Worked Example- Sockets (Chapter 12)
If I was to go to this in a browser, right there, you would see, and if I turned on developer console and I went to the network, let's make this a little bit bigger, you'd see that it retrieves this file "romeo.txt" and it gets back. What I'm getting is UTF-8 encoded data, most likely, and decode basically converts it to the internal format called Unicode that runs inside. And so, you know, this way we could write code that does stuff with this, but all we're really trying to do in this particular situation is show how you open a socket, send a command, and then retrieve the data.
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Hello everybody and welcome to some work sample code. If you’re interested in the source code you go to materials and download this “Sample Code.zip”. I have this downloaded, it’ll be in a folder called “code3” on my computer. This is where I’m at, “code3” folder and this has a ton of bits of code here. So if I do an “ls”, you’ll see I got all these files here and so we’ll just leave those there. And so this is the one I want to work through right now, it’s this “socket1.py” code. And basically, what we’re doing here is we’re simulating what is going to happen in a web browser. And the cool thing about the HTML, the HTTP protocol, is that we can do this by hand and I’m actually going to hack this HTTP protocol. This is going to go to “data.pr4e.org” and retrieve a document. And so, I’m going to do “telnet” to Now you can do this on a Mac and Linux. And if you put telnet on a Windows box, you can do it here, “data.pr4e.org”, and I want talk to port 80; and port 80 is a different port, it’s a nonstandard port, but what we’re doing here is talking to the HTTP port. And so I’m going to be able to hand send commands to the web server and retrieve a document. So, I’ve already copied this string, this “GET http - romeo.txt”. I’m copying that into my buffer because if I wait too long this won’t work. So here I go, and now I’m going to type that and I have to enter twice, and that literally was the HTTP protocol. What I typed there was the HTTP protocol and the web server responds with some metadata about the document – how much data there is; the kind of data is there. A blank line separates the header information from the the body of the document. If I was to go to this in a browser, right there, you would see, and if I turned on developer console and I went to the network, let’s make this a little bit bigger, you’d see that it retrieves this file “romeo.txt” and it gets back. That tells us, that shows us the headers and it shows us the response since this is all the same way of doing the same thing, and that is how to do the HTTP protocol. Okay? But now we’re going to do this in Python, and so here’s the code we’re going to write. So we’re going import the socket library and we’re going to make a socket. Now this doesn’t actually make a connection, think of a socket as a file handle that doesn’t have any data associated with it yet. And then what we’re going to do is we’re going to reach out and connect that socket to a destination across the Internet, with the domain name of “data.pr4e.org”. And the second parameter in this tuple, this is a function call with a single tuple as a parameter. And so tuple sub zero is “data.pr4e.org” and tuple sub one is the 80 which says I want to talk to port 80. That could fail. It will make the connection and if the port 80 is there, away it goes. And then we’re going to actually send the HTTP command, so GET, this is the HTTP rules, followed by an end of line, followed by a blank line. So you saw me do this there. This was what I typed here and then I had to type a blank line. Now, if you want to go read the RFCs for how to do this, you can figure this out. So the only other thing that’s kind of weird here is we have to add this dot encode. And that’s because there are strings inside of Python that are in Unicode and we have to send them out as what’s called UTF-8, and encode converts from Unicode internally to UTF-8. So, this command is a set of UTF-8 bytes that we’re then going to send. It still has that same set of characters in it, and now we’re going to send it. And that’s after we’ve made the connection, we’re going to send these two things and then we’re going to wait. And my mysock is like a file handle at that point because it’s been opened and we’ve sent data. The HTTP protocol told us what this we had to send and the fact that we did have to send it. So now I have just a simple “while” loop and I’m going to ask up to 512 characters and, you know, receive up to 512 characters and get that back. If I will know that this is the end of file, if they’ve got no data back, so if the length of the data, the byte array that I got back is less than 1 then I’m going to quit. Otherwise, I’m going to print the data and I’m going to use this decode which is kind of the opposite of this encode. What I’m getting is UTF-8 encoded data, most likely, and decode basically converts it to the internal format called Unicode that runs inside. So this is going to run a bunch of times pulling in the blocks basically 512, up to 512 characters at a time. Printing it out, and then when it’s all said and done we will close that connection. And so it’s not too exciting, “python3 socket1.py”, and you’ll see that it’s just going to, Python is now going to do what I did by hand. Now, of course, the interesting thing is these are all in strings, right? And so, you know, this way we could write code that does stuff with this, but all we’re really trying to do in this particular situation is show how you open a socket, send a command, and then retrieve the data.
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