Python 2 Vs Python 3 Complete Guide

1. Print Statement vs Function

Python 2:

• print is a statement, not a function. You can print without parentheses.

# Python 2 Example
print "Hello, world!"
print 42

Python 3:

• print is a function. Therefore, you need to use parentheses.

# Python 3 Example
print("Hello, world!")
print(42)

2. Integer Division

Python 2:

• Dividing two integers performs floor division by default (i.e., it discards the fractional part).

# Python 2 Example
result = 5 / 2
print result  # Outputs: 2

Python 3:

• Dividing two integers returns a float. Use // for floor division.

# Python 3 Example
result = 5 / 2
print(result)  # Outputs: 2.5

# Floor Division in Python 3
floor_result = 5 // 2
print(floor_result)  # Outputs: 2

3. Unicode Strings

Python 2:

• Strings are ASCII by default. Use u'' to define Unicode strings.

# Python 2 Example
ascii_str = 'Hello'
unicode_str = u'こんにちは'  # Japanese for "Hello"
print ascii_str
print unicode_str

Python 3:

• All strings are Unicode by default. No need to prefix with u.

# Python 3 Example
ascii_str = 'Hello'
unicode_str = 'こんにちは'  # Japanese for "Hello"
print(ascii_str)      # Outputs: Hello
print(unicode_str)    # Outputs: こんにちは

4. Iterators and Generators

Python 2:

• Functions like range(), xrange() (a memory-efficient version of range()), and built-in methods such as dict.keys(), dict.values(), and dict.items() return lists.

# Python 2 Example
keys = {'a': 1, 'b': 2}.keys()
values = {'a': 1, 'b': 2}.values()

print keys   # Outputs: ['a', 'b']
print values # Outputs: [1, 2]

numbers = range(10)
print numbers  # Outputs: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Python 3:

• Functions like range() return objects that generate values on-the-fly, which are more memory-efficient. Methods like dict.keys(), dict.values(), and dict.items() return view objects.

# Python 3 Example
keys = {'a': 1, 'b': 2}.keys()
values = {'a': 1, 'b': 2}.values()

print(keys)      # Outputs: dict_keys(['a', 'b'])
print(values)    # Outputs: dict_values([1, 2])

numbers = range(10)
print(numbers)   # Outputs: range(0, 10)

# To get a list in Python 3, you can explicitly convert
keys_list = list(keys)
values_list = list(values)
numbers_list = list(numbers)

print(keys_list)        # Outputs: ['a', 'b']
print(values_list)      # Outputs: [1, 2]
print(numbers_list)     # Outputs: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

5. Exception Handling Syntax

Python 2:

• The except clause uses a comma to separate the exception type from the variable.

# Python 2 Example
try:
    x = 1 / 0
except ZeroDivisionError, e:
    print "Caught an exception:", e

Python 3:

• The except clause uses the as keyword.

# Python 3 Example
try:
    x = 1 / 0
except ZeroDivisionError as e:
    print("Caught an exception:", e)

6. Input Function

Python 2:

• There are two input functions: raw_input() (returns a string) and input() (interprets the input as Python code).

# Python 2 Example
user_input = raw_input("Enter something:")
print type(user_input)  # Outputs: <type 'str'>

# If using input(), be cautious as it evaluates the input
user_eval_input = input("Enter a number:")
print type(user_eval_input)  # Outputs: <type 'int'> if you enter an integer

Python 3:

• The input() function behaves like raw_input() in Python 2 and always returns a string.

# Python 3 Example
user_input = input("Enter something:")
print(type(user_input))  # Outputs: <class 'str'>

7. File I/O

Python 2:

• Opening files with open() defaults to text mode. Use 'r', 'w', 'rb', etc., explicitly if needed.

# Python 2 Example
with open('example.txt', 'w') as file:
    file.write(u"こんにちは世界")  # Writing Unicode

with open('example.txt', 'r') as file:
    content = file.read()
    print content                # Outputs: u'\u3053\u3093\u306b\u3061\u308f \u4e16\u754c'

Python 3:

• Specify 't' (text mode) or 'b' (binary mode) explicitly. Text mode defaults to UTF-8 encoding.

# Python 3 Example
with open('example.txt', 'w', encoding='utf-8') as file:
    file.write("こんにちは世界")  # Writing Unicode

with open('example.txt', 'r', encoding='utf-8') as file:
    content = file.read()
    print(content)               # Outputs: こんにちは世界

8. Raising Exceptions

Python 2:

• Use the raise statement with a comma to raise exceptions.

# Python 2 Example
def divide(x, y):
    if y == 0:
        raise ValueError, "Cannot divide by zero"
    return x / y

print divide(10, 0)  # Throws ValueError: Cannot divide by zero

Python 3:

• Use the raise statement with parentheses.

# Python 3 Example
def divide(x, y):
    if y == 0:
        raise ValueError("Cannot divide by zero")
    return x / y

print(divide(10, 0))  # Throws ValueError: Cannot divide by zero

9. Comparison Operators for Sequence Types

Python 2:

• Sequence types (list, tuple) can be compared using <, >, <=, and >=. They compare lexicographically.

# Python 2 Example
list1 = [1, 2, 3]
list2 = [2, 3, 4]
print list1 > list2  # Outputs: False
print list1 <= list2  # Outputs: True

Python 3:

• Comparing non-singleton sequence types (like list and tuple) directly is deprecated and will raise a TypeError. Use == or != for equality checks.

# Python 3 Example
list1 = [1, 2, 3]
list2 = [2, 3, 4]
# print(list1 > list2)    # Throws TypeError in Python 3
print(list1 == list2)  # Outputs: False
print(list1 != list2)  # Outputs: True

10. exec and eval Statements

Python 2:

• These are statements, not functions.

# Python 2 Example
code = "print 'Hello, world!'"
exec code

expression = "2 + 2"
result = eval(expression)
print result  # Outputs: 4

Python 3:

• Both exec and eval are functions.

# Python 3 Example
code = "print('Hello, world!')"
exec(code)

expression = "2 + 2"
result = eval(expression)
print(result)  # Outputs: 4

11. next() Method

Python 2:

• Use iterator.next().

# Python 2 Example
my_iterable = iter([1, 2, 3])
print my_iterable.next()  # Outputs: 1
print my_iterable.next()  # Outputs: 2

Python 3:

• Use next(iterator).

# Python 3 Example
my_iterable = iter([1, 2, 3])
print(next(my_iterable))  # Outputs: 1
print(next(my_iterable))  # Outputs: 2

12. Libraries and Modules

• Some libraries and modules differ significantly between Python 2 and Python 3. For example, urllib in Python 3 is split into several submodules.

Python 2:

# Python 2 Example
import urllib
response = urllib.urlopen('http://www.example.com/')
print response.read()

Python 3:

# Python 3 Example
import urllib.request
response = urllib.request.urlopen('http://www.example.com/')
print(response.read().decode('utf-8'))  # Decoding the bytes response to string

13. xrange() in Python 2

• In Python 2, xrange() is used for generating a range of numbers lazily, similar to range() in Python 3.

Python 2:

# Python 2 Example
for i in xrange(10):
    print i

Python 3:

• xrange() does not exist. Use range().

# Python 3 Example
for i in range(10):
    print(i)

14. Byte Literals

Python 2:

• Prefix with b to define byte literals.

# Python 2 Example
bytes = b'hello'
print type(bytes)  # Outputs: <type 'str'>

Python 3:

• Byte literals are defined with the b prefix and have their own byte type.

# Python 3 Example
bytes = b'hello'
print(type(bytes))  # Outputs: <class 'bytes'>

# Convert bytes to string
str_from_bytes = bytes.decode('utf-8')
print(str_from_bytes)  # Outputs: hello

Migrating from Python 2 to Python 3

1. Use the 2to3 tool: This tool reads Python 2.x source code and applies a series of fixers to transform it into valid Python 3.x code.

   • Installation: pip install 2to3
   • Usage: 2to3 -w your_script.py

2. Manual Changes:

   • Update all print statements to functions.
   • Handle division changes: use / for floating-point division and // for floor division.
   • Ensure all string literals are Unicode by default and handle encoding/decoding accordingly.
   • Replace xrange() with range().
   • Update except syntax.
   • Update input() and avoid the insecure behavior of Python 2’s input().

Example Migration

Let's take a simple script in Python 2 and convert it to Python 3.

Python 2 Script:

# Python 2 Script
print "Hello, world!"

def read_file(filename):
    with open(filename, 'r') as file:
        content = file.read()
        return content

file_content = read_file('example.txt')
print file_content

Equivalent Python 3 Script:

# Python 3 Script
print("Hello, world!")

def read_file(filename):
    with open(filename, 'r', encoding='utf-8') as file:
        content = file.read()
        return content

file_content = read_file('example.txt')
print(file_content)

Summary of Key Differences

| Feature | Python 2 | Python 3 | |--------------------|-------------------------------------------|-------------------------------------------------| | Print | Statement (print "Hello") | Function (print("Hello")) | | Integer Division| Returns integer (5 / 2 = 2) | Returns float (5 / 2 = 2.5) | | Strings | ASCII by default; use u'' for Unicode | Unicode by default | | Range Function | Returns a list (range(10) = [0, ..., 9]) | Returns a range object | | Exception Handling | except ValueError, e: | except ValueError as e: | | Input Function | raw_input() for strings, input() for evaluation | input() for strings | | Comparison | Can compare sequences lexicographically | Only == and != for sequences | | exec and eval | Statements | Functions | | next() Method | iterator.next() | next(iterator) | | Byte Literals | b'str' (still exists but behaves differently) | b'str' with its own bytes type |