Make python-constraint optional

Since Qiskit#7213 we no longer have been using the CSPLayout pass by default
in the preset passmanagers or transpile(). This is because it has been
superseded by the VF2Layout pass which is now used everywhere. While we
will keep the CSPLayout pass around for the forseeable future there is
no need to install python-constraint by default anymore since it's only
user is the CSPLayout pass, which isn't going to be commonly used
anymore now that it's not used in the default compilation path anymore.
This commit removes the python-constraint library from the requirements
list and makes it an optional dependency.

Fixes Qiskit#7726

qiskit/transpiler/passes/layout/csp_layout.py

@@ -17,51 +17,60 @@
 """
 import random
 from time import time
-from constraint import Problem, RecursiveBacktrackingSolver, AllDifferentConstraint
 
 from qiskit.transpiler.layout import Layout
 from qiskit.transpiler.basepasses import AnalysisPass
-
-
-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)
-
-
+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
+            )
+
+
+@_optionals.HAS_CONSTRAINT.require_in_instance
 class CSPLayout(AnalysisPass):
     """If possible, chooses a Layout as a CSP, using backtracking."""
 
@@ -102,6 +111,8 @@ def run(self, dag):
         qubits = dag.qubits
         cxs = set()
 
+        from constraint import Problem, AllDifferentConstraint
+
         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

@@ -292,3 +292,9 @@ def _nlopt_callback(available):
     ("pdftocairo", "-v"),
     msg="This is part of the 'poppler' set of PDF utilities",
 )
+
+HAS_CONSTRAINT = _LazyImportTester(
+    "constraint",
+    name="python-constraint",
+    install="pip install python-constraint",
+)

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 passs 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(
@@ -89,7 +87,7 @@
         # 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",