Simple VCS Implementation (Incomplete WIP)

myvcs.py

#!/usr/local/bin/python
# -*- coding: utf-8 -*-
from __future__ import with_statement
import os
import json
import docopt

from shutil import rmtree, copytree

VCS_DIR_NAME = '.myvcs'
META_FILENAME = 'META'

SNAPSHOT_COUNTER_KEY = 'snapshot_counter'

META_DEFAULT = {
	SNAPSHOT_COUNTER_KEY: 1,
}

		
def vcs_ignore(curdir, contents):
	# Ignore all files / dirs called like VCS_DIR_NAME
	return [VCS_DIR_NAME]


def get_meta(path):
	try:
		with open(path, 'r') as meta_file:
			try:
				return json.load(meta_file)
			except ValueError:
				return META_DEFAULT
	except IOError:
		# The file does not exist. We'll create it when we're over.
		return META_DEFAULT

def set_meta(meta, path):
	try:
		with open(path, 'w+') as meta_file:
			json.dump(meta, meta_file)
	except IOError:
		print "There's no MyVCS repo here! Create one by running ./myvcs.py snap"


def main():
	"""MyVCS

		Usage:
		  myvcs.py snap
		  myvcs.py revert-to <snapshot_id>
	"""

	# We need these.
	cwd = os.getcwd()
	vcs_path = os.path.join(cwd, VCS_DIR_NAME)
	meta_path = os.path.join(vcs_path, META_FILENAME)

	# Parse the docstring to generate a docopt dict.
	options = docopt.docopt(main.__doc__)

	# Read metadata from file
	meta = get_meta(meta_path)
	
	if options['snap']:
		# Snap will likely change the metadata.
		# This same architecture will remain throughout the application
		meta = snap(meta, cwd, vcs_path)
	elif options['revert-to']:
		# Get snapshot_id from options dict
		snapshot_id = options['<snapshot_id>']
		meta = revert_to(snapshot_id, meta, cwd, vcs_path)

	# Save metadata to file
	set_meta(meta, meta_path)

	# Gracefully exit.
	exit()


def snap(meta, cwd, vcs_path):
	"""
	This method will read from metadata, 
	This method must return a new meta dict.
	"""

	# This snapshot's folder name is governed by what the META file says.
	snapshot_id = meta[SNAPSHOT_COUNTER_KEY]

	# The absolute path of the new snapshot's tree
	snapshot_path = os.path.join(vcs_path, str(snapshot_id))

	# The next time, it won't be the same id
	meta[SNAPSHOT_COUNTER_KEY] += 1

	# Use copytree to snapshot this dir
	copytree(cwd, snapshot_path, ignore=vcs_ignore)

	return meta

def revert_to(snapshot_id, meta, cwd, vcs_path):
	"""	
	All methods have a 1-1 mapping to console options, for now.

	Expects a snapshot_id as str.

	Like the method before it, it must return a new meta dict.

	"""

	# Get the path of the snapshot we're retrieving
	snapshot_path = os.path.join(vcs_path, snapshot_id)

	# Let's check if it does exist, otherwise, complain.
	if os.path.isdir(snapshot_path):
		print "There it is!"
	else:
		print "MyVCS ERROR: No such snapshot [snapshot_id = {}]".format(snapshot_id)

	return meta



if __name__ == '__main__':
	main()