Fix double-use of a generation

The first use exhausted it, so the second iteration was empty.

vdirsyncer/cli/fetchparams.py

@@ -74,20 +74,21 @@ def _fetch_value(opts, key):
         return rv
 
 
-def _strategy_command(*command):
+def _strategy_command(*command: str):
+    """Execute a user-specified command and return its output."""
     import subprocess
 
-    # normalize path of every path member
+    # Normalize path of every path member.
-    # if there is no path specified then nothing will happen
+    # If there is no path specified then nothing will happen.
-    command = map(expand_path, command)
+    # Makes this a list to avoid it being exhausted on the first iteration.
+    expanded_command = list(map(expand_path, command))
 
     try:
-        stdout = subprocess.check_output(command, universal_newlines=True)
+        stdout = subprocess.check_output(expanded_command, universal_newlines=True)
         return stdout.strip("\n")
     except OSError as e:
-        raise exceptions.UserError(
+        cmd = " ".join(expanded_command)
-            "Failed to execute command: {}\n{}".format(" ".join(command), str(e))
+        raise exceptions.UserError(f"Failed to execute command: {cmd}\n{str(e)}")
-        )
 
 
 def _strategy_prompt(text):

vdirsyncer/utils.py

@@ -19,7 +19,8 @@ SAFE_UID_CHARS = (
 _missing = object()
 
 
-def expand_path(p):
+def expand_path(p: str) -> str:
+    """Expand $HOME in a path and normalise slashes."""
     p = os.path.expanduser(p)
     p = os.path.normpath(p)
     return p