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