python字符串连接导致效率问题

dictt = {}
print id(dictt)
dictt.setdefault("key","value")
dictt.setdefault("key1","value")
dictt.setdefault("key2","value")
dictt.setdefault("key3","value")
dictt.setdefault("key4","value")
dictt.setdefault("key5","value")
dictt.setdefault("key6","value")
dictt.setdefault("key7","value")
for k,v in dictt.items():
print k,v
print id(dictt)

l = []
print id(l)

l.append("fsdf")
l.append("fsdf1")
l.append("fsdf2")
l.append("fsdf3")
l.append("fsdf4")
l.append("fsdf5")
print l
print id(l)

s = "abcd"
print "this id is %s"%id(s)
s = s + "efg"
print s
print id(s)

33433472
key3 value
key2 value
key1 value
key7 value
key6 value
key5 value
key4 value
key value
33433472
38612384
['fsdf', 'fsdf1', 'fsdf2', 'fsdf3', 'fsdf4', 'fsdf5']
38612384
this id is 38640416
abcdefg
38640480
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