We are currently solving the following issue within our repository. Here is the issue text:
--- BEGIN ISSUE ---
cmd && othercmd throws an exception
```
➤ ls &&Exception in thread Thread-35:
Traceback (most recent call last):
  File "/usr/lib/python3.6/threading.py", line 916, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.6/threading.py", line 864, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python3.6/site-packages/prompt_toolkit/interface.py", line 860, in run
    completions = list(buffer.completer.get_completions(document, complete_event))
  File "/usr/lib/python3.6/site-packages/xonsh/ptk/completer.py", line 49, in get_completions
    self.ctx)
  File "/usr/lib/python3.6/site-packages/xonsh/__amalgam__.py", line 124, in complete
    out = func(prefix, line, begidx, endidx, ctx)
  File "/usr/lib/python3.6/site-packages/xonsh/completers/__amalgam__.py", line 831, in complete_skipper
    ctx)
  File "/usr/lib/python3.6/site-packages/xonsh/__amalgam__.py", line 124, in complete
    out = func(prefix, line, begidx, endidx, ctx)
  File "/usr/lib/python3.6/site-packages/xonsh/completers/__amalgam__.py", line 1427, in complete_base
    complete_command(prefix, line, start, end, ctx))
TypeError: unsupported operand type(s) for |: 'tuple' and 'set'
```

```
➤ y -Q | egrep 'toolkit|xon'
community/python-prompt_toolkit 1.0.14-1
local/xonsh 0.5.9-1
```

Let me know if you need anything else.

--- END ISSUE ---

Below are some code segments, each from a relevant file. One or more of these files may contain bugs.
--- BEGIN FILES ---
Path: `xonsh/completers/base.py`
Content:
```
1 """Base completer for xonsh."""
2 import collections.abc as cabc
3 
4 from xonsh.completers.path import complete_path
5 from xonsh.completers.python import complete_python
6 from xonsh.completers.commands import complete_command
7 
8 
9 def complete_base(prefix, line, start, end, ctx):
10     """If the line is empty, complete based on valid commands, python names,
11     and paths.  If we are completing the first argument, complete based on
12     valid commands and python names.
13     """
14     if line.strip() == "":
15         out = complete_python(prefix, line, start, end, ctx) | complete_command(
16             prefix, line, start, end, ctx
17         )
18         paths = complete_path(prefix, line, start, end, ctx, False)
19         return (out | paths[0]), paths[1]
20     elif prefix == line:
21         python_comps = complete_python(prefix, line, start, end, ctx)
22         if isinstance(python_comps, cabc.Sequence):
23             return (
24                 python_comps[0] | complete_command(prefix, line, start, end, ctx),
25                 python_comps[1],
26             )
27         else:
28             return python_comps | complete_command(prefix, line, start, end, ctx)
29     return set()
30
```
--- 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/xonsh/completers/base.py b/xonsh/completers/base.py
--- a/xonsh/completers/base.py
+++ b/xonsh/completers/base.py
@@ -11,19 +11,21 @@
     and paths.  If we are completing the first argument, complete based on
     valid commands and python names.
     """
+    # get and unpack python completions
+    python_comps = complete_python(prefix, line, start, end, ctx)
+    if isinstance(python_comps, cabc.Sequence):
+        python_comps, python_comps_len = python_comps
+    else:
+        python_comps_len = None
+    # add command completions
+    out = python_comps | complete_command(prefix, line, start, end, ctx)
+    # add paths, if needed
     if line.strip() == "":
-        out = complete_python(prefix, line, start, end, ctx) | complete_command(
-            prefix, line, start, end, ctx
-        )
         paths = complete_path(prefix, line, start, end, ctx, False)
         return (out | paths[0]), paths[1]
     elif prefix == line:
-        python_comps = complete_python(prefix, line, start, end, ctx)
-        if isinstance(python_comps, cabc.Sequence):
-            return (
-                python_comps[0] | complete_command(prefix, line, start, end, ctx),
-                python_comps[1],
-            )
+        if python_comps_len is None:
+            return out
         else:
-            return python_comps | complete_command(prefix, line, start, end, ctx)
+            return out, python_comps_len
     return set()

{"golden_diff": "diff --git a/xonsh/completers/base.py b/xonsh/completers/base.py\n--- a/xonsh/completers/base.py\n+++ b/xonsh/completers/base.py\n@@ -11,19 +11,21 @@\n     and paths.  If we are completing the first argument, complete based on\n     valid commands and python names.\n     \"\"\"\n+    # get and unpack python completions\n+    python_comps = complete_python(prefix, line, start, end, ctx)\n+    if isinstance(python_comps, cabc.Sequence):\n+        python_comps, python_comps_len = python_comps\n+    else:\n+        python_comps_len = None\n+    # add command completions\n+    out = python_comps | complete_command(prefix, line, start, end, ctx)\n+    # add paths, if needed\n     if line.strip() == \"\":\n-        out = complete_python(prefix, line, start, end, ctx) | complete_command(\n-            prefix, line, start, end, ctx\n-        )\n         paths = complete_path(prefix, line, start, end, ctx, False)\n         return (out | paths[0]), paths[1]\n     elif prefix == line:\n-        python_comps = complete_python(prefix, line, start, end, ctx)\n-        if isinstance(python_comps, cabc.Sequence):\n-            return (\n-                python_comps[0] | complete_command(prefix, line, start, end, ctx),\n-                python_comps[1],\n-            )\n+        if python_comps_len is None:\n+            return out\n         else:\n-            return python_comps | complete_command(prefix, line, start, end, ctx)\n+            return out, python_comps_len\n     return set()\n", "issue": "cmd && othercmd throws an exception\n```\r\n\u27a4 ls &&Exception in thread Thread-35:\r\nTraceback (most recent call last):\r\n  File \"/usr/lib/python3.6/threading.py\", line 916, in _bootstrap_inner\r\n    self.run()\r\n  File \"/usr/lib/python3.6/threading.py\", line 864, in run\r\n    self._target(*self._args, **self._kwargs)\r\n  File \"/usr/lib/python3.6/site-packages/prompt_toolkit/interface.py\", line 860, in run\r\n    completions = list(buffer.completer.get_completions(document, complete_event))\r\n  File \"/usr/lib/python3.6/site-packages/xonsh/ptk/completer.py\", line 49, in get_completions\r\n    self.ctx)\r\n  File \"/usr/lib/python3.6/site-packages/xonsh/__amalgam__.py\", line 124, in complete\r\n    out = func(prefix, line, begidx, endidx, ctx)\r\n  File \"/usr/lib/python3.6/site-packages/xonsh/completers/__amalgam__.py\", line 831, in complete_skipper\r\n    ctx)\r\n  File \"/usr/lib/python3.6/site-packages/xonsh/__amalgam__.py\", line 124, in complete\r\n    out = func(prefix, line, begidx, endidx, ctx)\r\n  File \"/usr/lib/python3.6/site-packages/xonsh/completers/__amalgam__.py\", line 1427, in complete_base\r\n    complete_command(prefix, line, start, end, ctx))\r\nTypeError: unsupported operand type(s) for |: 'tuple' and 'set'\r\n```\r\n\r\n```\r\n\u27a4 y -Q | egrep 'toolkit|xon'\r\ncommunity/python-prompt_toolkit 1.0.14-1\r\nlocal/xonsh 0.5.9-1\r\n```\r\n\r\nLet me know if you need anything else.\n", "before_files": [{"content": "\"\"\"Base completer for xonsh.\"\"\"\nimport collections.abc as cabc\n\nfrom xonsh.completers.path import complete_path\nfrom xonsh.completers.python import complete_python\nfrom xonsh.completers.commands import complete_command\n\n\ndef complete_base(prefix, line, start, end, ctx):\n    \"\"\"If the line is empty, complete based on valid commands, python names,\n    and paths.  If we are completing the first argument, complete based on\n    valid commands and python names.\n    \"\"\"\n    if line.strip() == \"\":\n        out = complete_python(prefix, line, start, end, ctx) | complete_command(\n            prefix, line, start, end, ctx\n        )\n        paths = complete_path(prefix, line, start, end, ctx, False)\n        return (out | paths[0]), paths[1]\n    elif prefix == line:\n        python_comps = complete_python(prefix, line, start, end, ctx)\n        if isinstance(python_comps, cabc.Sequence):\n            return (\n                python_comps[0] | complete_command(prefix, line, start, end, ctx),\n                python_comps[1],\n            )\n        else:\n            return python_comps | complete_command(prefix, line, start, end, ctx)\n    return set()\n", "path": "xonsh/completers/base.py"}], "after_files": [{"content": "\"\"\"Base completer for xonsh.\"\"\"\nimport collections.abc as cabc\n\nfrom xonsh.completers.path import complete_path\nfrom xonsh.completers.python import complete_python\nfrom xonsh.completers.commands import complete_command\n\n\ndef complete_base(prefix, line, start, end, ctx):\n    \"\"\"If the line is empty, complete based on valid commands, python names,\n    and paths.  If we are completing the first argument, complete based on\n    valid commands and python names.\n    \"\"\"\n    # get and unpack python completions\n    python_comps = complete_python(prefix, line, start, end, ctx)\n    if isinstance(python_comps, cabc.Sequence):\n        python_comps, python_comps_len = python_comps\n    else:\n        python_comps_len = None\n    # add command completions\n    out = python_comps | complete_command(prefix, line, start, end, ctx)\n    # add paths, if needed\n    if line.strip() == \"\":\n        paths = complete_path(prefix, line, start, end, ctx, False)\n        return (out | paths[0]), paths[1]\n    elif prefix == line:\n        if python_comps_len is None:\n            return out\n        else:\n            return out, python_comps_len\n    return set()\n", "path": "xonsh/completers/base.py"}]}