Important differences between Python 2.x and Python 3.x with examples

Division operator

If we are porting our code or executing python 3.x code in python 2.x, it can be dangerous if integer division changes go unnoticed (since it doesn’t raise any error). It is preferred to use the floating value (like 7.0/5 or 7/5.0) to get the expected result when porting our code.

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print 7 / 5 
  
print -7 / 5     
  
    
  
''' 
  
Output in Python 2.x 
  
  
-2 
  
Output in Python 3.x : 
  
1.4 
  
-1.4 
  
    
  
# Refer below link for details 
  
  
''' 

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print function

This is the most well-known change. In this, the print keyword in Python 2.x is replaced by the print() function in Python 3.x. However, parentheses work in Python 2 if a space is added after print keyword because the interpreter evaluates it as an expression.

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print 'Hello, Geeks'      # Python 3.x doesn't support 
  
print('Hope You like these facts'
  
    
  
''' 
  
Output in Python 2.x : 
  
Hello, Geeks 
  
Hope You like these facts 
  
    
  
Output in Python 3.x : 
  
File "a.py", line 1 
  
    print 'Hello, Geeks' 
  
                       
  
SyntaxError: invalid syntax 
  
    
  
Refer below link for details 
  
  
''' 

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As we can see, if we use parentheses in python 2.x then there is no issue but if we don’t use parentheses in python 3.x, we get SyntaxError.



Unicode:

In Python 2, implicit str type is ASCII. But in Python 3.x implicit str type is Unicode.

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print(type('default string ')) 
  
print(type(b'string with b ')) 
  
    
  
''' 
  
Output in Python 2.x (Bytes is same as str) 
  
<type 'str'> 
  
<type 'str'> 
  
    
  
Output in Python 3.x (Bytes and str are different) 
  
<class 'str'> 
  
<class 'bytes'> 
  
''' 

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Python 2.x also supports Unicode

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print(type('default string ')) 
  
print(type(u'string with b ')) 
  
    
  
''' 
  
Output in Python 2.x (Unicode and str are different) 
  
<type 'str'> 
  
<type 'unicode'> 
  
    
  
Output in Python 3.x (Unicode and str are same) 
  
<class 'str'> 
  
<class 'str'> 
  
''' 

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xrange:

xrange() of Python 2.x doesn’t exist in Python 3.x. In Python 2.x, range returns a list i.e. range(3) returns [0, 1, 2] while xrange returns a xrange object i. e., xrange(3) returns iterator object which work similar to Java iterator and generates number when needed.

If we need to iterate over the same sequence multiple times, we prefer range() as range provides a static list. xrange() reconstructs the sequence every time. xrange() doesn’t support slices and other list methods. The advantage of xrange() is, it saves memory when the task is to iterate over a large range.

In Python 3.x, the range function now does what xrange does in Python 2.x, so to keep our code portable, we might want to stick to using range instead. So Python 3.x’s range function is xrange from Python 2.x.

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for x in xrange(1, 5): 
  
    print(x), 
  
    
  
for x in range(1, 5): 
  
    print(x), 
  
    
  
''' 
  
Output in Python 2.x 
  
1 2 3 4 1 2 3 4 
  
    
  
Output in Python 3.x 
  
NameError: name 'xrange' is not defined 
  
''' 

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Error Handling:



There is a small change in error handling in both versions. In python 3.x, ‘as’ keyword is required.

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try
  
    trying_to_check_error 
  
except NameError, err: 
  
    print err, 'Error Caused'   # Would not work in Python 3.x 
  
    
  
''' 
  
Output in Python 2.x: 
  
name 'trying_to_check_error' is not defined Error Caused 
  
    
  
Output in Python 3.x : 
  
File "a.py", line 3 
  
    except NameError, err: 
  
                    
  
SyntaxError: invalid syntax 
  
''' 

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try
  
     trying_to_check_error 
  
except NameError as err: # 'as' is needed in Python 3.x 
  
     print (err, 'Error Caused'
  
    
  
''' 
  
Output in Python 2.x: 
  
(NameError("name 'trying_to_check_error' is not defined",), 'Error Caused') 
  
    
  
Output in Python 3.x : 
  
name 'trying_to_check_error' is not defined Error Caused 
  
''' 

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__future__ module:

This is basically not a difference between the two versions, but a useful thing to mention here. The idea of __future__ module is to help migrate to Python 3.x.

If we are planning to have Python 3.x support in our 2.x code, we can use _future_ imports in our code.

For example, in the Python 2.x code below, we use Python 3.x’s integer division behavior using the __future__ module.

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# In below python 2.x code, division works 
  
# same as Python 3.x because we use  __future__ 
  
from __future__ import division 
  
    
  
print 7 / 5 
  
print -7 / 5 

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Output :

1.4 

-1.4 

Another example where we use brackets in Python 2.x using __future__ module:

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from __future__ import print_function     
  
    
  
print('GeeksforGeeks'

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Output:

GeeksforGeeks 

Refer this for more details of the __future__ module.

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