Fix ensure_venv to always install deps, even if venv exists

Previously, if a venv already existed, ensure_venv would just activate
it without checking/installing dependencies. Now it always runs pip/uv
install, which is fast when deps are already present (they get skipped)
but ensures new dependencies are installed.

Added test: test_ensure_venv_installs_new_deps

Author: benoitc

src/py.erl

@@ -763,24 +763,24 @@ ensure_venv(Path, RequirementsFile, Opts) ->
     PathStr = to_string(Path),
     ReqFileStr = to_string(RequirementsFile),
     Force = proplists:get_bool(force, Opts),
-    case venv_exists(PathStr) of
+    %% Create venv if needed
+    VenvReady = case venv_exists(PathStr) of
         true when not Force ->
-            %% Venv exists, just activate
-            activate_venv(PathStr);
+            ok;
         _ ->
-            %% Create venv
-            case create_venv(PathStr, Opts) of
+            create_venv(PathStr, Opts)
+    end,
+    case VenvReady of
+        ok ->
+            %% Always install/update dependencies (pip/uv skip existing)
+            case install_deps(PathStr, ReqFileStr, Opts) of
                 ok ->
-                    %% Install dependencies
-                    case install_deps(PathStr, ReqFileStr, Opts) of
-                        ok ->
-                            activate_venv(PathStr);
-                        {error, _} = Err ->
-                            Err
-                    end;
+                    activate_venv(PathStr);
                 {error, _} = Err ->
                     Err
-            end
+            end;
+        {error, _} = Err ->
+            Err
     end.
 
 %% @private Check if venv exists by looking for pyvenv.cfg

test/py_venv_SUITE.erl

@@ -32,6 +32,7 @@
     test_ensure_venv_creates_venv/1,
     test_ensure_venv_activates_existing/1,
     test_ensure_venv_with_requirements/1,
+    test_ensure_venv_installs_new_deps/1,
     test_ensure_venv_force_recreate/1,
     test_activate_venv/1,
     test_deactivate_venv/1,
@@ -46,6 +47,7 @@ groups() ->
         test_ensure_venv_creates_venv,
         test_ensure_venv_activates_existing,
         test_ensure_venv_with_requirements,
+        test_ensure_venv_installs_new_deps,
         test_ensure_venv_force_recreate,
         test_activate_venv,
         test_deactivate_venv,
@@ -161,6 +163,35 @@ test_ensure_venv_with_requirements(Config) ->
     true = is_binary(Version),
     ok.
 
+test_ensure_venv_installs_new_deps(Config) ->
+    TempDir = ?config(temp_dir, Config),
+    VenvPath = filename:join(TempDir, "venv"),
+    ReqFile = filename:join(TempDir, "requirements.txt"),
+
+    %% Create requirements with only six
+    ok = file:write_file(ReqFile, <<"six\n">>),
+
+    %% Create venv with six
+    ok = py:ensure_venv(VenvPath, ReqFile, [{installer, pip}]),
+
+    %% Verify six is installed
+    {ok, _} = py:eval(<<"__import__('six').__version__">>),
+
+    %% Verify toml is NOT installed yet (uncommon package)
+    {error, _} = py:eval(<<"__import__('toml').__version__">>),
+
+    %% Now update requirements to add toml
+    ok = file:write_file(ReqFile, <<"six\ntoml\n">>),
+
+    %% Deactivate and re-ensure - should install new deps without recreating venv
+    ok = py:deactivate_venv(),
+    ok = py:ensure_venv(VenvPath, ReqFile, [{installer, pip}]),
+
+    %% Verify both packages are now installed
+    {ok, _} = py:eval(<<"__import__('six').__version__">>),
+    {ok, _} = py:eval(<<"__import__('toml').__version__">>),
+    ok.
+
 test_ensure_venv_force_recreate(Config) ->
     TempDir = ?config(temp_dir, Config),
     VenvPath = filename:join(TempDir, "venv"),