last updated 07/02/2016

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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
Banker's Rounding
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:

from platform import python_version

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

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

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

Python 2.7.7
('a', 'b')
a b

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

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

Python 2.7.6

<type 'unicode'>

<type 'str'>

they are really the same

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¶

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¶

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

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.

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

Python 2.7.7
('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¶

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¶

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¶

print 'Python', python_version()

Python 2.7.6

print type(unicode('this is like a python3 str type'))

<type 'unicode'>

print type(b'byte type does not exist')

<type 'str'>

print 'they are really' + b' the same'

they are really the same

print type(bytearray(b'bytearray oddly does exist though'))

<type 'bytearray'>

Python 3¶

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

Python 3.4.1
strings are now utf-8 μnicoΔé!

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

Python 3.4.1 has <class 'bytes'>

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

and Python 3.4.1 also has <class 'bytearray'>

'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).

import timeit

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

Python 2¶

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¶

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

print(xrange(10))

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

NameError: name 'xrange' is not defined

The `contains` method for `range` objects in Python 3¶

Another thing worth mentioning is that range got a "new" __contains__ method in Python 3.x (thanks to Yuchen Ying, who pointed this out). The __contains__ method can speedup "look-ups" in Python 3.x range significantly for integer and Boolean types.

x = 10000000

def val_in_range(x, val):
    return val in range(x)

def val_in_xrange(x, val):
    return val in xrange(x)

print('Python', python_version())
assert(val_in_range(x, x/2) == True)
assert(val_in_range(x, x//2) == True)
%timeit val_in_range(x, x/2)
%timeit val_in_range(x, x//2)

Python 3.4.1
1 loops, best of 3: 742 ms per loop
1000000 loops, best of 3: 1.19 µs per loop

Based on the timeit results above, you see that the execution for the "look up" was about 60,000 faster when it was of an integer type rather than a float. However, since Python 2.x's range or xrange doesn't have a __contains__ method, the "look-up speed" wouldn't be that much different for integers or floats:

print 'Python', python_version()
assert(val_in_xrange(x, x/2.0) == True)
assert(val_in_xrange(x, x/2) == True)
assert(val_in_range(x, x/2) == True)
assert(val_in_range(x, x//2) == True)
%timeit val_in_xrange(x, x/2.0)
%timeit val_in_xrange(x, x/2)
%timeit val_in_range(x, x/2.0)
%timeit val_in_range(x, x/2)

Python 2.7.7
1 loops, best of 3: 285 ms per loop
1 loops, best of 3: 179 ms per loop
1 loops, best of 3: 658 ms per loop
1 loops, best of 3: 556 ms per loop

Below the "proofs" that the __contain__ method wasn't added to Python 2.x yet:

print('Python', python_version())
range.__contains__

Python 3.4.1

<slot wrapper '__contains__' of 'range' objects>

print 'Python', python_version()
range.__contains__

Python 2.7.7

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-7-05327350dafb> in <module>()
      1 print 'Python', python_version()
----> 2 range.__contains__

AttributeError: 'builtin_function_or_method' object has no attribute '__contains__'

print 'Python', python_version()
xrange.__contains__

Python 2.7.7

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-8-7d1a71bfee8e> in <module>()
      1 print 'Python', python_version()
----> 2 xrange.__contains__

AttributeError: type object 'xrange' has no attribute '__contains__'

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.

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

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

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

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¶

print 'Python', python_version()

Python 2.7.6

raise IOError, "file error"

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

IOError: file error

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¶

print('Python', python_version())

Python 3.4.1

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:

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¶

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¶

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¶

print 'Python', python_version()

my_generator = (letter for letter in 'abcdefg')

next(my_generator)
my_generator.next()

Python 2.7.6

'b'

Python 3¶

print('Python', python_version())

my_generator = (letter for letter in 'abcdefg')

next(my_generator)

Python 3.4.1

'a'

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¶

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¶

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¶

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¶

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¶

print 'Python', python_version() 

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

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

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

Python 2.7.12

16.0

17.0

Python 3.5.1

16

16

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¶

print 'Python', python_version() 

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

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

Python 3¶

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

Banker's Rounding¶

[back to the section-overview]

Python 3 adopted the now standard way of rounding decimals when it results in a tie (.5) at the last significant digits. Now, in Python 3, decimals are rounded to the nearest even number. Although it's an inconvenience for code portability, it's supposedly a better way of rounding compared to rounding up as it avoids the bias towards large numbers. For more information, see the excellent Wikipedia articles and paragraphs:

Python 2¶

print 'Python', python_version()

Python 2.7.12

round(15.5)

16.0

round(16.5)

17.0

Python 3¶

print('Python', python_version())

Python 3.5.1

round(15.5)

16

round(16.5)

16

feature	optional in	mandatory in	effect
nested_scopes	2.1.0b1	2.2	PEP 227: Statically Nested Scopes
generators	2.2.0a1	2.3	PEP 255: Simple Generators
division	2.2.0a2	3.0	PEP 238: Changing the Division Operator
absolute_import	2.5.0a1	3.0	PEP 328: Imports: Multi-Line and Absolute/Relative
with_statement	2.5.0a1	2.6	PEP 343: The “with” Statement
print_function	2.6.0a2	3.0	PEP 3105: Make print a function
unicode_literals	2.6.0a2	3.0	PEP 3112: Bytes literals in Python 3000

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¶

The __contains__ method for range objects in 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¶

Banker's Rounding¶

Python 2¶

Python 3¶

More articles about Python 2 and Python 3¶

The `future` module¶

The `contains` method for `range` objects in Python 3¶