Key differences between Python 2.7.x and Python 3.x

This article describe the main differences between Python 2 and 3.

From my perspective, this is the most important change of all:

Unicode

Python 2 has ASCII str() types, separate unicode(), but no byte type.
Now, in Python 3, we finally have Unicode (utf-8) strings, and 2 byte classes: byte and bytearrays.

Python 2

In [2]: print 'Python', python_version()

Python 2.7.6

In [3]: print type(unicode('this is like a python3 str type'))

<type 'unicode'>

In [4]: print type(b'byte type does not exist')

<type 'str'>

In [5]: print 'they are really' + b' the same'

they are really the same

In [7]: print type(bytearray(b'bytearray oddly does exist though'))

<type 'bytearray'>

Python 3

In [6]:

print('Python', python_version())
print('strings are now utf-8 \u03BCnico\u0394é!')

Python 3.4.1
strings are now utf-8 μnicoΔé!

In [8]:

print('Python', python_version(), end="")
print(' has', type(b' bytes for storing data'))

Python 3.4.1 has <class 'bytes'>

In [11]:

print('and Python', python_version(), end="")
print(' also has', type(bytearray(b'bytearrays')))

and Python 3.4.1 also has <class 'bytearray'>

In [13]:

'note that we cannot add a string' + b'bytes for data'

--------------------------------------------------------------------------- 

TypeError Traceback (most recent call last)
<ipython-input-13-d3e8942ccf81> in <module>()  

----> 1 'note that we cannot add a string' + b'bytes for data'

TypeError: Can't convert 'bytes' object to str implicitly

Full article: http://nbviewer.ipython.org/github/rasbt/python_reference/blob/master/tutorials/key_differences_between_python_2_and_3.ipynb