We are currently solving the following issue within our repository. Here is the issue text:
--- BEGIN ISSUE ---
repr for particle unions
The `__repr__` for particle unions should include the contents of `sub_types`

--- END ISSUE ---

Below are some code segments, each from a relevant file. One or more of these files may contain bugs.
--- BEGIN FILES ---
Path: `yt/data_objects/unions.py`
Content:
```
1 from yt.funcs import ensure_list
2 
3 class Union(object):
4     def __init__(self, name, sub_types):
5         self.name = name
6         self.sub_types = ensure_list(sub_types)
7 
8     def __iter__(self):
9         for st in self.sub_types:
10             yield st
11 
12 class MeshUnion(Union):
13     def __init__(self, name, sub_types):
14         super(MeshUnion, self).__init__(name, sub_types)
15
```

Path: `yt/data_objects/particle_unions.py`
Content:
```
1 from .unions import Union
2 
3 class ParticleUnion(Union):
4     def __init__(self, name, sub_types):
5         super(ParticleUnion, self).__init__(name, sub_types)
6
```
--- END FILES ---

Please first localize the bug based on the issue statement, and then generate a patch according to the `git diff` format fenced by three backticks.

Here is an example:

```diff
diff --git a/examples/server_async.py b/examples/server_async.py
--- a/examples/server_async.py
+++ b/examples/server_async.py
@@ -313,4 +313,4 @@
 
 
 if __name__ == "__main__":
-    asyncio.run(run_async_server("."), debug=True)
+    asyncio.run(run_async_server(), debug=True)
diff --git a/examples/server_sync.py b/examples/server_sync.py
--- a/examples/server_sync.py
+++ b/examples/server_sync.py
@@ -313,5 +313,5 @@
 
 
 if __name__ == "__main__":
-    server = run_sync_server(".")
+    server = run_sync_server()
     server.shutdown()

```

diff --git a/yt/data_objects/particle_unions.py b/yt/data_objects/particle_unions.py
--- a/yt/data_objects/particle_unions.py
+++ b/yt/data_objects/particle_unions.py
@@ -1,5 +1,6 @@
 from .unions import Union
 
 class ParticleUnion(Union):
+    _union_type = "particle"
     def __init__(self, name, sub_types):
         super(ParticleUnion, self).__init__(name, sub_types)
diff --git a/yt/data_objects/unions.py b/yt/data_objects/unions.py
--- a/yt/data_objects/unions.py
+++ b/yt/data_objects/unions.py
@@ -1,6 +1,7 @@
 from yt.funcs import ensure_list
 
 class Union(object):
+    _union_type = ""
     def __init__(self, name, sub_types):
         self.name = name
         self.sub_types = ensure_list(sub_types)
@@ -9,6 +10,11 @@
         for st in self.sub_types:
             yield st
 
+    def __repr__(self):
+        return "{} Union: '{}' composed of: {}".format(
+            self._union_type.capitalize(), self.name, self.sub_types)
+
 class MeshUnion(Union):
+    _union_type = "mesh"
     def __init__(self, name, sub_types):
         super(MeshUnion, self).__init__(name, sub_types)

{"golden_diff": "diff --git a/yt/data_objects/particle_unions.py b/yt/data_objects/particle_unions.py\n--- a/yt/data_objects/particle_unions.py\n+++ b/yt/data_objects/particle_unions.py\n@@ -1,5 +1,6 @@\n from .unions import Union\n \n class ParticleUnion(Union):\n+    _union_type = \"particle\"\n     def __init__(self, name, sub_types):\n         super(ParticleUnion, self).__init__(name, sub_types)\ndiff --git a/yt/data_objects/unions.py b/yt/data_objects/unions.py\n--- a/yt/data_objects/unions.py\n+++ b/yt/data_objects/unions.py\n@@ -1,6 +1,7 @@\n from yt.funcs import ensure_list\n \n class Union(object):\n+    _union_type = \"\"\n     def __init__(self, name, sub_types):\n         self.name = name\n         self.sub_types = ensure_list(sub_types)\n@@ -9,6 +10,11 @@\n         for st in self.sub_types:\n             yield st\n \n+    def __repr__(self):\n+        return \"{} Union: '{}' composed of: {}\".format(\n+            self._union_type.capitalize(), self.name, self.sub_types)\n+\n class MeshUnion(Union):\n+    _union_type = \"mesh\"\n     def __init__(self, name, sub_types):\n         super(MeshUnion, self).__init__(name, sub_types)\n", "issue": "repr for particle unions\nThe `__repr__` for particle unions should include the contents of `sub_types`\n", "before_files": [{"content": "from yt.funcs import ensure_list\n\nclass Union(object):\n    def __init__(self, name, sub_types):\n        self.name = name\n        self.sub_types = ensure_list(sub_types)\n\n    def __iter__(self):\n        for st in self.sub_types:\n            yield st\n\nclass MeshUnion(Union):\n    def __init__(self, name, sub_types):\n        super(MeshUnion, self).__init__(name, sub_types)\n", "path": "yt/data_objects/unions.py"}, {"content": "from .unions import Union\n\nclass ParticleUnion(Union):\n    def __init__(self, name, sub_types):\n        super(ParticleUnion, self).__init__(name, sub_types)\n", "path": "yt/data_objects/particle_unions.py"}], "after_files": [{"content": "from yt.funcs import ensure_list\n\nclass Union(object):\n    _union_type = \"\"\n    def __init__(self, name, sub_types):\n        self.name = name\n        self.sub_types = ensure_list(sub_types)\n\n    def __iter__(self):\n        for st in self.sub_types:\n            yield st\n\n    def __repr__(self):\n        return \"{} Union: '{}' composed of: {}\".format(\n            self._union_type.capitalize(), self.name, self.sub_types)\n\nclass MeshUnion(Union):\n    _union_type = \"mesh\"\n    def __init__(self, name, sub_types):\n        super(MeshUnion, self).__init__(name, sub_types)\n", "path": "yt/data_objects/unions.py"}, {"content": "from .unions import Union\n\nclass ParticleUnion(Union):\n    _union_type = \"particle\"\n    def __init__(self, name, sub_types):\n        super(ParticleUnion, self).__init__(name, sub_types)\n", "path": "yt/data_objects/particle_unions.py"}]}