Warn user about empty namespaces

Sometimes extension-related parameters seem to end up not persisted to
`setup.cfg`.

Issue #506, shows that it might happen for the `namespace` extension,
and via a reaction chain cause errors during updates.

The changes implemented in this commit try to prevent this reaction
chain of errors happening, while also instructing the user on how to
workaround in this situation.

src/pyscaffold/extensions/namespace.py

@@ -79,12 +79,13 @@ def prepare_namespace(namespace_str: str) -> List[str]:
 
 def enforce_namespace_options(struct: Structure, opts: ScaffoldOpts) -> ActionParams:
     """Make sure options reflect the namespace usage."""
-    opts.setdefault("namespace", None)
+    opts.setdefault("namespace", "")
 
     if opts["namespace"]:
         opts["ns_list"] = prepare_namespace(opts["namespace"])
         opts["root_pkg"] = opts["ns_list"][0]
         opts["qual_pkg"] = ".".join([opts["ns_list"][-1], opts["package"]])
+        opts["namespace"] = opts["namespace"].strip()
 
     return struct, opts
 
@@ -99,7 +100,11 @@ def add_namespace(struct: Structure, opts: ScaffoldOpts) -> ActionParams:
     Returns:
         Directory structure as dictionary of dictionaries and input options
     """
-    if not opts["namespace"]:
+    if not opts.get("namespace"):
+        msg = "Using the `Namespace` extension with an empty namespace string/None. "
+        msg += "You can try a valid string with the `--namespace` option in `putup` "
+        msg += "(PyScaffold CLI) the arguments in `create_project` (PyScaffold API)."
+        logger.warning(msg)
         return struct, opts
 
     namespace = opts["ns_list"][-1].split(".")

tests/extensions/test_namespace.py

@@ -27,12 +27,14 @@ def test_prepare_namespace():
         prepare_namespace("com.blue-yonder")
 
 
-def test_add_namespace():
+def test_add_namespace(isolated_logger, caplog):
     args = ["project", "-p", "package", "--namespace", "com.blue_yonder"]
     opts = parse_args(args)
     opts["ns_list"] = prepare_namespace(opts["namespace"])
     struct = {"src": {"package": {"file1": "Content"}}}
+
     ns_struct, _ = add_namespace(struct, opts)
+
     ns_pkg_struct = ns_struct["src"]
     assert "project" not in set(ns_pkg_struct.keys())
     assert "package" not in set(ns_pkg_struct.keys())
@@ -42,6 +44,23 @@ def test_add_namespace():
     submodules = set(ns_pkg_struct["com"]["blue_yonder"].keys())
     assert "package" in submodules
 
+    # warnings should not be logged
+    log = caplog.text
+    unexpected_log = ("empty namespace", "valid string", "the `--namespace` option")
+    for text in unexpected_log:
+        assert text not in log
+
+
+def test_add_namespace_empty(isolated_logger, caplog):
+    # When trying to add a namespace without actually providing a namespace string
+    add_namespace({}, {})
+
+    # then something should be logged,
+    log = caplog.text
+    expected_log = ("empty namespace", "valid string", "the `--namespace` option")
+    for text in expected_log:
+        assert text in log
+
 
 def test_create_project_with_namespace(tmpfolder):
     # Given options with the namespace extension,