
 from qiskit.transpiler.layout import Layout
 from qiskit.transpiler.basepasses import AnalysisPass
+from qiskit.utils import optionals as _optionals
 
 
-class CustomSolver(RecursiveBacktrackingSolver):
-    """A wrap to RecursiveBacktrackingSolver to support ``call_limit``"""
-
-    def __init__(self, call_limit=None, time_limit=None):
-        self.call_limit = call_limit
-        self.time_limit = time_limit
-        self.call_current = None
-        self.time_start = None
-        self.time_current = None
-        super().__init__()
-
-    def limit_reached(self):
-        """Checks if a limit is reached."""
-        if self.call_current is not None:
-            self.call_current += 1
-            if self.call_current > self.call_limit:
-                return True
-        if self.time_start is not None:
-            self.time_current = time() - self.time_start
-            if self.time_current > self.time_limit:
-                return True
-        return False
-
-    def getSolution(self, domains, constraints, vconstraints):
-        """Wrap RecursiveBacktrackingSolver.getSolution to add the limits."""
-        if self.call_limit is not None:
-            self.call_current = 0
-        if self.time_limit is not None:
-            self.time_start = time()
-        return super().getSolution(domains, constraints, vconstraints)
-
-    def recursiveBacktracking(self, solutions, domains, vconstraints, assignments, single):
-        """Like ``constraint.RecursiveBacktrackingSolver.recursiveBacktracking`` but
-        limited in the amount of calls by ``self.call_limit``"""
-        if self.limit_reached():
-            return None
-        return super().recursiveBacktracking(solutions, domains, vconstraints, assignments, single)
-
-
+@_optionals.HAS_CONSTRAINT.require_in_instance
 class CSPLayout(AnalysisPass):
     """If possible, chooses a Layout as a CSP, using backtracking."""
 
@@ -102,6 +63,9 @@ def run(self, dag):
         qubits = dag.qubits
         cxs = set()
 
+        from constraint import Problem, AllDifferentConstraint, RecursiveBacktrackingSolver
+        from qiskit.transpiler.passes.layout._csp_custom_solver import CustomSolver
+
         for gate in dag.two_qubit_ops():
             cxs.add((qubits.index(gate.qargs[0]), qubits.index(gate.qargs[1])))
         edges = set(self.coupling_map.get_edges())

qiskit/utils/optionals.py

@@ -47,6 +47,10 @@
 .. list-table::
     :widths: 25 75
 
+    * - .. py:data:: HAS_CONSTRAINT
+      - `python-constraint <https://github.com/python-constraint/python-constraint>__ is a
+        constraint satisfaction problem solver, used in the :class:`~.CSPLayout` transpiler pass.
+
     * - .. py:data:: HAS_CPLEX
       - The `IBM CPLEX Optimizer <https://www.ibm.com/analytics/cplex-optimizer>`__ is a
         high-performance mathematical programming solver for linear, mixed-integer and quadratic
@@ -142,7 +146,6 @@
       - `Z3 <https://github.com/Z3Prover/z3>`__ is a theorem prover, used in the
         :class:`.CrosstalkAdaptiveSchedule` and :class:`.HoareOptimizer` transpiler passes.
 
-
 External Command-Line Tools
 ---------------------------
 
@@ -207,6 +210,12 @@
     install="pip install qiskit-ignis",
 )
 
+HAS_CONSTRAINT = _LazyImportTester(
+    "constraint",
+    name="python-constraint",
+    install="pip install python-constraint",
+)
+
 HAS_CPLEX = _LazyImportTester(
     "cplex",
     install="pip install 'qiskit-terra[bip-mapper]'",

releasenotes/notes/constraint-optional-b6a2b2ee21211ccd.yaml

@@ -0,0 +1,12 @@
+---
+upgrade:
+  - |
+    The ``python-constraint`` dependency, which is used solely by the
+    :class:`~.CSPLayout` transpiler pass, is no longer in the requirements list
+    for the Qiskit Terra package. This is because the :class:`~.CSPLayout` pass
+    is no longer used by default in any of the preset pass managers for
+    :func:`~.transpile`. While the pass is still available, if you're using it
+    you will need to manually install ``python-contraint`` or when you
+    install ``qiskit-terra`` you can use the ``csp-layout`` extra, for example::
+
+        pip install "qiskit-terra[csp-layout]"

requirements-dev.txt

@@ -1,6 +1,7 @@
 setuptools-rust
 coverage>=4.4.0
 hypothesis>=4.24.3
+python-constraint>=1.4
 ipython<7.22.0
 ipykernel<5.5.2
 ipywidgets>=7.3.0

requirements.txt

@@ -5,7 +5,6 @@ psutil>=5
 scipy>=1.5
 sympy>=1.3
 dill>=0.3
-python-constraint>=1.4
 python-dateutil>=2.8.0
 stevedore>=3.0.0
 symengine>=0.9 ; platform_machine == 'x86_64' or platform_machine == 'aarch64' or platform_machine == 'ppc64le' or platform_machine == 'amd64' or platform_machine == 'arm64'

setup.py

@@ -33,9 +33,6 @@
     )
 
 
-csplayout_requirements = [
-    "python-constraint>=1.4",
-]
 visualization_extras = [
     "matplotlib>=3.3",
     "ipywidgets>=7.3.0",
@@ -49,6 +46,7 @@
     "z3-solver>=4.7",
 ]
 bip_requirements = ["cplex", "docplex"]
+csp_requirements = ["python-constraint>=1.4"]
 
 
 setup(
@@ -85,11 +83,11 @@
         "visualization": visualization_extras,
         "bip-mapper": bip_requirements,
         "crosstalk-pass": z3_requirements,
-        "csp-layout-pass": csplayout_requirements,
+        "csp-layout-pass": csp_requirements,
         # Note: 'all' does not include 'bip-mapper' because cplex is too fiddly and too little
         # supported on various Python versions and OSes compared to Terra.  You have to ask for it
         # explicitly.
-        "all": visualization_extras + z3_requirements + csplayout_requirements,
+        "all": visualization_extras + z3_requirements + csp_requirements,
     },
     project_urls={
         "Bug Tracker": "https://github.com/Qiskit/qiskit-terra/issues",