diff -r 33f2d56acc73 -r 93e9f448273c tests/test-rhg.t
--- a/tests/test-rhg.t	Mon Mar 01 16:18:42 2021 +0100
+++ b/tests/test-rhg.t	Mon Mar 01 20:36:06 2021 +0100
@@ -1,9 +1,10 @@
 #require rust
 
 Define an rhg function that will only run if rhg exists
+  $ RHG="$RUNTESTDIR/../rust/target/release/rhg"
   $ rhg() {
-  > if [ -f "$RUNTESTDIR/../rust/target/release/rhg" ]; then
-  >   "$RUNTESTDIR/../rust/target/release/rhg" "$@"
+  > if [ -f "$RHG" ]; then
+  >   "$RHG" "$@"
   > else
   >   echo "skipped: Cannot find rhg. Try to run cargo build in rust/rhg."
   >   exit 80
@@ -151,6 +152,27 @@
   $ rhg cat -r 1 copy_of_original
   original content
 
+Fallback to Python
+  $ rhg cat original
+  unsupported feature: `rhg cat` without `--rev` / `-r`
+  [252]
+  $ FALLBACK="--config rhg.on-unsupported=fallback"
+  $ rhg cat original $FALLBACK
+  original content
+
+  $ rhg cat original $FALLBACK --config rhg.fallback-executable=false
+  [1]
+
+  $ rhg cat original $FALLBACK --config rhg.fallback-executable=hg-non-existent
+  tried to fall back to a 'hg-non-existent' sub-process but got error $ENOENT$
+  unsupported feature: `rhg cat` without `--rev` / `-r`
+  [252]
+
+  $ rhg cat original $FALLBACK --config rhg.fallback-executable="$RHG"
+  Blocking recursive fallback. The 'rhg.fallback-executable = */rust/target/release/rhg' config points to `rhg` itself. (glob)
+  unsupported feature: `rhg cat` without `--rev` / `-r`
+  [252]
+
 Requirements
   $ rhg debugrequirements
   dotencode