RF: Roll our own expandvars, because op.expandvars doesn't replace unset

variables with the empty string

fsl/add_module/routines.py

@@ -10,6 +10,8 @@ program.
 
 
 import os.path        as op
+import                   os
+import                   re
 import                   math
 import                   zlib
 import                   shutil
@@ -26,11 +28,30 @@ except ImportError: progressbar = None
 import fsl.add_module.admin as admin
 
 
-def expand(path : Union[str, pathlib.Path]):
-    """Applies ``os.path.abspath``, ``os.path.expandvars`` and
+def expandvars(path: Union[str, pathlib.Path]) -> str:
+    """Alternative to ``os.path.expandvars`` which replaces unset variables
+    with an empty string.
+    """
+
+    path     = str(path)
+    patterns = [
+        r'(\$\{(\w+)\})',
+        r'(\$(\w+))',
+    ]
+
+    for pat in patterns:
+        matches = re.findall(pat, path)
+        for fullmatch, name in matches:
+            path = path.replace(fullmatch, os.environ.get(name, ''))
+
+    return path
+
+
+def expand(path : Union[str, pathlib.Path]) -> str:
+    """Applies ``os.path.abspath``, :func:`expandvars` and
     ``os.path.expanduser`` to ``path``.
     """
-    return op.abspath(op.expandvars(op.expanduser(path)))
+    return op.abspath(expandvars(op.expanduser(path)))
 
 
 class DownloadFailed(Exception):