Python for beginners in Tieto Czech: Differences between Python 2 and Python 3

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Differences betweenPython 2 and Python 3

print() is a function

Probably most visible diffenrence between Python 2 and Python 3. Command print has been replaced by function print()

Python 2

print 'Hello, World!'
print('Hello, World!')
print "text", ; print 'print more text on the same line'

Hello, World!
Hello, World!
text print more text on the same line

Python 3

print('Hello, World!')

print("some text,", end="")
print(' print more text on the same line')

Hello, World!
some text, print more text on the same line

print 'Hello, World!'

File "<ipython-input-3-139a7c5835bd>", line 1
    print 'Hello, World!'
                        ^
SyntaxError: invalid syntax

Division by integers

Integer division behaves differently in Python 2 and Python 3.

Python 2

print '3 / 2 =', 3 / 2
print '3 // 2 =', 3 // 2
print '3 / 2.0 =', 3 / 2.0
print '3 // 2.0 =', 3 // 2.0

3 / 2 = 1
3 // 2 = 1
3 / 2.0 = 1.5
3 // 2.0 = 1.0

Python 3

print('3 / 2 =', 3 / 2)
print('3 // 2 =', 3 // 2)
print('3 / 2.0 =', 3 / 2.0)
print('3 // 2.0 =', 3 // 2.0)

3 / 2 = 1.5
3 // 2 = 1
3 / 2.0 = 1.5
3 // 2.0 = 1.0

Unicode Support

Python 2 has separate types for str (), these are ASCII characters and then unicode (). In Python 3, all str () strings are Unicode characters by default. Next, Python 3 introduces byte support.

Raise an exception

In Python 2, there are two ways to raise an exception. Python 3 allows only one.

Python 2

raise IOError, "file error"

raise IOError, "file error"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IOError: file error

raise IOError("file error")

raise IOError("file error")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IOError: file error

Python 3

The following notation in Python 3 does not work:

raise IOError, "file error"

raise IOError, "file error"
  File "<stdin>", line 1
    raise IOError, "file error"
                 ^
SyntaxError: invalid syntax

The only correct entry is this:

raise IOError("file error")

raise IOError("file error")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: file error

Exception handling

The treatment of exceptions has also changed. See how exceptions are handled in Python 2 and Python 3.

Python 2

try:
    let_us_cause_a_NameError
except NameError, err:
    print err, '--> our error message'

name 'let_us_cause_a_NameError' is not defined --> our error message

Python 3

try:
    let_us_cause_a_NameError
except NameError as err:
    print(err, '--> our error message')

name 'let_us_cause_a_NameError' is not defined --> our error message

User inputs

In Python 2, there are two ways to get user input. input() function and raw_input() function. The problem with the input() function is that it can be potentially dangerous because it does not always return a string.

Python 2

Python 2.7.6
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

>>> my_input = input('enter a number: ')

enter a number: 123

>>> type(my_input)
<type 'int'>

>>> my_input = raw_input('enter a number: ')

enter a number: 123

>>> type(my_input)
<type 'str'>

Python 3

Python 3.7.0 (default, Aug 24 2018, 20:34:01)
[Clang 9.0.0 (clang-900.0.39.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> my_input = input('enter a number: ')
enter a number: 123
>>> type(my_input)
<class 'str'>

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