Python dict反序列化在python2.7中有效，在3.3中失败

Question

I'm using a sqlite3 table to store python dicts (utf8 content) and serialization is done with JSON. It works fine in python2.7 but fails in 3.3.

Schema:

CREATE TABLE mytable
  (id INTEGER, book TEXT NOT NULL, d JSON NOT NULL, priority INTEGER NOT NULL DEFAULT(3), 
   PRIMARY KEY (id, book))

When inserting values, the dict is serialized with json.dumps(d) . The faulty part is retrieving the previously saved values.

import sys
import sqlite3
import json

filename = 'mydb.db'
sqlite3.register_converter('JSON', json.loads)
conn = sqlite3.connect(filename, detect_types=sqlite3.PARSE_DECLTYPES|sqlite3.PARSE_COLNAMES)
c = conn.cursor()
c.execute('''SELECT book, id, d, priority FROM mytable''')
print(c.fetchall())

The above script works fine when executed with python2.7. However, using 3.3 a TypeError occures:

Traceback (most recent call last):
  File "tests/py3error_debug.py", line 15, in <module>
    c.execute('''SELECT book, id, d, priority FROM mytable''')
  File "/usr/local/Cellar/python3/3.3.2/Frameworks/Python.framework/Versions/3.3/lib/python3.3/json/__init__.py", line 319, in loads
    return _default_decoder.decode(s)
  File "/usr/local/Cellar/python3/3.3.2/Frameworks/Python.framework/Versions/3.3/lib/python3.3/json/decoder.py", line 352, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
TypeError: can't use a string pattern on a bytes-like object

I can't spot an essential difference between the 2.7 and 3.3 JSON modules (especially regarding json.loads ) and I'm running out of ideas.

Answer 1

According to Python 3 json module documentation :

The json module always produces str objects, not bytes objects. ...

According to Python 3 sqlite3.register_converter documentation :

Registers a callable to convert a bytestring from the database into a custom Python type. ...

If you try to load bytesstring json, you get TypeError:

>>> json.loads('"0"')
'0'
>>> json.loads(b'"0"')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.3/json/__init__.py", line 319, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3.3/json/decoder.py", line 352, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
TypeError: can't use a string pattern on a bytes-like object

You should convert bytes to str before call json.loads using bytes.decode .

sqlite3.register_converter('JSON', lambda x: json.loads(x.decode('utf-8')))