Checks if the environment.yml file is empty and if it contains a dictionary.

If the environment.yml is empty, an empty dictionary is instantiated.
This will rise an TypeError if the environment.yml file contains a list
or other data structure different than a dictionary.

repo2docker/buildpacks/conda/__init__.py

@@ -136,6 +136,12 @@ class CondaBuildPack(BaseImage):
             py_version = None
             with open(environment_yml) as f:
                 env = YAML().load(f)
+                # check if the env file is empty, if so instantiate an empty dictionary.
+                if env is None:
+                    env = {}
+                # check if the env file has a dictionary not a list or other data structure.
+                if not isinstance(env, dict):
+                    raise TypeError("environment.yml should contain a dictionary. Got %r" % env)
                 for dep in env.get('dependencies', []):
                     if not isinstance(dep, str):
                         continue

tests/test_env_yml.py

@@ -0,0 +1,23 @@
+"""
+Test if the environment.yml is empty or it constains other data structure than a dictionary
+"""
+import os
+import sys
+import pytest
+from repo2docker import buildpacks
+
+#@pytest.mark.xfail(reason="Not yet implemented")
+def test_empty_env_yml(tmpdir):
+    tmpdir.chdir()
+    p = tmpdir.join("environment.yml")
+    bp = buildpacks.CondaBuildPack()
+    with pytest.raises(Exception):
+        py_ver = bp.python_version()
+
+def test_no_dict_env_yml(tmpdir):
+    tmpdir.chdir()
+    q = tmpdir.join("environment.yml")
+    q.write("list/n ,string")
+    bq = buildpacks.CondaBuildPack()
+    with pytest.raises(Exception):
+        py_ver = bq.python_version()