Python 2 versus Python 3

Much of the example code you’ll find online is Python2, rather than Python3

For the most part, they are the same – so you can use those examples to learn from.

There are a lot of subtle differences that you don’t need to concern yourself with just yet.

But a couple that you’ll need to know right off the bat:

print()

In Python2, print is a “statement”, rather than a function. That means it didn’t require parentheses around what you want printed:

print something, something_else

This made it a bit less flexible and powerful.

But – if you try to use it that way in Python3, you’ll get an error:

In [15]: print "this"
  File "<ipython-input-15-70c8add5d16e>", line 1
    print "this"
               ^
SyntaxError: Missing parentheses in call to 'print'

So – if you get this error, simply add the parentheses:

In [16]: print("this")
this

Division

In python 3, the division operator is “smart” when you divide integers:

In [17]: 1 / 2
Out[17]: 0.5

However in Python2, integer division, will give you an integer result:

In [1]: 1/2
Out[1]: 0

In both versions, you can get “integer division” if you want it with a double slash:

In [1]: 1//2
Out[1]: 0

And in Python2, you can get the behavior of Python3 with “true division”:

In [2]: from __future__ import division

In [3]: 1/2
Out[3]: 0.5

For the most part, you just need to be a bit careful with the rare cases where Python2 code counts on integer division.

Iterators vs Lists

In Python2, a number of functions returned a full list of the contents. But most of the time, you didn’t need a list – you only needed a way to loop through all the items returned. Such an object is called an “iterable” – more about that later in the class. But for now, if you get an error like:

TypeError: 'dict_keys' object does not support indexing

Then you likely got an iterator, rather than a “proper” list. You can fix this by making a list out of it:

list(an_iterator)

the list constructor will make a list out of any iterable. So you can now index it, etc.

Other Python2 / Python3 differences

The most drastic difference (improvement!) is better Unicode support, and better bytes / Unicode separation.

Most of the other differences are essentially implementation details, like getting iterators instead of sequences – we’ll talk about that more when it comes up in a later lesson.

There are also a few syntax differences with more advanced topics: Exceptions, super(), etc.

We’ll talk about all that when we cover those topics as well.