last updated 05/27/2014

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

Many beginning Python users are wondering with which version of Python they should start. My answer to this question is usually something along the lines "just go with the version your favorite tutorial was written in, and check out the differences later on."

But what if you are starting a new project and have the choice to pick? I would say there is currently no "right" or "wrong" as long as both Python 2.7.x and Python 3.x support the libraries that you are planning to use. However, it is worthwhile to have a look at the major differences between those two most popular versions of Python to avoid common pitfalls when writing the code for either one of them, or if you are planning to port your project.

Sections

Using the __future__ module
The print function
Integer division
Unicode
xrange
Raising exceptions
Handling exceptions
The next() function and .next() method
For-loop variables and the global namespace leak
Comparing unorderable types
Parsing user inputs via input()
Returning iterable objects instead of lists
More articles about Python 2 and Python 3

The `future` module

Python 3.x introduced some Python 2-incompatible keywords and features that can be imported via the in-built __future__ module in Python 2. It is recommended to use __future__ imports it if you are planning Python 3.x support for your code. For example, if we want Python 3.x's integer division behavior in Python 2, we can import it via

from __future__ import division

More features that can be imported from the __future__ module are listed in the table below:

feature optional in mandatory in effect

nested_scopes

2.1.0b1

2.2

PEP 227: Statically Nested Scopes </tr>

generators

2.2.0a1

2.3

PEP 255: Simple Generators </tr>

division

2.2.0a2

3.0

PEP 238: Changing the Division Operator </tr>

absolute_import

2.5.0a1

3.0

PEP 328: Imports: Multi-Line and Absolute/Relative </tr>

with_statement

2.5.0a1

2.6

PEP 343: The “with” Statement </tr>

print_function

2.6.0a2

3.0

PEP 3105: Make print a function </tr>

unicode_literals

2.6.0a2

3.0

PEP 3112: Bytes literals in Python 3000 </tr> </table>

(Source: [https://docs.python.org/2/library/__future__.html](https://docs.python.org/2/library/__future__.html#module-__future__))



In [1]:

    
from platform import python_version

The print function

[back to the section-overview]

Very trivial, and the change in the print-syntax is probably the most widely known change, but still it is worth mentioning: Python 2's print statement has been replaced by the print() function, meaning that we have to wrap the object that we want to print in parantheses.

Python 2 doesn't have a problem with additional parantheses, but in contrast, Python 3 would raise a SyntaxError if we called the print function the Python 2-way without the parentheses.

Python 2



In [3]:

    
print 'Python', python_version()
print 'Hello, World!'
print('Hello, World!')
print "text", ; print 'print more text on the same line'









    



Python 2.7.6
Hello, World!
Hello, World!
text print more text on the same line

Python 3



In [2]:

    
print('Python', python_version())
print('Hello, World!')

print("some text,", end="") 
print(' print more text on the same line')









    



Python 3.4.1
Hello, World!
some text, print more text on the same line



In [3]:

    
print 'Hello, World!'









    



  File "<ipython-input-3-139a7c5835bd>", line 1
    print 'Hello, World!'
                        ^
SyntaxError: invalid syntax

Note:

Printing "Hello, World" above via Python 2 looked quite "normal". However, if we have multiple objects inside the parantheses, we will create a tuple, since print is a "statement" in Python 2, not a function call.



In [4]:

    
print 'Python', python_version()
print('a', 'b')
print 'a', 'b'









    



Python 2.7.6
('a', 'b')
a b

Integer division

[back to the section-overview]

This change is particularly dangerous if you are porting code, or if you are executing Python 3 code in Python 2, since the change in integer-division behavior can often go unnoticed (it doesn't raise a SyntaxError).
So, I still tend to use a float(3)/2 or 3/2.0 instead of a 3/2 in my Python 3 scripts to save the Python 2 guys some trouble (and vice versa, I recommend a from __future__ import division in your Python 2 scripts).

Python 2



In [4]:

    
print 'Python', python_version()
print '3 / 2 =', 3 / 2
print '3 // 2 =', 3 // 2
print '3 / 2.0 =', 3 / 2.0
print '3 // 2.0 =', 3 // 2.0









    



Python 2.7.6
3 / 2 = 1
3 // 2 = 1
3 / 2.0 = 1.5
3 // 2.0 = 1.0

Python 3



In [4]:

    
print('Python', python_version())
print('3 / 2 =', 3 / 2)
print('3 // 2 =', 3 // 2)
print('3 / 2.0 =', 3 / 2.0)
print('3 // 2.0 =', 3 // 2.0)









    



Python 3.4.1
3 / 2 = 1.5
3 // 2 = 1
3 / 2.0 = 1.5
3 // 2.0 = 1.0

Unicode

[back to the section-overview]

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

xrange

[back to the section-overview]

The usage of xrange() is very popular in Python 2.x for creating an iterable object, e.g., in a for-loop or list/set-dictionary-comprehension.
The behavior was quite similar to a generator (i.e., "lazy evaluation"), but here the xrange-iterable is not exhaustible - meaning, you could iterate over it infinitely.

Thanks to its "lazy-evaluation", the advantage of the regular range() is that xrange() is generally faster if you have to iterate over it only once (e.g., in a for-loop). However, in contrast to 1-time iterations, it is not recommended if you repeat the iteration multiple times, since the generation happens every time from scratch!

In Python 3, the range() was implemented like the xrange() function so that a dedicated xrange() function does not exist anymore (xrange() raises a NameError in Python 3).



In [5]:

    
import timeit

n = 10000
def test_range(n):
    for i in range(n):
        pass
    
def test_xrange(n):
    for i in xrange(n):
        pass

Python 2



In [6]:

    
print 'Python', python_version()

print '\ntiming range()'
%timeit test_range(n)

print '\n\ntiming xrange()'
%timeit test_xrange(n)









    



Python 2.7.6

timing range()
1000 loops, best of 3: 433 µs per loop


timing xrange()
1000 loops, best of 3: 350 µs per loop

Python 3



In [7]:

    
print('Python', python_version())

print('\ntiming range()')
%timeit test_range(n)









    



Python 3.4.1

timing range()
1000 loops, best of 3: 520 µs per loop



In [8]:

    
print(xrange(10))









    



---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-8-5d8f9b79ea70> in <module>()
----> 1 print(xrange(10))

NameError: name 'xrange' is not defined

Note about the speed differences in Python 2 and 3

Some people pointed out the speed difference between Python 3's range() and Python2's xrange(). Since they are implemented the same way one would expect the same speed. However the difference here just comes from the fact that Python 3 generally tends to run slower than Python 2.



In [3]:

    
def test_while():
    i = 0
    while i < 20000:
        i += 1
    return



In [4]:

    
print('Python', python_version())
%timeit test_while()









    



Python 3.4.1
100 loops, best of 3: 2.68 ms per loop



In [6]:

    
print 'Python', python_version()
%timeit test_while()









    



Python 2.7.6
1000 loops, best of 3: 1.72 ms per loop

Raising exceptions

[back to the section-overview]

Where Python 2 accepts both notations, the 'old' and the 'new' syntax, Python 3 chokes (and raises a SyntaxError in turn) if we don't enclose the exception argument in parentheses:

Python 2



In [7]:

    
print 'Python', python_version()









    



Python 2.7.6



In [8]:

    
raise IOError, "file error"









    



---------------------------------------------------------------------------
IOError                                   Traceback (most recent call last)
<ipython-input-8-25f049caebb0> in <module>()
----> 1 raise IOError, "file error"

IOError: file error



In [9]:

    
raise IOError("file error")









    



---------------------------------------------------------------------------
IOError                                   Traceback (most recent call last)
<ipython-input-9-6f1c43f525b2> in <module>()
----> 1 raise IOError("file error")

IOError: file error

Python 3



In [9]:

    
print('Python', python_version())









    



Python 3.4.1



In [10]:

    
raise IOError, "file error"









    



  File "<ipython-input-10-25f049caebb0>", line 1
    raise IOError, "file error"
                 ^
SyntaxError: invalid syntax

The proper way to raise an exception in Python 3:



In [11]:

    
print('Python', python_version())
raise IOError("file error")









    



Python 3.4.1






    



---------------------------------------------------------------------------
OSError                                   Traceback (most recent call last)
<ipython-input-11-c350544d15da> in <module>()
      1 print('Python', python_version())
----> 2 raise IOError("file error")

OSError: file error

Handling exceptions

[back to the section-overview]

Also the handling of exceptions has slightly changed in Python 3. In Python 3 we have to use the "as" keyword now

Python 2



In [10]:

    
print 'Python', python_version()
try:
    let_us_cause_a_NameError
except NameError, err:
    print err, '--> our error message'









    



Python 2.7.6
name 'let_us_cause_a_NameError' is not defined --> our error message

Python 3



In [12]:

    
print('Python', python_version())
try:
    let_us_cause_a_NameError
except NameError as err:
    print(err, '--> our error message')









    



Python 3.4.1
name 'let_us_cause_a_NameError' is not defined --> our error message

The next() function and .next() method

[back to the section-overview]

Since next() (.next()) is such a commonly used function (method), this is another syntax change (or rather change in implementation) that is worth mentioning: where you can use both the function and method syntax in Python 2.7.5, the next() function is all that remains in Python 3 (calling the .next() method raises an AttributeError).

Python 2



In [11]:

    
print 'Python', python_version()

my_generator = (letter for letter in 'abcdefg')

next(my_generator)
my_generator.next()









    



Python 2.7.6






    Out[11]:





'b'

Python 3



In [13]:

    
print('Python', python_version())

my_generator = (letter for letter in 'abcdefg')

next(my_generator)









    



Python 3.4.1






    Out[13]:





'a'



In [14]:

    
my_generator.next()









    



---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-14-125f388bb61b> in <module>()
----> 1 my_generator.next()

AttributeError: 'generator' object has no attribute 'next'

For-loop variables and the global namespace leak

[back to the section-overview]

Good news is: In Python 3.x for-loop variables don't leak into the global namespace anymore!

This goes back to a change that was made in Python 3.x and is described in What’s New In Python 3.0 as follows:

"List comprehensions no longer support the syntactic form [... for var in item1, item2, ...]. Use [... for var in (item1, item2, ...)] instead. Also note that list comprehensions have different semantics: they are closer to syntactic sugar for a generator expression inside a list() constructor, and in particular the loop control variables are no longer leaked into the surrounding scope."

Python 2



In [12]:

    
print 'Python', python_version()

i = 1
print 'before: i =', i

print 'comprehension: ', [i for i in range(5)]

print 'after: i =', i









    



Python 2.7.6
before: i = 1
comprehension:  [0, 1, 2, 3, 4]
after: i = 4

Python 3



In [15]:

    
print('Python', python_version())

i = 1
print('before: i =', i)

print('comprehension:', [i for i in range(5)])

print('after: i =', i)









    



Python 3.4.1
before: i = 1
comprehension: [0, 1, 2, 3, 4]
after: i = 1

Comparing unorderable types

[back to the section-overview]

Another nice change in Python 3 is that a TypeError is raised as warning if we try to compare unorderable types.

Python 2



In [2]:

    
print 'Python', python_version()
print "[1, 2] > 'foo' = ", [1, 2] > 'foo'
print "(1, 2) > 'foo' = ", (1, 2) > 'foo'
print "[1, 2] > (1, 2) = ", [1, 2] > (1, 2)









    



Python 2.7.6
[1, 2] > 'foo' =  False
(1, 2) > 'foo' =  True
[1, 2] > (1, 2) =  False

Python 3



In [16]:

    
print('Python', python_version())
print("[1, 2] > 'foo' = ", [1, 2] > 'foo')
print("(1, 2) > 'foo' = ", (1, 2) > 'foo')
print("[1, 2] > (1, 2) = ", [1, 2] > (1, 2))









    



Python 3.4.1






    



---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-16-a9031729f4a0> in <module>()
      1 print('Python', python_version())
----> 2 print("[1, 2] > 'foo' = ", [1, 2] > 'foo')
      3 print("(1, 2) > 'foo' = ", (1, 2) > 'foo')
      4 print("[1, 2] > (1, 2) = ", [1, 2] > (1, 2))

TypeError: unorderable types: list() > str()

Parsing user inputs via input()

[back to the section-overview]

Fortunately, the input() function was fixed in Python 3 so that it always stores the user inputs as str objects. In order to avoid the dangerous behavior in Python 2 to read in other types than strings, we have to use raw_input() instead.

Python 2

Python 2.7.6 
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

>>> my_input = input('enter a number: ')

enter a number: 123

>>> type(my_input)
<type 'int'>

>>> my_input = raw_input('enter a number: ')

enter a number: 123

>>> type(my_input)
<type 'str'>

Python 3

Python 3.4.1 
[GCC 4.2.1 (Apple Inc. build 5577)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

>>> my_input = input('enter a number: ')

enter a number: 123

>>> type(my_input) <class 'str'> </pre></div>

Returning iterable objects instead of lists

[back to the section-overview]

As we have already seen in the xrange section, some functions and methods return iterable objects in Python 3 now - instead of lists in Python 2.

Since we usually iterate over those only once anyway, I think this change makes a lot of sense to save memory. However, it is also possible - in contrast to generators - to iterate over those multiple times if needed, it is aonly not so efficient.

And for those cases where we really need the list-objects, we can simply convert the iterable object into a list via the list() function.

Python 2



In [2]:

    
print 'Python', python_version() 

print range(3) 
print type(range(3))









    



Python 2.7.6
[0, 1, 2]
<type 'list'>

Python 3



In [7]:

    
print('Python', python_version())

print(range(3))
print(type(range(3)))
print(list(range(3)))









    



Python 3.4.1
range(0, 3)
<class 'range'>
[0, 1, 2]

Some more commonly used functions and methods that don't return lists anymore in Python 3:

zip()
map()
filter()
dictionary's .keys() method
dictionary's .values() method
dictionary's .items() method

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

Sections

The __future__ module

The print function

Python 2

Python 3

Integer division

Python 2

Python 3

Unicode

Python 2

Python 3

xrange

Python 2

Python 3

Note about the speed differences in Python 2 and 3

Raising exceptions

Python 2

Python 3

Handling exceptions

Python 2

Python 3

The next() function and .next() method

Python 2

Python 3

For-loop variables and the global namespace leak

Python 2

Python 3

Comparing unorderable types

Python 2

Python 3

Parsing user inputs via input()

Python 2

Python 3

Returning iterable objects instead of lists

Python 2

Python 3

More articles about Python 2 and Python 3

The `future` module