13.4. Retrieving an image over HTTP

In the above example, we retrieved a plain text file which had newlines in the file and we simply copied the data to the screen as the program ran. We can use a similar program to retrieve an image across using HTTP. Instead of copying the data to the screen as the program runs, we accumulate the data in a string, trim off the headers, and then save the image data to a file as follows:

import socket import time HOST = 'data.pr4e.org' PORT = 80 mysock = socket.socket(socket.AF_INET, socket.SOCK_STREAM) mysock.connect((HOST, PORT)) mysock.sendall(b'GET http://data.pr4e.org/cover3.jpg HTTP/1.0\r\n\r\n') count = 0 picture = b"" while True: data = mysock.recv(5120) if len(data) < 1: break #time.sleep(0.25) count = count + len(data) print(len(data), count) picture = picture + data mysock.close() # Look for the end of the header (2 CRLF) pos = picture.find(b"\r\n\r\n") print('Header length', pos) print(picture[:pos].decode()) # Skip past the header and save the picture data picture = picture[pos+4:] fhand = open("stuff.jpg", "wb") fhand.write(picture) fhand.close()

You can see that for this url, the Content-Type header indicates that body of the document is an image (image/jpeg). Once the program completes, you can view the image data by opening the file stuff.jpg in an image viewer.

Q-2: The ________ header indicates that the boday of the document is a jpeg image.

As the program runs, you can see that we don’t get 5120 characters each time we call the recv() method. We get as many characters as have been transferred across the network to us by the web server at the moment we call recv(). In this example, we either get as few as 3200 characters each time we request up to 5120 characters of data.

Your results may be different depending on your network speed. Also note that on the last call to recv() we get 3167 bytes, which is the end of the stream, and in the next call to recv() we get a zero-length string that tells us that the server has called close() on its end of the socket and there is no more data forthcoming.

We can slow down our successive recv() calls by uncommenting the call to time.sleep(). This way, we wait a quarter of a second after each call so that the server can “get ahead” of us and send more data to us before we call recv() again.

Now other than the first and last calls to recv(), we now get 5120 characters each time we ask for new data.

There is a buffer between the server making send() requests and our application making recv() requests. When we run the program with the delay in place, at some point the server might fill up the buffer in the socket and be forced to pause until our program starts to empty the buffer. The pausing of either the sending application or the receiving application is called “flow control.”

Q-3: Which of the following methods will slow down the requests made in the program above?

• time.sleep()
• Uncommenting time.sleep() will make the program wait a quarter of a second before sending the next call.
• mysock.sendall()
• This command does exactly what it says: it sends all the data included in the ().
• picture.find()
• This command does exactly what it says: it finds the contents of the () in the picture variable.
• mysock.recv()
• The recv() method receives the message, it is what we want to slow down our requests to. How do we do that?

You have attempted of activities on this page