fix: Return JSON on environment creation dry run (#3627)

Signed-off-by: Julien Jerphanion <git@jjerphan.xyz>

libmamba/src/api/create.cpp

@@ -4,6 +4,8 @@
 //
 // The full license is in the file LICENSE, distributed with this software.
 
+#include <iostream>
+
 #include "mamba/api/configuration.hpp"
 #include "mamba/api/create.hpp"
 #include "mamba/api/install.hpp"
@@ -29,6 +31,7 @@ namespace mamba
 
         auto& create_specs = config.at("specs").value<std::vector<std::string>>();
         auto& use_explicit = config.at("explicit_install").value<bool>();
+        auto& json_format = config.at("json").get_cli_config<bool>();
 
         auto channel_context = ChannelContext::make_conda_compatible(ctx);
 
@@ -78,6 +81,21 @@ namespace mamba
                 );
             }
         }
+        else
+        {
+            if (create_specs.empty() && json_format)
+            {
+                // Just print the JSON
+                nlohmann::json output;
+                output["actions"]["FETCH"] = nlohmann::json::array();
+                output["actions"]["PREFIX"] = ctx.prefix_params.target_prefix;
+                output["dry_run"] = true;
+                output["prefix"] = ctx.prefix_params.target_prefix;
+                output["success"] = true;
+                std::cout << output.dump(2) << std::endl;
+                return;
+            }
+        }
 
         if (ctx.env_lockfile)
         {

micromamba/tests/test_create.py

@@ -1343,3 +1343,18 @@ def test_parsable_env_history_with_metadata(tmp_home, tmp_root_prefix, tmp_path)
 
     # Must not hang
     helpers.umamba_list("-p", env_prefix, "--json")
+
+
+def test_create_dry_run_json(tmp_path):
+    # Non-regression test for https://github.com/mamba-org/mamba/issues/3583
+    env_prefix = tmp_path / "env-create_dry_run_json"
+    res = helpers.create("-p", env_prefix, "--dry-run", "--json")
+
+    expected_output = {
+        "actions": {"FETCH": [], "PREFIX": str(env_prefix)},
+        "dry_run": True,
+        "prefix": str(env_prefix),
+        "success": True,
+    }
+
+    assert res == expected_output