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

3 files changed, 120 insertions, 72 deletions

diff --git a/lib/spack/spack/cmd/common/arguments.py b/lib/spack/spack/cmd/common/arguments.py
index 8a38d09f03..66ed953d6e 100644
--- a/lib/spack/spack/cmd/common/arguments.py
+++ b/lib/spack/spack/cmd/common/arguments.py
@@ -18,9 +18,22 @@ from spack.util.pattern import Args
 
 __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 @@ class DeptypeAction(argparse.Action):
         setattr(namespace, self.dest, deptype)
 
 
-_arguments['constraint'] = Args(
-    'constraint', nargs=argparse.REMAINDER, action=ConstraintAction,
-    help='constraint to select a subset of installed packages')
-
-_arguments['yes_to_all'] = 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',
-    help='recursively traverse spec dependencies')
-
-_arguments['recurse_dependents'] = 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(
-    '--clean',
-    action='store_false',
-    default=spack.config.get('config:dirty'),
-    dest='dirty',
-    help='unset harmful variables in the build environment (default)')
-
-_arguments['deptype'] = 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(
-    '--dirty',
-    action='store_true',
-    default=spack.config.get('config:dirty'),
-    dest='dirty',
-    help='preserve user environment in the spack build environment (danger!)')
-
-_arguments['long'] = Args(
-    '-l', '--long', action='store_true',
-    help='show dependency hashes as well as versions')
-
-_arguments['very_long'] = Args(
-    '-L', '--very-long', action='store_true',
-    help='show full dependency hashes as well as versions')
-
-_arguments['tags'] = Args(
-    '-t', '--tags', action='append',
-    help='filter a package query by tags')
-
-_arguments['jobs'] = Args(
-    '-j', '--jobs', action=SetParallelJobs, type=int, dest='jobs',
-    help='explicitly set number of parallel jobs')
-
-_arguments['install_status'] = 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(
-    '-n', '--no-checksum', action='store_true', default=False,
-    help="do not use checksums to verify downloaded files (unsafe)")
+@arg
+def constraint():
+    return Args(
+        'constraint', nargs=argparse.REMAINDER, action=ConstraintAction,
+        help='constraint to select a subset of installed packages')
+
+
+@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')
+
+
+@arg
+def recurse_dependencies():
+    return Args(
+        '-r', '--dependencies', action='store_true',
+        dest='recurse_dependencies',
+        help='recursively traverse spec dependencies')
+
+
+@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')
+
+
+@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)')
+
+
+@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))
+
+
+@arg
+def dirty():
+    return Args(
+        '--dirty',
+        action='store_true',
+        default=spack.config.get('config:dirty'),
+        dest='dirty',
+        help="preserve user environment in spack's build environment (danger!)"
+    )
+
+
+@arg
+def long():
+    return Args(
+        '-l', '--long', action='store_true',
+        help='show dependency hashes as well as versions')
+
+
+@arg
+def very_long():
+    return Args(
+        '-L', '--very-long', action='store_true',
+        help='show full dependency hashes as well as versions')
+
+
+@arg
+def tags():
+    return Args(
+        '-t', '--tags', action='append',
+        help='filter a package query by tags')
+
+
+@arg
+def jobs():
+    return Args(
+        '-j', '--jobs', action=SetParallelJobs, type=int, dest='jobs',
+        help='explicitly set number of parallel jobs')
+
+
+@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)')
+
+
+@arg
+def no_checksum():
+    return Args(
+        '-n', '--no-checksum', action='store_true', default=False,
+        help="do not use checksums to verify downloaded files (unsafe)")
diff --git a/lib/spack/spack/test/cmd/install.py b/lib/spack/spack/test/cmd/install.py
index 0d3d5f5de1..e8240a87b1 100644
--- a/lib/spack/spack/test/cmd/install.py
+++ b/lib/spack/spack/test/cmd/install.py
@@ -30,14 +30,6 @@ env = SpackCommand('env')
 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
 
diff --git a/lib/spack/spack/test/data/config.yaml b/lib/spack/spack/test/data/config.yaml
index 2d2c2e356a..0611251189 100644
--- a/lib/spack/spack/test/data/config.yaml
+++ b/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