Different behaviour for io in pickle with string content - python-2.7

Question

When working with pickled data I encountered a different behavior for the io.open and __builtin__.open. Consider the following simple example:
import pickle
payload = 'foo'
fn = 'test.pickle'
pickle.dump(payload, open(fn, 'w'))
a = pickle.load(open(fn, 'r'))
This works as expected. But running this code here:
import pickle
import io
payload = 'foo'
fn = 'test.pickle'
pickle.dump(payload, io.open(fn, 'w'))
a = pickle.load(io.open(fn, 'r'))
gives the following Traceback:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "D:\WinPython-32bit-2.7.8.1\python-2.7.8\lib\site-packages\spyderlib\widgets\externalshell\sitecustomize.py", line 580, in runfile
execfile(filename, namespace)
File "D:/**.py", line 15, in <module>
pickle.dump(payload, io.open(fn, 'w'))
File "D:\WinPython-32bit-2.7.8.1\python-2.7.8\lib\pickle.py", line 1370, in dump
Pickler(file, protocol).dump(obj)
File "D:\WinPython-32bit-2.7.8.1\python-2.7.8\lib\pickle.py", line 224, in dump
self.save(obj)
File "D:\WinPython-32bit-2.7.8.1\python-2.7.8\lib\pickle.py", line 286, in save
f(self, obj) # Call unbound method with explicit self
File "D:\WinPython-32bit-2.7.8.1\python-2.7.8\lib\pickle.py", line 488, in save_string
self.write(STRING + repr(obj) + '\n')
TypeError: must be unicode, not str
As I want to be future-compatible, how can I circumwent this misbehavior? Or, what else am I doing wrong here?
I stumbled over this when dumping dictionaries with keys of type string.
My python version is:
'2.7.8 (default, Jun 30 2014, 16:03:49) [MSC v.1500 32 bit (Intel)]'

Answer 1

The difference is not supprising, because io.open() explicitly deals with Unicode strings when using text mode. The documentation is quite clear about this:
Note: Since this module has been designed primarily for Python 3.x, you have to be aware that all uses of “bytes” in this document refer to the str type (of which bytes is an alias), and all uses of “text” refer to the unicode type. Furthermore, those two types are not interchangeable in the io APIs.
and
Python distinguishes between files opened in binary and text modes, even when the underlying operating system doesn’t. Files opened in binary mode (including 'b' in the mode argument) return contents as bytes objects without any decoding. In text mode (the default, or when 't' is included in the mode argument), the contents of the file are returned as unicode strings, the bytes having been first decoded using a platform-dependent encoding or using the specified encoding if given.
You need to open files in binary mode. The fact that it worked without with the built-in open() at all is actually more luck than wisdom; if your pickles contained data with \n and/or \r bytes the pickle loading may well fail. The Python 2 default pickle happens to be a text protocol but the output should still be considered as binary.
In all cases, when writing pickle data, use binary mode:
pickle.dump(payload, open(fn, 'wb'))
a = pickle.load(open(fn, 'rb'))
or
pickle.dump(payload, io.open(fn, 'wb'))
a = pickle.load(io.open(fn, 'rb'))