python - How to pickle unicodes and save them in utf-8 databases

Question

I have a database (mysql) where I want to store pickled data.

The data can be for instance a dictionary, which may contain unicode, e.g.

data = {1 : u'é'}

and the database (mysql) is in utf-8.

When I pickle,

import pickle
pickled_data = pickle.dumps(data)
print type(pickled_data) # returns <type 'str'>

the resulting pickled_data is a string.

When I try to store this in a database (e.g. in a Textfield) this can causes problems. In particular, I'm getting at some point a

UnicodeDecodeError "'utf8' codec can't decode byte 0xe9 in position X"

when trying to save the pickled_data in the database. This makes sense because pickled_data can have non-utf-8 characters. My question is how do I store pickled_data on a utf-8 database?

I see two possible candidates:

1. Encode the result of the pickle.dump to utf-8 and store it. When I want to pickle.load, I have to decode it.

2. Store the pickled string in binary format (how?), which forces all characters to be within ascii.

My issue is that I'm not seeing what are the consequences of choosing one of this options in the long run. Since the change already requires some effort, I'm driven to ask for an opinion on this issue, asking for eventual better candidates.

(P.S. This is for instance useful in Django)

Answer 1

Pickle data is opaque, binary data, even when you use protocol version 0:

>>> pickle.dumps(data, 0)
'(dp0
I1
Vxe9
p1
s.'

When you try to store that in a TextField, Django will try to decode that data to UTF8 to store it; this is what fails because this is not UTF-8 encoded data; it is binary data instead:

>>> pickled_data.decode('utf8')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mj/Development/venvs/stackoverflow-2.7/lib/python2.7/encodings/utf_8.py", line 16, in decode
    return codecs.utf_8_decode(input, errors, True)
UnicodeDecodeError: 'utf8' codec can't decode byte 0xe9 in position 9: invalid continuation byte

The solution is to not try to store this in a TextField. Use a BinaryField instead:

A field to store raw binary data. It only supports bytes assignment. Be aware that this field has limited functionality. For example, it is not possible to filter a queryset on a BinaryField value.

You have a bytes value (Python 2 strings are byte strings, renamed to bytes in Python 3).

If you insist on storing the data in a text field, explicitly decode it as latin1; the Latin 1 codec maps bytes one-on-one to Unicode codepoints:

>>> pickled_data.decode('latin1')
u'(dp0
I1
Vxe9
p1
s.'

and make sure you encode it again before unpickling again:

>>> encoded = pickled_data.decode('latin1')
>>> pickle.loads(encoded)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mj/Development/Libraries/buildout.python/parts/opt/lib/python2.7/pickle.py", line 1381, in loads
    file = StringIO(str)
UnicodeEncodeError: 'ascii' codec can't encode character u'xe9' in position 9: ordinal not in range(128)
>>> pickle.loads(encoded.encode('latin1'))
{1: u'xe9'}

Do note that if you let this value go to the browser and back again in a text field, the browser is likely to have replaced characters in that data. Internet Explorer will replace characters with , for example, because it assumes it is dealing with text.

Not that you ever should allow accepting pickle data from a network connection in any case, because that is a security hole waiting for exploitation.