argparse: lazily construct common arguments

Continuing to shave small bits of time off startup --
`spack.cmd.common.arguments` constructs many `Args` objects at module
scope, which has to be done for all commands that import it.  Instead of
doing this at load time, do it lazily.

- [x] construct Args objects lazily

- [x] remove the module-scoped argparse fixture

- [x] make the mock config scope set dirty to False by default (like the
  regular scope)

This *seems* to reduce load time slightly

lib/spack/spack/cmd/common/arguments.py

@@ -18,9 +18,22 @@
 
 __all__ = ['add_common_arguments']
 
+#: dictionary of argument-generating functions, keyed by name
 _arguments = {}
 
 
+def arg(fn):
+    """Decorator for a function that generates a common argument.
+
+    This ensures that argument bunches are created lazily. Decorate
+    argument-generating functions below with @arg so that
+    ``add_common_arguments()`` can find them.
+
+    """
+    _arguments[fn.__name__] = fn
+    return fn
+
+
 def add_common_arguments(parser, list_of_arguments):
     """Extend a parser with extra arguments
 
@@ -32,7 +45,8 @@ def add_common_arguments(parser, list_of_arguments):
         if argument not in _arguments:
             message = 'Trying to add non existing argument "{0}" to a command'
             raise KeyError(message.format(argument))
-        x = _arguments[argument]
+
+        x = _arguments[argument]()
         parser.add_argument(*x.flags, **x.kwargs)
 
 
@@ -118,64 +132,104 @@ def __call__(self, parser, namespace, values, option_string=None):
         setattr(namespace, self.dest, deptype)
 
 
-_arguments['constraint'] = Args(
+@arg
+def constraint():
+    return Args(
         'constraint', nargs=argparse.REMAINDER, action=ConstraintAction,
         help='constraint to select a subset of installed packages')
 
-_arguments['yes_to_all'] = Args(
+
+@arg
+def yes_to_all():
+    return Args(
         '-y', '--yes-to-all', action='store_true', dest='yes_to_all',
         help='assume "yes" is the answer to every confirmation request')
 
-_arguments['recurse_dependencies'] = Args(
-    '-r', '--dependencies', action='store_true', dest='recurse_dependencies',
+
+@arg
+def recurse_dependencies():
+    return Args(
+        '-r', '--dependencies', action='store_true',
+        dest='recurse_dependencies',
         help='recursively traverse spec dependencies')
 
-_arguments['recurse_dependents'] = Args(
+
+@arg
+def recurse_dependents():
+    return Args(
         '-R', '--dependents', action='store_true', dest='dependents',
         help='also uninstall any packages that depend on the ones given '
         'via command line')
 
-_arguments['clean'] = Args(
+
+@arg
+def clean():
+    return Args(
         '--clean',
         action='store_false',
         default=spack.config.get('config:dirty'),
         dest='dirty',
         help='unset harmful variables in the build environment (default)')
 
-_arguments['deptype'] = Args(
+
+@arg
+def deptype():
+    return Args(
         '--deptype', action=DeptypeAction, default=dep.all_deptypes,
         help="comma-separated list of deptypes to traverse\ndefault=%s"
         % ','.join(dep.all_deptypes))
 
-_arguments['dirty'] = Args(
+
+@arg
+def dirty():
+    return Args(
         '--dirty',
         action='store_true',
         default=spack.config.get('config:dirty'),
         dest='dirty',
-    help='preserve user environment in the spack build environment (danger!)')
+        help="preserve user environment in spack's build environment (danger!)"
+    )
 
-_arguments['long'] = Args(
+
+@arg
+def long():
+    return Args(
         '-l', '--long', action='store_true',
         help='show dependency hashes as well as versions')
 
-_arguments['very_long'] = Args(
+
+@arg
+def very_long():
+    return Args(
         '-L', '--very-long', action='store_true',
         help='show full dependency hashes as well as versions')
 
-_arguments['tags'] = Args(
+
+@arg
+def tags():
+    return Args(
         '-t', '--tags', action='append',
         help='filter a package query by tags')
 
-_arguments['jobs'] = Args(
+
+@arg
+def jobs():
+    return Args(
         '-j', '--jobs', action=SetParallelJobs, type=int, dest='jobs',
         help='explicitly set number of parallel jobs')
 
-_arguments['install_status'] = Args(
+
+@arg
+def install_status():
+    return Args(
         '-I', '--install-status', action='store_true', default=False,
         help='show install status of packages. packages can be: '
         'installed [+], missing and needed by an installed package [-], '
         'or not installed (no annotation)')
 
-_arguments['no_checksum'] = Args(
+
+@arg
+def no_checksum():
+    return Args(
         '-n', '--no-checksum', action='store_true', default=False,
         help="do not use checksums to verify downloaded files (unsafe)")

lib/spack/spack/test/cmd/install.py

@@ -30,14 +30,6 @@
 add = SpackCommand('add')
 
 
-@pytest.fixture(scope='module')
-def parser():
-    """Returns the parser for the module command"""
-    parser = argparse.ArgumentParser()
-    spack.cmd.install.setup_parser(parser)
-    return parser
-
-
 @pytest.fixture()
 def noop_install(monkeypatch):
     def noop(*args, **kwargs):
@@ -115,7 +107,9 @@ def test_install_package_already_installed(
     (['--clean'], False),
     (['--dirty'], True),
 ])
-def test_install_dirty_flag(parser, arguments, expected):
+def test_install_dirty_flag(arguments, expected):
+    parser = argparse.ArgumentParser()
+    spack.cmd.install.setup_parser(parser)
     args = parser.parse_args(arguments)
     assert args.dirty == expected
 

lib/spack/spack/test/data/config.yaml

@@ -11,4 +11,4 @@ config:
   misc_cache: ~/.spack/cache
   verify_ssl: true
   checksum: true
-  dirty: True
+  dirty: false