Replacing import os

>>> import fslib
>>>
>>> fs = fslib.FileSystem(fslib.OSFS('/'))
>>> fs.dir_exists('/etc/')
True
>>> fs.listdir('/')
['/etc', '/dev', '/proc']

Forcing read-only

In order to ensure that user code doesn’t change any physical file, fslib provides a simple wrapper that prevents writing to the filesystem.

>>> import fslib
>>> import fslib.stacking
>>>
>>> fs = fslib.FileSystem(fslib.stacking.ReadOnlyFS(fslib.OSFS('/')))
>>> fs.read_one_line('/etc/hostname')
"ithor"
>>> fs.open('/tmp/x', 'w')
OSError: [Errno 30] Read-only file system: '/tmp/x'

UnionFS-like mounting

fslib provides a UnionFS-like wrapper as fslib.stacking.UnionFS. This backend will provide a merged view of several branch, redirecting all writes to the branch(es) declared writable=True.

>>> import fslib, fslib.builders, fslib.stacking
>>>
>>> mem_fs = fslib.builders.make_memory_fake()
>>> union_fs = fslib.stacking.UnionFS()
>>> union_fs.add_branch(mem_fs, ref='memory', rank=0, writable=True)
>>> union_fs.add_branch(fslib.stacking.ReadOnlyFS(fslib.OSFS('/')), ref='os', rank=1)
>>>
>>> fs = fslib.FileSystem(backend=union_fs)
>>> fs.writelines('/tmp/x', ['aa', 'bb'])
>>>
>>> open('/tmp/x', 'r')
IOError: [Errno 2] No such file or directory: '/tmp/x'
>>> fs.file_exists('/tmp/x')
True
>>> fs.readlines('/tmp/x')
['aa', 'bb']
>>> fs.access('/tmp/x', os.F_OK)
True

Unix-like filesystem tree

It is possible to “overlay” physical or virtual file systems to present a simple, unified structure to the program.

>>> import fslib, fslib.stacking
>>> mnt = fslib.MountFS()
>>> mnt.mount_fs('/', fslib.stacking.ReadOnlyFS(fslib.OSFS('/')))
>>> mnt.mount_fs('/home/xelnor/.myapp', fslib.stacking.MemoryFS())
>>> mnt.mount_fs('/home/xelnor/.myapp/cache', fslib.OSFS('/tmp/myapp/shared_cache'))

With this setup:

• All reads/writes to /home/xelnor/.myapp/cache will actually occur within /tmp/myapp/shared_cache

• All reads/writes within /home/xelnor/.myapp (except for /cache) will occur in memory

• No write will be permitted anywhere else.