What’s new in Python 3

At the time of writing Python 2 is still in use, but it's a good idea to upgrade to Python 3 or make your project available for both versions. January 1, 2020, is The End Of Life Date for Python 2.7, meaning it will not be maintained after that.

Python 3 is different from the previous Python releases as it is not compatible with older versions. Making Python 3 backward incompatible was needed to implement a variety of great features that we will talk about in this article.

Let's start with the simplest change.

The Print function

The old print statement has been replaced with a function print(). Calling print as a statement in Python 3 throws a SyntaxError. All features of the print statement are covered by print function so there is no loss of functionality. Example:

Python 2

print 'Python', python_version()
>>> Python 2.7.6
print 'Hello  World'
>>> Hello World

Python 3

print('Python', python_version())
>>> Python 3.4
print('Hello, World!')
>>> Hello World

Old: print x,
New: print(x, ent=’ ‘) 

Integers

In Python 2 there were two kinds of integers: short integers (32-64bits) and long integers (limited by available memory). Python 3 unifies Long Integers and Integers into one type Integer. In Python 2 / is an integer division, in Python 3 / results in a float and // is used for integer division. Example:

Python 2

3 / 2
>>> 1
3 // 2
>>> 1

Python 3

3 / 2
>>> 1.5
3 // 2
>>> 1

Unicode

Python 2 and 3 have two types of strings, byte sequences (ASCII characters) and Unicode strings. The Unicode string represents a series of characters that are represented as a number (0 - 1,114,111). In Python 2 you had to add u in front of the string to make it Unicode. Python 3 makes this more accessible by making all strings Unicode by default. If you want a byte sequence instead of the Unicode string you need to mark the string with a b. Example:

Python 2

name = u'John'
>>> type(name)
<type 'unicode'>

Python 3

name = 'John'
>>> type(name)
<class 'str'>

Following the Python 3 documentation, the most important tip for creating Unicode aware programs is:

Software should only work with Unicode strings internally, decoding the input data as soon as possible and encoding the output only at the end.

Exceptions

There should be one -- and preferably only one -- obvious way to do it

Python 2 raise statement violates that principle with accepting raising exceptions with and without parentheses. In Python 3 you will get a SyntaxError if the exception arguments are not enclosed in parentheses. Example:

Python 2

raise IOError, 'error in file'
>>> File "<stdin>", line 1, in <module>
IOError: error in file
#or
raise IOError('error in file')
>>> File "<stdin>", line 1, in <module>
IOError: error in file

Python 3

#wrong
raise IOError, 'error in file'
>>> SyntaxError: invalid syntax
#right
raise IOError('error in file')
>>>  File "<stdin>", line 1, in <module>
OSError: error in file

The handling of exceptions has slightly changed in Python 3. You must now use an as keyword. Example:

Python 2

try:
    call_function()
except Exception, err:
    print err

Python 3

try:
    call_function()
except Exception as err:
    print err

xrange

The range() function in Python 3 was implemented as Python 2 xrange(). The xrange() function was removed - calling xrange() in Python 3 will raise a NameError.

Returning iterable objects

Iterators have better memory consumption than the lists, that's why in Python 3 the return values of some functions are changed.
Instead of returning lists, these functions now return iterable objects:

  • map() returns an iterator map object.
  • filter() - returns an iterator filter object.
  • zip() - returns an iterator zip object.
  • range() - returns an iterator range object.

Example:

Python 2

type(range(3))
>>> <type 'list'>

Python 3

type(range(3))
>>> <class 'range'>

In case you really a need list instead of an iterable object, simply convert object to list using the list() function.

Ordering Comparisons

Comparing unorderable types raises a TypeError in Python 3. As an example in Python 2 you can compare string and integer objects, but in Python 3 it fails with the a TypeError. Example:
Python 2

5 < '5'
>>> True

Python 3

5 < '5'
Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
TypeError: '<' not supported between instances of 'int' and 'str'

Conclusion

There are more changes and new features in Python 3 which you should check out.

As most of the top libraries are now Python 3 compatible (around 344 of 360 most popular packages), it's safe to pick Python 3 for most of your new projects and consider migrating your existing codebase over from Python 2 to Python 3.

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