bagit-python

bagit is a Python library and command line utility for working with BagIt style packages.

Installation

bagit.py is a single-file python module that you can drop into your project as needed or you can install globally with:

pip install bagit

Python v2.6+ is required.

Command Line Usage

When you install bagit you should get a command line program called bagit.py which you can use to turn an existing directory into a bag:

bagit.py --contact-name 'John Kunze' /directory/to/bag

You can pass in key/value metadata for the bag using options like --contact-name above, which get persisted to the bag-info.txt. For a complete list of bag-info.txt properties you can use as commmand line arguments see --help.

Since calculating checksums can take a while when creating a bag, you may want to calculate them in parallel if you are on a multicore machine. You can do that with the --processes option:

bagit.py --processes 4 /directory/to/bag

To specify which checksum algorithm(s) to use when generating the manifest, use the --md5, --sha1, --sha256 and/or --sha512 flags (MD5 is generated by default).

bagit.py --sha1 /path/to/bag
bagit.py --sha256 /path/to/bag
bagit.py --sha512 /path/to/bag

If you would like to validate a bag you can use the --validate flag.

bagit.py --validate /path/to/bag

If you would like to take a quick look at the bag to see if it seems valid by just examining the structure of the bag, and comparing its payload-oxum (byte count and number of files) then use the --fast flag.

bagit.py --validate --fast /path/to/bag

And finally, if you'd like to parallelize validation to take advantage of multiple CPUs you can:

bagit.py --validate --processes 4 /path/to/bag

Library Usage

You can also use bagit programatically in your own Python programs.

Create

To create a bag you would do this:

bag = bagit.make_bag('mydir', {'Contact-Name': 'John Kunze'})

make_bag returns a Bag instance. If you have a bag already on disk and would like to create a Bag instance for it, simply call the constructor directly:

bag = bagit.Bag('/path/to/bag')

Update Bag Metadata

You can change the metadata persisted to the bag-info.txt by using the info property on a Bag.

# load the bag
bag = bagit.Bag('/path/to/bag')

# update bag info metadata
bag.info['Internal-Sender-Description'] = 'Updated on 2014-06-28.'
bag.info['Authors'] = ['John Kunze', 'Andy Boyko']
bag.save()

Update Bag Manifests

By default save will not update manifests. This guards against a situation where a call to save to persist bag metadata accidentally regenerates manifests for an invalid bag. If you have modified the payload of a bag by adding, modifying or deleting files in the data directory, and wish to regenerate the manifests set the manifests parameter to True when calling save.

import shutil, os

# add a file
shutil.copyfile('newfile', '/path/to/bag/data/newfile')

# remove a file
os.remove('/path/to/bag/data/file')

# persist changes
bag.save(manifests=True)

The save method takes an optional processes parameter which will determine how many processes are used to regenerate the checksums. This can be handy on multicore machines.

Validation

If you would like to see if a bag is valid, use its is_valid method:

bag = bagit.Bag('/path/to/bag')
if bag.is_valid():
    print "yay :)"
else:
    print "boo :("

If you'd like to get a detailed list of validation errors, execute the validate method and catch the BagValidationError exception. If the bag's manifest was invalid (and it wasn't caught by the payload oxum) the exception's details property will contain a list of ManifestErrors that you can introspect on. Each ManifestError, will be of type ChecksumMismatch, FileMissing, UnexpectedFile.

So for example if you want to print out checksums that failed to validate you can do this:

bag = bagit.Bag("/path/to/bag")

try:
  bag.validate()

except bagit.BagValidationError, e:
  for d in e.details:
    if isinstance(d, bag.ChecksumMismatch):
      print "expected %s to have %s checksum of %s but found %s" % \
        (e.path, e.algorithm, e.expected, e.found)

To iterate through a bag's manifest and retrieve checksums for the payload files use the bag's entries dictionary:

bag = bagit.Bag("/path/to/bag")

for path, fixity in bag.entries.items():
  print "path:%s md5:%s" % (path, fixity["md5"])

Development

% git clone git://github.com/LibraryOfCongress/bagit-python.git
% cd bagit-python
% python test.py

If you'd like to see how increasing parallelization of bag creation on your system effects the time to create a bag try using the included bench utility:

% ./bench.py

License

Note: By contributing to this project, you agree to license your work under the same terms as those that govern this project's distribution.