{"CallResponse":[0, {"Signature":[{"sig":{"name":"len","description":"calculates the length of its input","extra_description":"","search_terms":[],"required_positional":[],"optional_positional":[],"rest_positional":null,"vectorizes_over_list":false,"named":[{"long":"help","short":"h","arg":null,"required":false,"desc":"Display the help message for this command","var_id":null,"default_value":null}],"input_type":"String","output_type":"Int","input_output_types":[],"allow_variants_without_examples":false,"is_filter":false,"creates_scope":false,"allows_unknown_args":false,"category":"Default"},"examples":[]}]}]}
```

The plugin prints its signature serialized as JSON. We'll reformat for readability.

```json
{
  "Signature": [
    {
      "sig": {
        "name": "len",
        "description": "calculates the length of its input",
        "extra_description": "",
        "search_terms": [],
        "required_positional": [],
        "optional_positional": [],
        "rest_positional": null,
        "vectorizes_over_list": false,
        "named": [
          {
            "long": "help",
            "short": "h",
            "arg": null,
            "required": false,
            "desc": "Display the help message for this command",
            "var_id": null,
            "default_value": null
          }
        ],
        "input_type": "String",
        "output_type": "Int",
        "input_output_types": [],
        "allow_variants_without_examples": false,
        "is_filter": false,
        "creates_scope": false,
        "allows_unknown_args": false,
        "category": "Default"
      },
      "examples": []
    }
  ]
}
```

This signature tells Nu everything it needs to pass data in and out of the plugin as well as format the help message and support type aware tab completion. A full description of these fields is beyond the scope of this tutorial, but the response is simply a serialized form of the [`PluginSignature`](https://docs.rs/nu-protocol/latest/nu_protocol/struct.PluginSignature.html) struct in the `nu-plugin` crate.

Now let's try simulating an invocation. Above we tested the plugin within Nu by executing the command `"hello" | len` and we got the response `5`. Of course this hides all of the typed data handling that makes Nu so powerful.

```nu
$ echo '{"Hello":{"protocol":"nu-plugin","version":"0.90.2","features":[]}}{"Call":[0,{"Run":{"name":"len","call":{"head":{"start":100953,"end":100957},"positional":[],"named":[]},"input":{"Value":{"String":{"val":"hello","span":{"start":100953,"end":100957}}}}}}]}' | target/release/nu_plugin_len --stdio
json{"Hello":{"protocol":"nu-plugin","version":"0.90.2","features":[]}}
{"PipelineData":{"Value":{"Int":{"val":5,"span":{"start":100953,"end":100957}}}}}
```

We invoked our plugin and passed a [`Run`](plugin_protocol_reference.md#run-plugin-call) plugin call that looks like the following on stdin:

```json
{
  "Run": {
    "name": "len",
    "call": {
      "head": {
        "start": 100953,
        "end": 100957
      },
      "positional": [],
      "named": []
    },
    "input": {
      "Value": {
        "String": {
          "val": "hello",
          "span": {
            "start": 100953,
            "end": 100957
          }
        }
      }
    }
  }
}
```

That is, we passed len the string "hello" and it replied with the following [`PipelineData`](plugin_protocol_reference.md#pipelinedata-plugin-call-response) response:

```json
{
  "PipelineData": {
    "Value": {
      "Int": {
        "val": 5,
        "span": {
          "start": 100953,
          "end": 100957
        }
      }
    }
  }
}
```

with the integer 5 along with preserving source span information that may be useful for error messages later.

When implementing a plugin in a non-Rust language like Python, you must manage this input and output serialization. Please refer to the [protocol documentation](plugin_protocol_reference.md) for more specific details on the protocol itself.

## Creating a plugin (in Python)