init: make spack.cmd.all_commands lazy

- `spack.cmd.all_commands` does a directory listing on
  `lib/spack/spack/cmd`, regardless of whether it is needed

- make this lazy so that the directory listing won't happen unless it's
  necessary.

lib/spack/docs/conf.py

@@ -77,7 +77,8 @@
 # Find all the `cmd-spack-*` references and add them to a command index
 #
 import spack
-command_names = spack.cmd.all_commands
+import spack.cmd
+command_names = spack.cmd.all_commands()
 documented_commands = set()
 for filename in glob('*rst'):
     with open(filename) as f:

lib/spack/spack/cmd/__init__.py

@@ -83,11 +83,26 @@ def cmd_name(python_name):
     return python_name.replace('_', '-')
 
 
-for file in os.listdir(spack.paths.command_path):
-    if file.endswith(".py") and not re.search(ignore_files, file):
-        cmd = re.sub(r'.py$', '', file)
-        all_commands.append(cmd_name(cmd))
-all_commands.sort()
+#: global, cached list of all commands -- access through all_commands()
+_all_commands = None
+
+
+def all_commands():
+    """Get a sorted list of all spack commands.
+
+    This will list the lib/spack/spack/cmd directory and find the
+    commands there to construct the list.  It does not actually import
+    the python files -- just gets the names.
+    """
+    global _all_commands
+    if _all_commands is None:
+        _all_commands = []
+        for file in os.listdir(spack.paths.command_path):
+            if file.endswith(".py") and not re.search(ignore_files, file):
+                cmd = re.sub(r'.py$', '', file)
+                _all_commands.append(cmd_name(cmd))
+        _all_commands.sort()
+    return _all_commands
 
 
 def remove_options(parser, *options):

lib/spack/spack/cmd/commands.py

@@ -131,7 +131,7 @@ def rst(args):
 
 @formatter
 def names(args):
-    for cmd in spack.cmd.all_commands:
+    for cmd in spack.cmd.all_commands():
         print(cmd)
 
 

lib/spack/spack/main.py

@@ -108,14 +108,14 @@ def set_working_dir():
 
 def add_all_commands(parser):
     """Add all spack subcommands to the parser."""
-    for cmd in spack.cmd.all_commands:
+    for cmd in spack.cmd.all_commands():
         parser.add_command(cmd)
 
 
 def index_commands():
     """create an index of commands by section for this help level"""
     index = {}
-    for command in spack.cmd.all_commands:
+    for command in spack.cmd.all_commands():
         cmd_module = spack.cmd.get_module(command)
 
         # make sure command modules have required properties
@@ -174,7 +174,7 @@ def format_help_sections(self, level):
             self.actions = self._subparsers._actions[-1]._get_subactions()
 
         # make a set of commands not yet added.
-        remaining = set(spack.cmd.all_commands)
+        remaining = set(spack.cmd.all_commands())
 
         def add_group(group):
             formatter.start_section(group.title)

lib/spack/spack/test/cmd/commands.py

@@ -39,7 +39,7 @@
 def test_commands_by_name():
     """Test default output of spack commands."""
     out = commands()
-    assert out.strip().split('\n') == sorted(spack.cmd.all_commands)
+    assert out.strip().split('\n') == sorted(spack.cmd.all_commands())
 
 
 def test_subcommands():