little adjustments for dependency management

corneto/backend/__init__.py

@@ -20,15 +20,12 @@ def available_backends():
 _picos_mip_solvers = ["gurobi", "cplex", "scip", "glpk"]
 
 
-DEFAULT_BACKEND: Backend = (
-    available_backends()[0] if len(available_backends()) > 0 else NoBackend()
-)
+DEFAULT_BACKEND, = available_backends()[:1] or (NoBackend(),)
 DEFAULT_SOLVER = None
 
 if not DEFAULT_BACKEND:
-    s.LOGGER.warn(
-        "None of the supported backends found. You can install the default backend with `pip install cvxpy`."
-    )
+    s.LOGGER.warn(DEFAULT_BACKEND._error)
+
 else:
     if isinstance(DEFAULT_BACKEND, CvxpyBackend):
         from cvxpy import installed_solvers

corneto/backend/_base.py

@@ -1335,7 +1335,13 @@ def hstack(self, arg_list: Iterable[CExpression]) -> CExpression:
 
 class NoBackend(Backend):
     def __init__(self) -> None:
-        self._error = "No backend available. Please install on of the supported backends, e.g. `pip install cvxpy`."
+        self._error = (
+            "No backend found. You can install one of the "
+            "supported backend by `pip install cvxpy` or `pip install picos`."
+        )
+
+    def __bool__(self) -> bool:
+        return False
 
     def _load(self):
         return None

pyproject.toml

@@ -27,6 +27,7 @@ python = ">=3.9"
 numpy = ">=1.25"
 cvxpy-base = {version = "^1.5.0", optional = true}
 scipy = {version = ">=1.11.0", optional = true}
+picos = {version = "^2.4.1", optional = true}
 
 [tool.poetry.group.dev.dependencies]
 scipy = "^1.11.0"
@@ -59,6 +60,7 @@ pydocstyle = "^6.3.0"
 
 [tool.poetry.extras]
 highs = ["cvxpy-base"]
+picos = ["picos"]
 os = ["cvxpy-base", "scipy", "pyscipopt"]
 
 [tool.ruff]