allow trimming to an empty database (--keep=0)

IPython/core/historyapp.py

@@ -1,124 +1,126 b''
 # encoding: utf-8
 """
 An application for managing IPython history.
 
 To be invoked as the `ipython history` subcommand.
 """
 from __future__ import print_function
 
 import os
 import sqlite3
 
 from IPython.config.application import Application
 from IPython.core.application import BaseIPythonApplication
 from IPython.utils.traitlets import Bool, Int, Dict
 
 trim_hist_help = """Trim the IPython history database to the last 1000 entries.
 
 This actually copies the last 1000 entries to a new database, and then replaces
 the old file with the new. Use the `--keep=` argument to specify a number
 other than 1000.
 """
 
 class HistoryTrim(BaseIPythonApplication):
     description = trim_hist_help
     
     backup = Bool(False, config=True,
         help="Keep the old history file as history.sqlite.<N>")
     
     keep = Int(1000, config=True,
         help="Number of recent lines to keep in the database.")
     
     flags = Dict(dict(
         backup = ({'HistoryTrim' : {'backup' : True}},
             backup.get_metadata('help')
         )
     ))
 
     aliases=Dict(dict(
         keep = 'HistoryTrim.keep'
     ))
     
     def start(self):
         profile_dir = self.profile_dir.location
         hist_file = os.path.join(profile_dir, 'history.sqlite')
         con = sqlite3.connect(hist_file)
 
         # Grab the recent history from the current database.
         inputs = list(con.execute('SELECT session, line, source, source_raw FROM '
                                 'history ORDER BY session DESC, line DESC LIMIT ?', (self.keep+1,)))
         if len(inputs) <= self.keep:
             print("There are already at most %d entries in the history database." % self.keep)
             print("Not doing anything. Use --keep= argument to keep fewer entries")
             return
         
         print("Trimming history to the most recent %d entries." % self.keep)
         
         inputs.pop() # Remove the extra element we got to check the length.
         inputs.reverse()
+        if inputs:
         first_session = inputs[0][0]
         outputs = list(con.execute('SELECT session, line, output FROM '
                                    'output_history WHERE session >= ?', (first_session,)))
         sessions = list(con.execute('SELECT session, start, end, num_cmds, remark FROM '
                                     'sessions WHERE session >= ?', (first_session,)))
         con.close()
         
         # Create the new history database.
         new_hist_file = os.path.join(profile_dir, 'history.sqlite.new')
         i = 0
         while os.path.exists(new_hist_file):
             # Make sure we don't interfere with an existing file.
             i += 1
             new_hist_file = os.path.join(profile_dir, 'history.sqlite.new'+str(i))
         new_db = sqlite3.connect(new_hist_file)
         new_db.execute("""CREATE TABLE IF NOT EXISTS sessions (session integer
                             primary key autoincrement, start timestamp,
                             end timestamp, num_cmds integer, remark text)""")
         new_db.execute("""CREATE TABLE IF NOT EXISTS history
                         (session integer, line integer, source text, source_raw text,
                         PRIMARY KEY (session, line))""")
         new_db.execute("""CREATE TABLE IF NOT EXISTS output_history
                         (session integer, line integer, output text,
                         PRIMARY KEY (session, line))""")
         new_db.commit()
 
 
+        if inputs:
         with new_db:
             # Add the recent history into the new database.
             new_db.executemany('insert into sessions values (?,?,?,?,?)', sessions)
             new_db.executemany('insert into history values (?,?,?,?)', inputs)
             new_db.executemany('insert into output_history values (?,?,?)', outputs)
         new_db.close()
 
         if self.backup:
             i = 1
             backup_hist_file = os.path.join(profile_dir, 'history.sqlite.old.%d' % i)
             while os.path.exists(backup_hist_file):
                 i += 1
                 backup_hist_file = os.path.join(profile_dir, 'history.sqlite.old.%d' % i)
             os.rename(hist_file, backup_hist_file)
             print("Backed up longer history file to", backup_hist_file)
         else:
             os.remove(hist_file)
         
         os.rename(new_hist_file, hist_file)
 
 
 class HistoryApp(Application):
     name = u'ipython-history'
     description = "Manage the IPython history database."
 
     subcommands = Dict(dict(
         trim = (HistoryTrim, HistoryTrim.description.splitlines()[0]),
     ))
 
     def start(self):
         if self.subapp is None:
             print("No subcommand specified. Must specify one of: %s" % \
                                                     (self.subcommands.keys()))
             print()
             self.print_description()
             self.print_subcommands()
             self.exit(1)
         else:
             return self.subapp.start()