Which Python?¶
There are two versions of Python we might possibly use for this course: 2.7.x or 3.x.
For a number of reasons, we have chosen to teach this course in Python 2.7. For the time being, Python 2 is the version you are most likely to see in a professiona situation. Every company we spoke with here in Seattle uses it, and the large pre-existing code base built in Python 2 means that it will be a while before Python 3 is the default Python to learn.
That being said, there are some things you can do now to begin writing code that will be more portable forward to Python 3. This class will be taught with those steps in mind.
Bytes and Unicode¶
One of the largest changes from Python 2 to Python three concerns the difference in handling bytes and unicode.
In Python 2, a string literal is written thusly:
'I am a string'
It is extremely common in Python 2 code to use string literals to represent textual objects–things meant to be read as text by humans. This is fine so long as the only characters in your text are in the ASCII character set (basic English). However, this is less and less often the case. It is very common these days to see international characters in text that passes through your programs.
Python 2 can handle these international characters using a unicode literal:
u'I am a string with élan'
Enocding a unicode literal transforms it to a string literal:
>>> foo = u'I am a string with élan'
>>> bar = foo.encode('utf8')
>>> bar
'I am a string with \xc3\xa9lan'
This shows that in fact, a string is a sequence of bytes.
Python 2 allows for implicit conversion between unicode and bytes, but Python 3 does not.
In Python 3 you must hold text in your Python programs as unicode objects,
and only encode to bytes when passing that data out (to file, network,
buffer).
Similarly, you must decode bytes into unicode when you bring data in to your program.
To help build the rigor needed to support this while still writing Python 2, we will follow this convention:
- All textual data in our programs should be explicitly unicode:
u'élan'. - All binary data will be written explicitly as byte string:
b'\xc3\xa9lan.
Integer vs. Float Division¶
In Python there are two division operators: / and //. In Python 3 these
are division and integer division (also known as floored division):
>>> 1 / 2
0.5
>>> 1 // 2
0
>>> 3 / 2
1.5
>>> 3 // 2
1
But in Python 2, things are a bit different. The // operator works
similarly (it still gives you the floored result). The / operator is a
completely different story. The return values for both depend on what you give
them, and the / operator may work as floored division if you give use
integers:
>>> 1 // 2
0
>>> 1 / 2
0
>>> 1.0 / 2
0.5
>>> 1.0 // 2
0.0
You should always be explicit which version of division you are using. If you
want integer or floored results, Explicitly use the // operator.
And remember to be aware that much existing code does not carefully make this distinction. You should always be alert for potential problems arising from this discrepancy.
Test Test Test¶
Subtle differences like floored versus true division are sure to result in bugs during transition from Python 2 to Python 3.
For this reason, one of the keys to successful porting is establishing excellent test coverage.
The more of your code is covered by extensive, thorough tests, the more easily you can rest assured that a port from 2 to 3 will go as smoothly as possible.
For this reason, we will emphasize testing in this class.
You will write unit tests, integration tests and functional tests.
Establishing a habit of solid testing is the best tool you can use to ensure safe transitions to Python 3.
More to Learn¶
There are other techniques we will touch on as the class wears on, but that’s enough to go on for now.
Please take some time to watch this video by Tres Seaver from PyCon 2014 about porting code to Python 3 to get a sense of the larger picture.
In addition, you will want to bookmark Porting to Python 3 by Lennart Regebro. In particular, the chapter on preparing for Python 3 is an excellent guide to strategies you can use today.