13.6. Reading binary files using
Sometimes you want to retrieve a non-text (or binary) file such as an
image or video file. The data in these files is generally not useful to
print out, but you can easily make a copy of a URL to a local file on
your hard disk using
The pattern is to open the URL and use
read to download the
entire contents of the document into a string variable
img) then write that information to a local file as
This program reads all of the data in at once across the network and
stores it in the variable
img in the main memory of your
computer, then opens the file
cover.jpg and writes the data
out to your disk. The
wb argument for
open() opens a binary file
for writing only. This program will work if the size of the file is less than
the size of the memory of your computer.
However if this is a large audio or video file, this program may crash or at least run extremely slowly when your computer runs out of memory. In order to avoid running out of memory, we retrieve the data in blocks (or buffers) and then write each block to your disk before retrieving the next block. This way the program can read any size file without using up all of the memory you have in your computer.
In this example, we read only 100,000 characters at a time and then
write those characters to the
cover.jpg file before
retrieving the next 100,000 characters of data from the web.