Add check command, remove _ast import

- The check command checks to see if there is anything missing from the
  specified output file.

- All types in the `_ast' module are also exported from the `ast'
  module, so there is no need to use `_ast' explicitly.

- Don't raise an exception when a value from the specification doesn't
  match a predicate, the `check' command will take care of this.

- Place some functions outside of their previous classes. These are used
  by several classes/methods and they need no state from these classes.

topple/topple.py

@@ -1,6 +1,5 @@
 from __future__ import print_function
 
-import _ast
 import ast
 import os
 import os.path
@@ -21,12 +20,34 @@ def write_settings(specs, out, values=None):
           .format(file=specs.OUTPUT_FILE), file=out)
 
     for key, spec in specs.SETTINGS:
-        value = values.get(key, spec['initial'])
-
-        if (spec['predicate'](value)):
+        value = values.get(key, spec[1])
         print(key, repr(value), sep=' = ', file=out)
+
+
+def get_node_value(node):
+    if isinstance(node, ast.Name):
+        return ast.literal_eval(node.id)
+    elif isinstance(node, ast.Str):
+        return node.s
+    elif isinstance(node, ast.Num):
+        return node.n
     else:
-            raise RuntimeError('Invalid type for %s: %s' % (key, repr(value)))
+        return node
+
+
+def get_settings_from(filename):
+    res = {}
+
+    with open(filename, 'r') as f:
+        tree = ast.parse('\n'.join(f.readlines()), filename)
+        for node in ast.iter_child_nodes(tree):
+            if isinstance(node, ast.Assign):
+                for target in node.targets:
+                    res[target.id] = get_node_value(node.value)
+            else:
+                print(node, node._fields)
+
+    return res
 
 
 class PreviewMixin(object):
@@ -62,35 +83,29 @@ class Generate(pycommand.CommandBase, PreviewMixin):
 class Update(pycommand.CommandBase, PreviewMixin):
     usagestr = 'usage: topple update [--preview|-p]'
 
-    def _get_value(self, node):
-        if isinstance(node, _ast.Name):
-            return eval(node.id)
-        elif isinstance(node, _ast.Str):
-            return node.s
-        elif isinstance(node, _ast.Num):
-            return node.n
-        else:
-            return node
+    def run(self):
+        sys.path.insert(0, os.getcwd())
+        filename = 'settings_local.py'
+        res = get_settings_from(filename)
+        specs = get_setting_specs(self.parentFlags['module'] or 'toppler')
+        self.write(specs.OUTPUT_FILE, specs, res)
 
-    def _get_values(self, filename):
-        res = {}
 
-        with open(filename, 'r') as f:
-            tree = ast.parse('\n'.join(f.readlines()), filename)
-            for node in ast.iter_child_nodes(tree):
-                if isinstance(node, _ast.Assign):
-                    res[node.targets[0].id] = self._get_value(node.value)
-                else:
-                    print(node, node._fields)
-
-        return res
+class Check(pycommand.CommandBase):
+    usagestr = 'usage: topple check'
 
     def run(self):
         sys.path.insert(0, os.getcwd())
         filename = 'settings_local.py'
-        res = self._get_values(filename)
+        res = get_settings_from(filename)
         specs = get_setting_specs(self.parentFlags['module'] or 'toppler')
-        self.write(specs.OUTPUT_FILE, specs, res)
+
+        for key, spec in specs.SETTINGS:
+            if key not in res:
+                print('Missing setting:', key)
+            elif not spec[0](res[key]):
+                print('Value "%s" for %s does not pass predicate `%s\''
+                      % (res[key], key, spec[0].func_name))
 
 
 class Topple(pycommand.CommandBase):
@@ -99,6 +114,7 @@ class Topple(pycommand.CommandBase):
         'Commands:\n'
         '  generate   Generate a new settings file.\n'
         '  update     Update an existing settings file.\n'
+        '  check      Check the validity of an existing settings file.'
     )
     optionList = (('module', ('m', '<modulename>',
                               'Use specified python module.')),)
@@ -117,6 +133,8 @@ class Topple(pycommand.CommandBase):
             cmd = Generate(argv=self.args[1:])
         elif self.args[0] == 'update':
             cmd = Update(argv=self.args[1:])
+        elif self.args[0] == 'check':
+            cmd = Check(argv=self.args[1:])
         else:
             print('Undefined command: {cmd}'.format(cmd=self.args[0]))
             return 1