Interpolate environment variables

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

compose/config/config.py

@@ -8,6 +8,7 @@ import six
 
 from compose.cli.utils import find_candidates_in_parent_dirs
 
+from .interpolation import interpolate_environment_variables
 from .errors import (
     ConfigurationError,
     CircularReference,
@@ -132,11 +133,11 @@ def get_config_path(base_dir):
 
 def load(config_details):
     dictionary, working_dir, filename = config_details
+    dictionary = interpolate_environment_variables(dictionary)
+
     service_dicts = []
 
     for service_name, service_dict in list(dictionary.items()):
-        if not isinstance(service_dict, dict):
-            raise ConfigurationError('Service "%s" doesn\'t have any configuration options. All top level keys in your docker-compose.yml must map to a dictionary of configuration options.' % service_name)
         loader = ServiceLoader(working_dir=working_dir, filename=filename)
         service_dict = loader.make_service_dict(service_name, service_dict)
         validate_paths(service_dict)
@@ -429,9 +430,9 @@ def resolve_volume_paths(volumes, working_dir=None):
 
 def resolve_volume_path(volume, working_dir):
     container_path, host_path = split_path_mapping(volume)
-    container_path = os.path.expanduser(os.path.expandvars(container_path))
+    container_path = os.path.expanduser(container_path)
     if host_path is not None:
-        host_path = os.path.expanduser(os.path.expandvars(host_path))
+        host_path = os.path.expanduser(host_path)
         return "%s:%s" % (expand_path(working_dir, host_path), container_path)
     else:
         return container_path

compose/config/interpolation.py

@@ -0,0 +1,69 @@
+import os
+from string import Template
+from collections import defaultdict
+
+import six
+
+from .errors import ConfigurationError
+
+
+def interpolate_environment_variables(config):
+    return dict(
+        (service_name, process_service(service_name, service_dict))
+        for (service_name, service_dict) in config.items()
+    )
+
+
+def process_service(service_name, service_dict):
+    if not isinstance(service_dict, dict):
+        raise ConfigurationError(
+            'Service "%s" doesn\'t have any configuration options. '
+            'All top level keys in your docker-compose.yml must map '
+            'to a dictionary of configuration options.' % service_name
+        )
+
+    return dict(
+        (key, interpolate_value(service_name, key, val))
+        for (key, val) in service_dict.items()
+    )
+
+
+def interpolate_value(service_name, config_key, value):
+    try:
+        return recursive_interpolate(value)
+    except InvalidInterpolation as e:
+        raise ConfigurationError(
+            'Invalid interpolation format for "{config_key}" option '
+            'in service "{service_name}": "{string}"'
+            .format(
+                config_key=config_key,
+                service_name=service_name,
+                string=e.string,
+            )
+        )
+
+
+def recursive_interpolate(obj):
+    if isinstance(obj, six.string_types):
+        return interpolate(obj, os.environ)
+    elif isinstance(obj, dict):
+        return dict(
+            (key, recursive_interpolate(val))
+            for (key, val) in obj.items()
+        )
+    elif isinstance(obj, list):
+        return map(recursive_interpolate, obj)
+    else:
+        return obj
+
+
+def interpolate(string, mapping):
+    try:
+        return Template(string).substitute(defaultdict(lambda: "", mapping))
+    except ValueError:
+        raise InvalidInterpolation(string)
+
+
+class InvalidInterpolation(Exception):
+    def __init__(self, string):
+        self.string = string