py-2-vs-3.md

Python 2 Vs Python 3

Python 2.x is legacy.

Python 2.x support will end on 2020, you will only get security updates, no New features

There would be not offical Python 2.8.

Python 3.x is the present and future of the language

---

print

Python 2

print is a statement.

>>> print('python')
python
>>> print x, y
1 2
>>> print(x, y)
(1, 2)

Python 3

print is a function.

>>> print(x, y)
1 2
>>> print("hello")
hello

---

unicode

Python 2

>>> type('python')
<type 'str'>
>>> type(unicode('python'))
<type 'unicode'>
>>> type('python')
<type 'unicode'>

>>> print 'string \u03BC'
string \u03BC

Python 3

>>> type('hello')
<class 'str'>
>>> type(u'hello')
<class 'str'>


>>> print('string \u03BC')
string μ

>>> r = 5
>>> π = 3.17
>>> c = 2 * π * r
>>> c
31.7

---

Division

Python 2

>>> 3 / 2
1
>>> 3 // 2
1
>>> 3 // float(2)
1.0
>>> 3 / 2 * 1.0
1.0
>>> 3 / (2 * 1.0)
1.5

Python 3

>>> 3 / 2
1.5
>>> 3 // 2
1
>>> int(3 / 2)
1

---

range

Python 2

range() creates the list of integers in memory.

xrange() is an efficient way to produce integers without consuming a lot of memory.

>>> x = range(20)
>>> x
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
>>> sys.getsizeof(x)
232
>>> y = xrange(20)
>>> y
xrange(20)
>>> sys.getsizeof(y)
40

Python 3

Has only range() which works like xrange().

>>> x = range(10)
>>> sys.getsizeof(x)
48
>>> xrange
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'xrange' is not defined

---

classes

Python 2

Old style classes.

class Account():
    pass

New style classes.

class Account(object):
    pass

Can use descriptors.

Has elegant mro for base class lookup.

Always use new style classes

Python 3

Only new style classes.

class Account():
    pass

---

iterators everywhere

Python 2

A lot of built-ins returned lists by default.

my_dict = {'name': 'Dan', 'age': 25}

>>> my_dict.keys()
['age', 'name']

>>> my_dict.items()
[('age', 25), ('name', 'Dan')]

>>> my_dict.values()
[25, 'Dan']

If you need iterators, you need to use iter* methods.

>>> my_dict.iterkeys()
<dictionary-keyiterator object at 0x207e050>

>>> my_dict.iteritems()
<dictionary-itemiterator object at 0x207e0a8>

>>> my_dict.itervalues()
<dictionary-valueiterator object at 0x207e158>

Python 3

Python 3 decided that it would be best to always return iterators for efficiency.

my_dict = {'name': 'Dan', 'age': 25}

>>> my_dict.keys()
dict_keys(['age', 'name'])

>>> my_dict.items()
dict_items([('age', 25), ('name', 'Dan')])

>>> my_dict.values()
dict_values([25, 'Dan'])

Other functions like map(), filter(), range(), zip() return iterators.