Support inline default values.

Signed-off-by: Daniel Nephin <dnephin@docker.com>

compose/config/config.py

@@ -413,31 +413,35 @@ def load_services(config_details, config_file):
     return build_services(service_config)
 
 
-def interpolate_config_section(filename, config, section, environment):
-    validate_config_section(filename, config, section)
-    return interpolate_environment_variables(config, section, environment)
+def interpolate_config_section(config_file, config, section, environment):
+    validate_config_section(config_file.filename, config, section)
+    return interpolate_environment_variables(
+            config_file.version,
+            config,
+            section,
+            environment)
 
 
 def process_config_file(config_file, environment, service_name=None):
     services = interpolate_config_section(
-        config_file.filename,
+        config_file,
         config_file.get_service_dicts(),
         'service',
-        environment,)
+        environment)
 
     if config_file.version in (V2_0, V2_1):
         processed_config = dict(config_file.config)
         processed_config['services'] = services
         processed_config['volumes'] = interpolate_config_section(
-            config_file.filename,
+            config_file,
             config_file.get_volumes(),
             'volume',
-            environment,)
+            environment)
         processed_config['networks'] = interpolate_config_section(
-            config_file.filename,
+            config_file,
             config_file.get_networks(),
             'network',
-            environment,)
+            environment)
 
     if config_file.version == V1:
         processed_config = services

compose/config/interpolation.py

@@ -7,14 +7,35 @@ from string import Template
 import six
 
 from .errors import ConfigurationError
+from compose.const import COMPOSEFILE_V1 as V1
+from compose.const import COMPOSEFILE_V2_0 as V2_0
+
+
 log = logging.getLogger(__name__)
 
 
-def interpolate_environment_variables(config, section, environment):
+class Interpolator(object):
+
+    def __init__(self, templater, mapping):
+        self.templater = templater
+        self.mapping = mapping
+
+    def interpolate(self, string):
+        try:
+            return self.templater(string).substitute(self.mapping)
+        except ValueError:
+            raise InvalidInterpolation(string)
+
+
+def interpolate_environment_variables(version, config, section, environment):
+    if version in (V2_0, V1):
+        interpolator = Interpolator(Template, environment)
+    else:
+        interpolator = Interpolator(TemplateWithDefaults, environment)
 
     def process_item(name, config_dict):
         return dict(
-            (key, interpolate_value(name, key, val, section, environment))
+            (key, interpolate_value(name, key, val, section, interpolator))
             for key, val in (config_dict or {}).items()
         )
 
@@ -24,9 +45,9 @@ def interpolate_environment_variables(config, section, environment):
     )
 
 
-def interpolate_value(name, config_key, value, section, mapping):
+def interpolate_value(name, config_key, value, section, interpolator):
     try:
-        return recursive_interpolate(value, mapping)
+        return recursive_interpolate(value, interpolator)
     except InvalidInterpolation as e:
         raise ConfigurationError(
             'Invalid interpolation format for "{config_key}" option '
@@ -37,25 +58,44 @@ def interpolate_value(name, config_key, value, section, mapping):
                 string=e.string))
 
 
-def recursive_interpolate(obj, mapping):
+def recursive_interpolate(obj, interpolator):
     if isinstance(obj, six.string_types):
-        return interpolate(obj, mapping)
-    elif isinstance(obj, dict):
+        return interpolator.interpolate(obj)
+    if isinstance(obj, dict):
         return dict(
-            (key, recursive_interpolate(val, mapping))
+            (key, recursive_interpolate(val, interpolator))
             for (key, val) in obj.items()
         )
-    elif isinstance(obj, list):
-        return [recursive_interpolate(val, mapping) for val in obj]
-    else:
-        return obj
+    if isinstance(obj, list):
+        return [recursive_interpolate(val, interpolator) for val in obj]
+    return obj
 
 
-def interpolate(string, mapping):
-    try:
-        return Template(string).substitute(mapping)
-    except ValueError:
-        raise InvalidInterpolation(string)
+class TemplateWithDefaults(Template):
+    idpattern = r'[_a-z][_a-z0-9]*(?::?-[_a-z0-9]+)?'
+
+    # Modified from python2.7/string.py
+    def substitute(self, mapping):
+        # Helper function for .sub()
+        def convert(mo):
+            # Check the most common path first.
+            named = mo.group('named') or mo.group('braced')
+            if named is not None:
+                if ':-' in named:
+                    var, _, default = named.partition(':-')
+                    return mapping.get(var) or default
+                if '-' in named:
+                    var, _, default = named.partition('-')
+                    return mapping.get(var, default)
+                val = mapping[named]
+                return '%s' % (val,)
+            if mo.group('escaped') is not None:
+                return self.delimiter
+            if mo.group('invalid') is not None:
+                self._invalid(mo)
+            raise ValueError('Unrecognized named group in pattern',
+                             self.pattern)
+        return self.pattern.sub(convert, self.template)
 
 
 class InvalidInterpolation(Exception):