init: only imoprt jsonschema if needed

- It turns out that jsonschema is one of the more expensive imports.

- move imports of jsonschema into functions to avoid the performance hits
  for calls that don't need config.

lib/spack/spack/config.py

@@ -59,9 +59,7 @@
 from six import iteritems
 
 import yaml
-import jsonschema
 from yaml.error import MarkedYAMLError
-from jsonschema import Draft4Validator, validators
 
 import llnl.util.tty as tty
 from llnl.util.filesystem import mkdirp
@@ -72,7 +70,6 @@
 from spack.error import SpackError
 from spack.util.ordereddict import OrderedDict
 
-
 # Hacked yaml for configuration files preserves line numbers.
 import spack.util.spack_yaml as syaml
 
@@ -132,6 +129,7 @@ def _extend_with_default(validator_class):
        commented out.
 
     """
+    import jsonschema
     validate_properties = validator_class.VALIDATORS["properties"]
     validate_pattern_properties = validator_class.VALIDATORS[
         "patternProperties"]
@@ -157,16 +155,12 @@ def set_pp_defaults(validator, properties, instance, schema):
                 validator, properties, instance, schema):
             yield err
 
-    return validators.extend(validator_class, {
+    return jsonschema.validators.extend(validator_class, {
         "properties": set_defaults,
         "patternProperties": set_pp_defaults
     })
 
 
-#: the validator we use for Spack config files
-DefaultSettingValidator = _extend_with_default(Draft4Validator)
-
-
 class ConfigScope(object):
     """This class represents a configuration scope.
 
@@ -192,6 +186,7 @@ def get_section(self, section):
         return self.sections[section]
 
     def write_section(self, section):
+        import jsonschema
         filename = self.get_section_filename(section)
         data = self.get_section(section)
         try:
@@ -545,8 +540,14 @@ def _validate_section(data, schema):
     on Spack YAML structures.
 
     """
+    import jsonschema
+    if not hasattr(_validate_section, 'validator'):
+        DefaultSettingValidator = _extend_with_default(
+            jsonschema.Draft4Validator)
+        _validate_section.validator = DefaultSettingValidator
+
     try:
-        DefaultSettingValidator(schema).validate(data)
+        _validate_section.validator(schema).validate(data)
     except jsonschema.ValidationError as e:
         raise ConfigFormatError(e, data)