What is the difference between Python 2 and Python 3?

Below are the key differences which I have observed and encountered frequently while working on Python 2 and Python 3.

Integer division

Python2 : The return type of a division (/) operation depends on its operands. If both operands are of type int, floor division is performed and an int is returned. If either operand is a float, classic division is performed and a float is returned. The // operator is also provided for doing floor division no matter what the operands are.

>>> 3/2

1 

>>> -3/2

-2 

>>> 3//2

1 

>>> -3//2

-2

Python3 : Division (/) always returns a float. To do floor division and get an integer result (discarding any fractional result) you need to use // operator.

>>> 3/2

1.5 

>>> -3/2

-1.5 

>>> 3//21 >>> -3//2-2

Input function

Python2 : When you use input() function, Python dynamically converts the data type based on your input. So input() function can return anything int, str, float, bool, list etc based on your input. For instance,

>>> val = input("Enter any value: ")

Enter any value: 7

>>> type(val)

int 

>>> val = input("Enter any value: ")

Enter any value: 7.0

>>> type(val)

float 

>>> val = input("Enter any value: ")

Enter any value: 'abc'

>>> type(val)

str 

>>> val = input("Enter any value: ")

Enter any value: True

>>> type(val)

bool 

>>> val = input("Enter any value: ")

Enter any value: [1,2,3,4,5]

>>> type(val)

list

When you use raw_input(), Python will simply return string irrespective of your input value data type.

>>> val = raw_input("Enter any value: ")

Enter any value: 7

>>> type(val)

str 

>>> val = raw_input("Enter any value: ")

Enter any value: 7.0

>>> type(val)

str 

>>> val = raw_input("Enter any value: ")

Enter any value: 'abc'

>>> type(val)

str 

and so on..

Python3 : In Python 3, input() function returns string type (acts like raw_input()). It’s confusing sometimes when you are transiting from Python 2 to Python 3.

>>> val = input("Enter any value: ")

Enter any value: 7

>>> type(val)

str

Note when you are using input() in Python 3 : If you are using Python 2.7, it’s good idea to use raw_input() when you are getting user inputs.

In Python 3, you would use input(). Both the functions interprets all input as a string based on which version of Python you are using.

Sometimes, I feel instead of Python doing type change dynamically it’s always safe to handle it manually. In Python 3, when you use the input() func, every input will be interpreted as string and sometimes it creates unexpected results. Consider the below example,

>>> a = input("Enter first number: ")

Enter first number: 3.4 

>>> b = input("Enter second number: ")

Enter second number: 5.7 

>>> print a + b

3.45.7 

# It's because

>>> type(a), type(b)

(str, str) 

# In order to fix this you need to apply float() function when user is prompted for input.

Round function

Python2 : Return the floating point value number rounded to n digits after the decimal point. If n digits is omitted, it defaults to zero. The result is a floating point number. Values are rounded to the closest multiple of 10 to the power minus n digits; if two multiples are equally close, rounding is done away from 0 (so, for example, round(0.5) is 1.0 and round(-0.5) is -1.0).

>>> round(3.5)

4.0

Python3 : Return number rounded to n digits precision after the decimal point. If n digits is omitted or is None, it returns the nearest integer to its input.

>>> round(3.5)

4

Print function

Python2 : Extra pair of parenthesis was not mandatory.

>>> print "Hello"

Hello

>>> print 'Hello'

Hello