appendと+の違い

for_hana.py

import pprint
print '----try not append----'
a = []
print 'object id of variable \'a\' => %d' % id(a)
print 'content of variable \'a\': %s' % pprint.pformat(a)
a = a + [1]
print 'added value 1 with + operator'
print 'object id of variable \'a\' has changed: %d' % id(a)
print 'content of variable \'a\': %s' % pprint.pformat(a)

print '----try append----'
b = []
print 'object id of variable \'b\' => %d' % id(a)
print 'content of variable \'b\': %s' % pprint.pformat(b)
b.append(2)
print 'added value 2 with append() method'
print 'object id of variable \'b\' has not changed: %d' % id(a)
print 'content of variable \'b\': %s' % pprint.pformat(b)