Building a WebSocket client with PycURL involves using the PycURL library to connect to a WebSocket server and send and receive data using the WebSocket protocol.

PycURL does not have built-in support for the WebSocket protocol, but it can be used to connect to a WebSocket server by making a standard HTTP request and upgrading the connection to a WebSocket connection.

Here’s an example of using PycURL to connect to a WebSocket server and send a message:

import pycurl
from io import BytesIO

buffer = BytesIO()
c = pycurl.Curl()
c.setopt(c.URL, 'ws://echo.websocket.org')
c.setopt(c.HTTPHEADER, ['Connection: Upgrade', 'Upgrade: websocket'])
c.setopt(c.POSTFIELDS, 'Hello, WebSocket!')
c.setopt(c.WRITEDATA, buffer)
c.perform()
c.close()

print(buffer.getvalue())

In this example, we first import the PycURL library and create a new Curl object. We then set the URL option to the WebSocket server we want to connect to. We also set the HTTPHEADER option to a list of headers that indicate that we want to upgrade the connection to a WebSocket connection. The POSTFIELDS option is set to the message that we want to send to the server. The WRITEDATA option is set to a buffer object that will hold the response from the server. Finally, we call the perform() method to send the message and the close() method to close the connection to the server.

It’s important to note that this is a very basic example and in real-world usage, you will want to set some other options such as the WebSocket subprotocol, the WebSocket extensions, and the WebSocket version, and also you will want to handle the WebSocket connection in a more robust way, for example, by using a library such as websocket-client or async-websockets that provides a more pythonic interface for handling WebSockets.

It’s also worth noting that PycURL is not typically used for WebSocket communication and other libraries such as websocket-client or async-websockets are more suited for this task and provide a more pythonic interface for handling WebSockets.