This chapter is about binary files, and the previous two exercises were not, at least, not directly. There simply is not much that you can exercise with where binary files are concerned; the main problems are with handling byte values, which is what the previous two exercises were concerned with. But to round off what these two exercises did, let’s now use what you developed in them to compress files.
Write a program that asks for an input file, that must exist, and an output file, that should not exist. Then it asks whether you want to compress or decompress. If you choose compress, the input file is compressed using the method developed above, and written as the output file. If you choose decompress, the input file is decompressed under the assumption that it was compressed with the method developed above, and written as the output file. So you should be able to get the original file again by first compressing and then decompressing.
You best read the whole file in memory before (de)compressing, so that you do not get into problems when a byte string ends in half a byte instead of a full byte after compression. You also best treat both the input file and the output file as binary files.
Instead of a program that asks for its input, write two functions that both take the location of an input file (str) and the location of an output file (str).
Write a function compress_file that reads the given input file in binary mode, compresses its content with the method developed in the exercise Compression, and writes the result to the given output file.
Write a function decompress_file that reads the given input file in binary mode, decompresses its content with the method developed in the exercise Decompression, and writes the result to the given output file.
Both functions read the entire input file into memory before (de)compressing it, so that it does not matter that a compressed byte string may end in half a byte. Both functions treat every byte of the input file as the character with that ordinal value, so that any file can be compressed, not just text files. Both functions must return a tuple with two elements: i) the number of bytes read from the input file and ii) the number of bytes written to the output file.
Both functions must also check that the input file exists and that the output file does not exist yet. If the input file does not exist, an AssertionError must be raised with the message input file does not exist. If the output file already exists, an AssertionError must be raised with the message output file already exists.
Compressing a file and decompressing the result must give back the original file.
In the interactive session below we assume the text file data.txt is located in the current directory, and that the files data.cmp and data.new do not exist yet.
>>> compress_file('data.txt', 'data.cmp')
(13, 11)
>>> open('data.cmp', 'rb').read()
b'\x04\x81\xbb@,\xf0wI\xba\x02\x10'
>>> decompress_file('data.cmp', 'data.new')
(11, 13)
>>> open('data.new', 'rb').read() == open('data.txt', 'rb').read()
True
>>> compress_file('data.xxx', 'data.yyy')
Traceback (most recent call last):
AssertionError: input file does not exist
>>> compress_file('data.txt', 'data.cmp')
Traceback (most recent call last):
AssertionError: output file already exists