Python2 đã dừng lại ở Python 2.7; Python3 thì vẫn đang tiếp tục phát triển. Phiên bản RC gần nhất là 3.6 và tiếp tục đang phát triển 3.7 beta. Python2 và Python3 có nhiều điểm khác biệt về syntax. Vậy nên học phiên bản nào Python 2 hay Python3 ?

Chắc chắn là một lập trình viên hay công ty sản xuất phần mềm thì chúng ta dễ dàng lựa chọn sử dụng Python3 thay cho Python2. Nhưng nếu là một newbie thì nên dùng bản nào để học ?

Dưới đây là một số khác biệt cơ bản giữa Python2 và Python3:

  1. Print statement vs. print function
  2. Differences between range and xrange functions
  3. Raising and handling Exceptions
  4. Comparison of unorderable types
  5. bytes vs string
  6. Integer division
  7. Unicode
  8. dictionary method
  9. Data Input

Khác biệt lớn nhất là:

Python2 dùng print

Python3 dùng print()

Python 2 Python 3 
>>> print "Hello World"
Hello World
 
>>> print("Hello World")
Hello World

>>> #print a newline
>>> print


>>> #print a newline,
>>> #using parentheses
>>> print()
  

>>> #add trailing comma to
>>> #remove newline
>>> print "No newline", 
    No newline

>>> #Appends nothing 
>>> #instead of a newline
>>> print("No newline", end="")
 No newline
>>> 
>>> #print to stderr
>>> print >>sys.stderr, "Error"   
  Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'sys' is not defined
 
>>> #Specifies the 
>>> #output buffer
>>>print("Error", file=sys.stderr)
>>> 
Traceback (most recent call last):
File "C:/Python34/test.py", 
 line 1, in <module>
 print("Error", file=sys.stderr)
 NameError: name 'sys' 
 is not defined
>>>
  
>>> #sep specifies the separator
>>> #here ',' is used as a separator
>>> print("Python", "JS", sep=",")
          Python,JS
  
>>> #prints as XYZ, there
>>> #is nothing in sep
>>> print("X", "Y", "Z", sep="")
    XYZ
  
>>> #flush the output buffer
>>> print("Python", flush=True)
  Python

Differences between range and xrange functions

Python 2 Python 3 
>>> range(1, 10)
   [1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> range(1, 10)
   range(1, 10)
>>> 

>>> xrange(1, 10)
xrange(1, 15)
 
>>> xrange(1, 10)
  Traceback (most recent call last):
File "<pyshell#9>", 
line 1, in <module>
xrange(1, 10)
NameError: name 'xrange' 
is not defined
      
>>> isinstance(xrange(1, 15), xrange)
True
>>>

>>> isinstance(range(1, 15), range)
True
>>>

Raising and handling Exceptions

Python 2 Python 3 
>>> 3 < 5.5
    True
>>>
 
>>> 3 < 5.5
True
>>>
 
>>> [3, 5] > 'xyz'
      False
>>>

>>> [3, 5] > 'xyz'
 Traceback (most recent call last):
 File "<pyshell#14>", 
 line 1, in <module>
 [3, 5] > 'xyz'
TypeError: unorderable types: 
list() > str()
>>>

>>> (3, 5) > 'xyz'
   True
>>>
 
>>> (3, 5) > 'xyz'
Traceback (most recent call last):
File "<pyshell#15>", 
line 1, in <module>
(3, 5) > 'xyz'
TypeError: unorderable types: 
tuple() > str()>>> 

>>> [3, 5] > (3, 5)
      False
>>>

>>> [3, 5] > (3, 5)
  Traceback (most recent call last):
File "<pyshell#21>", 
line 1, in <module>
[3, 5] > (3, 5)
TypeError: unorderable types: 
list() > tuple()
>>>

>>> 50 < [2, 'x'] < 'xyz' < (2, 'x')
	   True
>>>

>>> 50 < [2, 'x'] < 'xyz' < (2, 'x')
  Traceback (most recent call last):
File "<pyshell#23>", 
line 1, in <module>
50 < [2, 'x'] < 'xyz' < (2, 'x')
TypeError: unorderable types: 
int() < list()
>>>

Comparison of unorderable types

Python 2 Python 3 
>>> print "[2, 3]>'xyz'= ", [2, 3]>'xyz'
[2, 3]>'xyz' =  False
>>> print "(2, 3)>'xyz' = ", (2, 3)>'xyz'
(2, 3)>'xyz' =  True
>>> print "[2, 3]>(2, 3) = ", [2, 3]>(2, 3)
[2, 3] > (2, 3) =  False

print("[1, 2]>'foo' = ", [1, 2]>'foo')
print("(1, 2)>'foo' = ", (1, 2)>'foo')
print("[1, 2]>(1, 2) = ", [1, 2]>(1, 2))
Traceback (most recent call last):
  File "C:/Python34/test.py", line 1, 
  in 
    print("[1, 2] > 'foo' = ", 
	[1, 2] > 'foo')
TypeError: unorderable types: 
list() > str()

bytes vs string

Python 2 Python 3 
>>> a = "Python"
>>> len(a)
    6
>>> a[0]
    'P'
>>>

>>> a = "Python"
>>> len(a)
   6
>>> a[0]
   'P'
>>>
  
>>> 
#To treat a string as a 
#sequence of bytes, 
#you need to cast:
>>> a = bytes("Python", "utf-8")
>>> a
   b'Python'
>>> a[0]
   80
>>>
  
>>> #bytes(foo, 'utf-8') means to encode <em>foo</em>
>>> #in UTF-8. treat the result as a 
>>> #sequence of unsigned  8bit integers. You can 
>>> #also convert bytes to a string,
>>> #as following
>>> a = bytes("Python", "utf-8")
>>> b = str(a, "utf-8")
>>> b
    'Python'
>>>

Integer division

Python 2 Python 3 
>>> import sys
>>> print(sys.version)
       2.7.9 (default, Apr  2 2015, 
	   15:33:21)[GCC 4.9.2]
>>> print "6/3 =", 6/3
    6/3 = 2
>>> print "6//3 =", 6//3
    6//3 = 2
>>> print "6/3.0 =", 6/3.0
    6/3.0 = 2.0
>>> print "6//3.0 =", 6//3.0
    6//3.0 = 2.0
>>>

>>> import sys
>>> print(sys.version)
       3.4.2 (v3.4.2:ab2c023a9432, 
	    Oct  6 2014, 22:15:05)
	    [MSC v.1600 32 bit (Intel)]
>>>
>>> print("6/3 =", 6/3)
    6/3 = 2.0
>>> print("6//3 =", 6//3)
    6//3 = 2
>>> print("6/3.0 =", 6/3.0)
    6/3.0 = 2.0
>>> print("6//3.0 =", 6//3.0)
    6//3.0 = 2.0
>>>

Unicode

Python2 không hỗ trợ Unicode, muốn in ra Unicode phải thêm một số thao tác nữa.

Python 2 Python 3 
>>> import sys
>>> print(sys.version)
   2.7.9 (default, Apr  2 2015, 
   15:33:21) GCC 4.9.2]
>>> print type(unicode
("Python Unicode"))
<type 'unicode'>
>>>

>>> import sys
>>> print(sys.version)
  3.4.2 (v3.4.2:ab2c023a9432,
   Oct  6 2014, 22:15:05) 
   MSC v.1600 32 bit (Intel)]
>>>
 
>>> print type(b"Python")
   <type 'str'>
>>>
   
>>> print "Python" + b" Unicode"
Python Unicode
>>>
 
>>> print(" Python", type(b" Unicode"))
Python <class 'bytes'>
>>>

dictionary method

In Python 2 dict.iteritems(): Return an iterator over the dictionary’s (key, value) pairs.

In Python 3 dict.items(): Return a copy of the dictionary’s list of (key, value) pairs.

Python 2 Python 3 
>>> x = {1: "red", 2: "blue"}
>>> print(x)
   {1: 'red', 2: 'blue'}
>>> print(x.items())
dict_items([(1, 'red'), (2, 'blue')])
>>> print(x.iteritems())
    Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'dict' object has no 
attribute 'iteritems'
>>> print([i for i in x.iteritems()])
 [(1, 'red'), (2, 'blue')]
>>>
 
>>> x = {1: "red", 2: "blue"}
>>> print(x)
  {1: 'red', 2: 'blue'}
>>> print(x.items)
<built-in method items of dict object 
at 0x7fe191a89748>
>>> print(x.items())
dict_items([(1, 'red'), (2, 'blue')])
>>> print([i for i in x.items()])
  [(1, 'red'), (2, 'blue')]
>>>
   

Trong Python2: dict.keys, dict.values, dict.items trả về một list.

Python3 không trả về một list mà trả về object dict_items. Muốn chuyển về list cần thực hiện convert sang dạng list

>>> #In Python 2 dict.has_key()
 is used to test.
>>> #whether the dict has 
a certain key.
>>> x = {1: "red", 2: "blue"}
>>> print(x.has_key(1))
True
>>>

>>> #But this method has been
>>> #removed in Python 3.
>>> #Instead, the in operator
>>> #Is used.
>>> x = {1: "red", 2: "blue"}
>>> print(a.has_key(1))
 #Don't run it, unless you want this:
 Traceback (most recent call last):
 File "<stdin>", line 1, in <module> 
 AttributeError: 'dict' object has
 no attribute 'has_key'
>>> print(1 in x)
        True
>>>

Data Input

Python2 Python3 
>>>data_input2 = raw_input()

>>>data_input3 = input()