json是用来转换python object 和json format 的，字符编码有gb2312，gb18030/gbk，utf-8等。
在 Python 中出现的 str 都是用字符集编码的 ansi 字符串。Python 本身并不知道 str 的编码，需要由开发者指定正确的字符集 decode。
因为 Python 认为 16 位的 unicode 才是字符的唯一内码，而大家常用的字符集如 gb2312，gb18030/gbk，utf-8，以及 ascii 都是字符的二进制（字节）编码形式。把字符从 unicode 转换成二进制编码，当然是要 encode。
# 从 str 转换成 unicode
print s.decode('utf-8')
# 从 unicode 转换成 str
print u.encode('utf-8')
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-3: ordinal not in range(128)
json.dump(obj, fp, skipkeys=False, ensure_ascii=True, check_circular=True, allow_nan=True, cls=None, indent=None,separators=None, encoding="utf-8", default=None, sort_keys=False, **kw)Serialize obj as a JSON formatted stream to fp (a
.write()-supporting file-like object) using this conversion table.
json.dumps(obj, skipkeys=False, ensure_ascii=True, check_circular=True, allow_nan=True, cls=None, indent=None,separators=None, encoding="utf-8", default=None, sort_keys=False, **kw)Serialize obj to a JSON formatted
strusing this conversion table. If ensure_ascii is false, the result may contain non-ASCII characters and the return value may be aunicodeinstance.
json.load(fp[, encoding[, cls[, object_hook[, parse_float[, parse_int[, parse_constant[, object_pairs_hook[,**kw]]]]]]]])Deserialize fp (a
.read()-supporting file-like object containing a JSON document) to a Python object using thisconversion table.
json.loads(s[, encoding[, cls[, object_hook[, parse_float[, parse_int[, parse_constant[, object_pairs_hook[,**kw]]]]]]]])Deserialize s (a
strorunicodeinstance containing a JSON document) to a Python object using this conversion table.
json有上面四种function，注意带s和不带s的区别，json.dumps是把python object转换为json format,反之，json.loads是把json str 转换为python object。
python object 有dict , list 等。
下面给出输出一个大的json字符串到文件，并以可读的格式输出。
#!/usr/bin/env python
import urllib2
import json
import sys
url = 'world.taobao.com/search/json.htm'
url = 'http://' + url
keyword = sys.argv[1]
url = url + '?q=' + keyword
print url
request = urllib2.Request(url)
response = urllib2.urlopen(request)
content = response.read()
if isinstance(content, basestring):
print "content is string"
else:
print "content is not string"
content = json.loads(content, encoding='gbk')
content = json.dumps(content, encoding='gbk', ensure_ascii=False, indent=4, separators=(',', ': '))
content = content.encode('utf-8')
file = keyword
f = open(file, 'w')
f.write(content)
f.close()