# Python Handlers
Python file handlers are Python files which the server executes in response to
requests made to the corresponding URL. This is hooked up to a route like
`("*", "*.py", python_file_handler)`, meaning that any .py file will be
treated as a handler file (note that this makes it easy to write unsafe
handlers, particularly when running the server in a web-exposed setting).
The Python files must define a function named `main` with the signature:
main(request, response)
...where `request` is [a wptserve `Request`
object](/tools/wptserve/docs/request) and `response` is [a wptserve `Response`
object](/tools/wptserve/docs/response).
This function must return a value in one of the following four formats:
((status_code, reason), headers, content)
(status_code, headers, content)
(headers, content)
content
Above, `headers` is a list of (field name, value) pairs, and `content` is a
string or an iterable returning strings.
The `main` function may also update the response manually. For example, one may
use `response.headers.set` to set a response header, and only return the
content. One may even use this kind of handler, but manipulate the output
socket directly. The `writer` property of the response exposes a
`ResponseWriter` object that allows writing specific parts of the request or
direct access to the underlying socket. If used, the return value of the
`main` function and the properties of the `response` object will be ignored.
The wptserver implements a number of Python APIs for controlling traffic.
```eval_rst
.. toctree::
:maxdepth: 1
/tools/wptserve/docs/request
/tools/wptserve/docs/response
/tools/wptserve/docs/stash
```
## Example: Dynamic HTTP headers
The following code defines a Python handler that allows the requester to
control the value of the `Content-Type` HTTP response header:
```python
def main(request, response):
content_type = request.GET.first('content-type')
headers = [('Content-Type', content_type)]
return (200, 'my status text'), headers, 'my response content'
```
If saved to a file named `resources/control-content-type.py`, the WPT server
will respond to requests for `resources/control-content-type.py` by executing
that code.
This could be used from a [testharness.js test](../testharness) like so:
```html
Demonstrating the WPT server's Python handler feature
```