Introduce default_args context manager (#39964)

This adds a rather trivial context manager that lets you deduplicate repeated
arguments in directives, e.g.

```python
depends_on("py-x@1", when="@1", type=("build", "run"))
depends_on("py-x@2", when="@2", type=("build", "run"))
depends_on("py-x@3", when="@3", type=("build", "run"))
depends_on("py-x@4", when="@4", type=("build", "run"))
```

can be condensed to

```python
with default_args(type=("build", "run")):
    depends_on("py-x@1", when="@1")
    depends_on("py-x@2", when="@2")
    depends_on("py-x@3", when="@3")
    depends_on("py-x@4", when="@4")
```

The advantage is it's clear for humans, the downside it's less clear for type checkers due to type erasure.

lib/spack/docs/packaging_guide.rst

@@ -3503,6 +3503,56 @@ is equivalent to:
 Constraints from nested context managers are also combined together, but they are rarely
 needed or recommended.
 
+.. _default_args:
+
+------------------------
+Common default arguments
+------------------------
+
+Similarly, if directives have a common set of default arguments, you can
+group them together in a ``with default_args()`` block:
+
+.. code-block:: python
+
+   class PyExample(PythonPackage):
+
+       with default_args(type=("build", "run")):
+           depends_on("py-foo")
+           depends_on("py-foo@2:", when="@2:")
+           depends_on("py-bar")
+           depends_on("py-bz")
+
+The above is short for:
+
+.. code-block:: python
+
+   class PyExample(PythonPackage):
+
+       depends_on("py-foo", type=("build", "run"))
+       depends_on("py-foo@2:", when="@2:", type=("build", "run"))
+       depends_on("py-bar", type=("build", "run"))
+       depends_on("py-bz", type=("build", "run"))
+
+.. note::
+
+   The ``with when()`` context manager is composable, while ``with default_args()``
+   merely overrides the default. For example:
+
+   .. code-block:: python
+
+      with default_args(when="+feature"):
+          depends_on("foo")
+          depends_on("bar")
+          depends_on("baz", when="+baz")
+
+   is equivalent to:
+
+   .. code-block:: python
+
+      depends_on("foo", when="+feature")
+      depends_on("bar", when="+feature")
+      depends_on("baz", when="+baz")  # Note: not when="+feature+baz"
+
 .. _install-method:
 
 ------------------

lib/spack/spack/directives.py

@@ -137,6 +137,7 @@ class DirectiveMeta(type):
     _directive_dict_names: Set[str] = set()
     _directives_to_be_executed: List[str] = []
     _when_constraints_from_context: List[str] = []
+    _default_args: List[dict] = []
 
     def __new__(cls, name, bases, attr_dict):
         # Initialize the attribute containing the list of directives
@@ -199,6 +200,16 @@ def pop_from_context():
         """Pop the last constraint from the context"""
         return DirectiveMeta._when_constraints_from_context.pop()
 
+    @staticmethod
+    def push_default_args(default_args):
+        """Push default arguments"""
+        DirectiveMeta._default_args.append(default_args)
+
+    @staticmethod
+    def pop_default_args():
+        """Pop default arguments"""
+        return DirectiveMeta._default_args.pop()
+
     @staticmethod
     def directive(dicts=None):
         """Decorator for Spack directives.
@@ -259,7 +270,13 @@ def _decorator(decorated_function):
             directive_names.append(decorated_function.__name__)
 
             @functools.wraps(decorated_function)
-            def _wrapper(*args, **kwargs):
+            def _wrapper(*args, **_kwargs):
+                # First merge default args with kwargs
+                kwargs = dict()
+                for default_args in DirectiveMeta._default_args:
+                    kwargs.update(default_args)
+                kwargs.update(_kwargs)
+
                 # Inject when arguments from the context
                 if DirectiveMeta._when_constraints_from_context:
                     # Check that directives not yet supporting the when= argument

lib/spack/spack/multimethod.py

@@ -26,6 +26,7 @@
 """
 import functools
 import inspect
+from contextlib import contextmanager
 
 from llnl.util.lang import caller_locals
 
@@ -271,6 +272,13 @@ def __exit__(self, exc_type, exc_val, exc_tb):
         spack.directives.DirectiveMeta.pop_from_context()
 
 
+@contextmanager
+def default_args(**kwargs):
+    spack.directives.DirectiveMeta.push_default_args(kwargs)
+    yield
+    spack.directives.DirectiveMeta.pop_default_args()
+
+
 class MultiMethodError(spack.error.SpackError):
     """Superclass for multimethod dispatch errors"""
 

lib/spack/spack/package.py

@@ -85,7 +85,7 @@
     UpstreamPackageError,
 )
 from spack.mixins import filter_compiler_wrappers
-from spack.multimethod import when
+from spack.multimethod import default_args, when
 from spack.package_base import (
     DependencyConflictError,
     build_system_flags,