kj7ny wrote: > I am attempting to incrementally back up files using python. How can I > compare a file on a hard drive to a file in a python created zip file > to tell if the file has changed? Or should I be using some other > method to determine if a file has changed? >
You can easily determine size and date from the hard drive/filesystem and from zip files. Look at the Python docs for the os and zipfile modules. That would be the fastest way to detect changes for most files you will encounter. Zip files have the CRC of each file (generated from the original uncompressed file) for checksumming purposes. You can calculate the CRC for the file on the hard drive and then compare the CRC values. Calculating the CRC should be faster than decompressing the ZIPped file or compressing the file on the hard drive. While not perfect it should definitely give you a very good indication of changed content. Python doesn't any built-in functions for CRC calculations, AFAIK. There are third party modules around. A better plan would be to store a list of MD5 or SHA1 hashes for each file in your archive. These functions are built in to Python and are more suitable for detection large differences in files (these are hash functions while CRC is a checksum function and thus are less likely to have collisions). You can store the list of hashes in a separate file, or as a file in the ZIP file, or even in the ZIP file header. Your last resort would be to decompress each file and compare it with the one on disk. This will take a lot of time and memory. Not at all advisable but definitely foolproof. There are other issues you will have to contend with, such as renaming of files and directories, and deletions. If I were you, I'd take a look at the work done on programs like rsync, unison (both file synchronization programs), or CVS and Subversion (both version or source control programs) to see how they deal with such issues. -- http://mail.python.org/mailman/listinfo/python-list
