Detailed explanation of the difference between Python3.x and Python2.x

Python3.x is a revolutionary upgrade of Python, which abolishes many previous features and introduces new features. Not fully compatible with old code from 1.x and 2.x.

Python2.x is backward compatible, and the latest version is Python 2.7. Among them, versions 2.6 and 2.7 were released to allow Python to smoothly transition to 3.X, and also added some 3.X features.

Currently, many libraries do not support 3.x (for example: Django, wxPython), and most applications are still based on 2.x. Beginners are recommended to use version 2.7.

Here are some main differences between Python 3.X and 2.X

1.Performance
Py3.0 runs pystone benchmark 30% slower than Py2.5. Guido believes that Py3.0 has great room for optimization, and can
to achieve good optimization results.
The performance of Py3.1 is 15% slower than that of Py2.5, and there is still a lot of room for improvement.

2. Encoding
Py3.X source code files use utf-8 encoding by default, which makes the following code legal:
>>> China = 'china'
>>>print(China)
china

3. Grammar
1) Removed <> and replaced all with !=
2) Remove `` and use repr()
instead. 3) Add as and with keywords, as well as True, False, None
4) Integer division returns a floating point number. To get an integer result, please use //
5) Add nonlocal statement. Peripheral (non-global) variables can be assigned directly using noclocal x
6) Remove the print statement and add the print() function to achieve the same function. The same is true for the exec statement, which has been changed to the exec() function
For example:
2.X: print "The answer is", 2*2
3.X: print("The answer is", 2*2)
2.X: print x, 3.X: print(x, end=" ") # Use spaces to replace lines
2.X: print 2.X: print # Output new line
3.X: print() ​ 2.X: print >>sys.stderr, "fatal error"
3.X: print("fatal error", file=sys.stderr)
2.X: print (x, y) 3.X: print((x, y)) # Different from print(x, y)!
7) Changed the behavior of sequential operators, such as x 8) The input function has changed, raw_input has been deleted and replaced with input:
2.X:guess = int(raw_input('Enter an integer : ')) # Method to read keyboard input
3.X:guess = int(input('Enter an integer : '))

9) Remove tuple parameter unpacking. You cannot define the function like def(a, (b, c)):pass
10) For the new octal word variable, the oct() function has been modified accordingly.
2.The method of X is as follows:
>>> 0666
  438
>>> oct(438)
'0666'
3.X like this:
>>> 0666
SyntaxError: invalid token (, line 1)
>>> 0o666
  438
>>> oct(438)
'0o666'
11) Added binary literals and bin() function
>>> bin(438)
'0b110110110'
>>> _438 = '0b110110110'
>>> _438
'0b110110110'
12) Extended iterable unpacking. In Py3.X, a, b, *rest = seq and *rest, a = seq are legal, and only require two points: rest is a list
Objects and seq are iterable.
13) The new super() can no longer pass parameters to super(),
>>> class C(object):
            def __init__(self, a):
                      print('C', a)
>>> class D(C):
            def __init(self, a):
                super().__init__(a) # Call super() without parameters
>>> D(8)
C 8
<__main__.D object at 0x00D7ED90>
14) New metaclass syntax:
Class Foo(*bases, **kwds):
Pass
15) Support class decorator. Usage is the same as function decorator:
>>> def foo(cls_a):
            def print_func(self):
                                  print('Hello, world!')
           cls_a.print = print_func
           return cls_a
>>> @foo
Class C(object):
Pass
>>> C().print()
Hello, world!
Class decorator can be used to play the trick of changing the civet cat into a prince. For more information please refer to PEP 3129

4. Strings and byte strings
1) Now there is only one type of string, str, but it is almost the same as the 2.x version of unicode.

2) Regarding byte strings, please refer to item 2 of "Data Type"

5.Data type
1) Py3.X has removed the long type, and now there is only one integer type - int, but it behaves like the 2. 2) A new bytes type is added, corresponding to the octet string of version 2.X. The method of defining a bytes literal is as follows:
>>> b = b'china'
>>> type(b)

str objects and bytes objects can be converted to each other using the .encode() (str -> bytes) or .decode() (bytes -> str) method.
>>> s = b.decode()
>>> s
'china'
>>> b1 = s.encode()
>>> b1
b'china'
3) The .keys(), .items and .values() methods of dict return iterators, while the previous iterkeys() and other functions have been abandoned. Also removed are
dict.has_key(), replace it with in

6. Object-oriented
1) Introduce abstract base classes (Abstract Base Classes, ABCs).
2) Container classes and iterator classes are ABCsized, so there are many more types in the cellections module than in Py2.5.
>>> import collections
>>> print('\n'.join(dir(collections)))
​ Callable
Container
Hashable
ItemsView
Iterable
Iterator
​ KeysView
Mapping
MappingView
MutableMapping
MutableSequence
MutableSet
NamedTuple
Sequence
Set
Sized
ValuesView
__all__
__builtins__
__doc__
__file__
__name__
_abcoll
_itemgetter
_sys
defaultdict
deque
In addition, numeric types are also ABCsized. On these two points, see PEP 3119 and PEP 3141.
3) The next() method of iterator is renamed to __next__(), and the built-in function next() is added to call the __next__() method of iterator
4) Two decorators, @abstractmethod and @abstractproperty, have been added to make it more convenient to write abstract methods (properties).

7.Exception
1) All exceptions inherit from BaseException and StardardError
is removed 2) Removed the sequence behavior and .message attribute of the exception class
3) Use raise Exception(args) instead of raise Exception, args syntax
4) The syntax for catching exceptions has changed, and the as keyword has been introduced to identify exception instances. In Py2.5:
>>> try:
...     raise NotImplementedError('Error')
...except NotImplementedError, error:

... ... print error.message
...
Error
In Py3.0:
>>> try:
Raise NotImplementedError('Error')
         except NotImplementedError as error: #Note this as
                                     print(str(error))
Error
5) Exception chain, because __context__ is not implemented in version 3.0a1

8. Module changes
1) The cPickle module has been removed and can be replaced by the pickle module. Eventually we will have a transparent and efficient module.
2) Removed imageop module
3) Removed audiodev, Bastion, bsddb185, exceptions, linuxaudiodev, md5, MimeWriter, mimify, popen2,
rexec, sets, sha, stringold, strop, sunaudiodev, timing and xmllib modules
4) Removed the bsddb module (released separately, available from http://www.jcea.es/programacion/pybsddb.htm)
5) Removed new module
6) The os.tmpnam() and os.tmpfile() functions have been moved to the tmpfile module
7) The tokenize module now works with bytes. The main entry point is no longer generate_tokens, but tokenize.tokenize()

9. Others
1) xrange() is renamed to range(). If you want to use range() to obtain a list, you must explicitly call:
>>> list(range(10))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
2) The bytes object cannot hash, nor does it support the b.lower(), b.strip() and b.split() methods, but for the latter two, you can use b.strip(b’
\n\t\r \f’) and b.split(b’ ‘) to achieve the same purpose
3) zip(), map() and filter() all return iterators. And apply(), callable(), coerce(), execfile(), reduce() and reload
() functions have been removed

Now you can use hasattr() to replace callable(). The syntax of hasattr() is: hasattr(string, '__name__')

4) string.letters and related .lowercase and .uppercase have been removed, please use string.ascii_letters instead
5) If x < y cannot be compared, a TypeError exception is thrown. Version 2.x is to return a pseudo-random Boolean value
6) The __getslice__ series members are abandoned. a[i:j] is converted to a.__getitem__(slice(I, j)) or __setitem__ and
depending on the context __delitem__ calls
7) The file class is abandoned, in Py2.5:
>>> file

In Py3.X:
>>> file
Traceback (most recent call last):
File "", line 1, in
file
NameError: name 'file' is not defined

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