Add docstring explaining how to enable the `storemagic` extension.

Renamed for clarity:
  extensions/pspersistence.py -> extensions/storemagic.py

IPython/extensions/storemagic.py

@@ -1,178 +1,187 b''
 # -*- coding: utf-8 -*-
 """
 %store magic for lightweight persistence.
 
 Stores variables, aliases etc. in PickleShare database.
+
+To enable this functionality, run::
+
+  %load_ext storemagic
+
+in your IPython session.  If you always want it enabled, you can list it in
+your default profile `ipython_config.py` file::
+
+  c.InteractiveShellApp.extensions = ['storemagic']
 """
 
 from IPython.core.error import TryNext, UsageError
 from IPython.utils import pickleshare
 
 import inspect,pickle,os,sys,textwrap
 from IPython.core.fakemodule import FakeModule
 
 def restore_aliases(ip):
     staliases = ip.db.get('stored_aliases', {})
     for k,v in staliases.items():
         #print "restore alias",k,v # dbg
         #self.alias_table[k] = v
         ip.alias_manager.define_alias(k,v)
 
 
 def refresh_variables(ip):
     db = ip.db
     for key in db.keys('autorestore/*'):
         # strip autorestore
         justkey = os.path.basename(key)
         try:
             obj = db[key]
         except KeyError:
             print "Unable to restore variable '%s', ignoring (use %%store -d to forget!)" % justkey
             print "The error was:",sys.exc_info()[0]
         else:
             #print "restored",justkey,"=",obj #dbg
             ip.user_ns[justkey] = obj
 
 
 def restore_dhist(ip):
     ip.user_ns['_dh'] = ip.db.get('dhist',[])
 
 def restore_data(ip):
     refresh_variables(ip)
     restore_aliases(ip)
     restore_dhist(ip)
 
 def magic_store(self, parameter_s=''):
     """Lightweight persistence for python variables.
 
     Example:
 
     ville@badger[~]|1> A = ['hello',10,'world']\\
     ville@badger[~]|2> %store A\\
     ville@badger[~]|3> Exit
 
     (IPython session is closed and started again...)
 
     ville@badger:~$ ipython -p pysh\\
     ville@badger[~]|1> print A
 
     ['hello', 10, 'world']
 
     Usage:
 
     %store          - Show list of all variables and their current values\\
     %store <var>    - Store the *current* value of the variable to disk\\
     %store -d <var> - Remove the variable and its value from storage\\
     %store -z       - Remove all variables from storage\\
     %store -r       - Refresh all variables from store (delete current vals)\\
     %store foo >a.txt  - Store value of foo to new file a.txt\\
     %store foo >>a.txt - Append value of foo to file a.txt\\
 
     It should be noted that if you change the value of a variable, you
     need to %store it again if you want to persist the new value.
 
     Note also that the variables will need to be pickleable; most basic
     python types can be safely %stored.
 
     Also aliases can be %store'd across sessions.
     """
 
     opts,argsl = self.parse_options(parameter_s,'drz',mode='string')
     args = argsl.split(None,1)
     ip = self.shell
     db = ip.db
     # delete
     if opts.has_key('d'):
         try:
             todel = args[0]
         except IndexError:
             raise UsageError('You must provide the variable to forget')
         else:
             try:
                 del db['autorestore/' + todel]
             except:
                 raise UsageError("Can't delete variable '%s'" % todel)
     # reset
     elif opts.has_key('z'):
         for k in db.keys('autorestore/*'):
             del db[k]
 
     elif opts.has_key('r'):
         refresh_variables(ip)
 
 
     # run without arguments -> list variables & values
     elif not args:
         vars = self.db.keys('autorestore/*')
         vars.sort()
         if vars:
             size = max(map(len,vars))
         else:
             size = 0
 
         print 'Stored variables and their in-db values:'
         fmt = '%-'+str(size)+'s -> %s'
         get = db.get
         for var in vars:
             justkey = os.path.basename(var)
             # print 30 first characters from every var
             print fmt % (justkey,repr(get(var,'<unavailable>'))[:50])
 
     # default action - store the variable
     else:
         # %store foo >file.txt or >>file.txt
         if len(args) > 1 and args[1].startswith('>'):
             fnam = os.path.expanduser(args[1].lstrip('>').lstrip())
             if args[1].startswith('>>'):
                 fil = open(fnam,'a')
             else:
                 fil = open(fnam,'w')
             obj = ip.ev(args[0])
             print "Writing '%s' (%s) to file '%s'." % (args[0],
               obj.__class__.__name__, fnam)
 
 
             if not isinstance (obj,basestring):
                 from pprint import pprint
                 pprint(obj,fil)
             else:
                 fil.write(obj)
                 if not obj.endswith('\n'):
                     fil.write('\n')
 
             fil.close()
             return
 
         # %store foo
         try:
             obj = ip.user_ns[args[0]]
         except KeyError:
             # it might be an alias
             # This needs to be refactored to use the new AliasManager stuff.
             if args[0] in self.alias_manager:
                 name = args[0]
                 nargs, cmd = self.alias_manager.alias_table[ name ]
                 staliases = db.get('stored_aliases',{})
                 staliases[ name ] = cmd
                 db['stored_aliases'] = staliases
                 print "Alias stored: %s (%s)" % (name, cmd)
                 return
             else:
                 raise UsageError("Unknown variable '%s'" % args[0])
 
         else:
             if isinstance(inspect.getmodule(obj), FakeModule):
                 print textwrap.dedent("""\
                 Warning:%s is %s
                 Proper storage of interactively declared classes (or instances
                 of those classes) is not possible! Only instances
                 of classes in real modules on file system can be %%store'd.
                 """ % (args[0], obj) )
                 return
             #pickled = pickle.dumps(obj)
             self.db[ 'autorestore/' + args[0] ] = obj
             print "Stored '%s' (%s)" % (args[0], obj.__class__.__name__)
 
 def load_ipython_extension(ip):
     ip.define_magic('store', magic_store)
     restore_data(ip)