python-2-vs-python-3.md

FAQ: Python 2 and Python 3

Difference between python 2 and 3

Python 2 and Python 3 are mostly the same during our discussions. However there are some differences if you explore further. During this semester, we base our discussions and exercises on Python 3. We highlight some main differences here for your reference. To read further, you can click here

syntax of print

Python 2’s print statement has been replaced by theprint()function, meaning that we have to wrap the object that we want to print in parentheses. Python 2 doesn’t have a problem with additional parentheses, but in contrast, Python 3 would raise aSyntaxErrorif we called the print function the Python 2-way without the parentheses. However, if we have multiple objects inside the parentheses, we will create a tuple, sinceprintis a “statement” in Python 2, not a function call.

str, bytes, encoding and decoding

Python 2 has ASCIIstr()types, separateunicode(), but nobytetype. Now, in Python 3, we finally have Unicode (utf-8)str, and 2 byte classes:byteandbytearray.

Sometimes you need to convert between bytes and str (unicode string). You can use bytes.decode() or str.encode() for the conversion. Read more on the official unicode support doc.

Division

Python 2 interprets / as integer division, whereas Python 3 interprets it as decimal division. Here's the mapping:

	Python 2	Python 3
Floating point division	float(a)/b	a / b
Integer division	a / b	a // b
Division for remainder	%	%

Return iterable instead of list

When you read tutorials online, one frequent confusion is your code returns iterable but the sample code returns a list. You need to call next() on a iterable to find the next element. This is an efficiency oriented design in Python3. Given the small amount of data we are handling in this course, you can use the "brute force" way to convert valid Python2 code into Python3. The common trick is to wrap previous codes by list(). For example, if a = map(...) works in Python2, you simply write a = list(map(...)) and that works in Python3. Having another layer of list won't harm (in terms of grammar correctness).

Rounding to closest even number

Python 3 rounds to the closest even number. This causes little difference in most of your program.

%python3
>>> round(16.5)
16
>>> round(17.5)
18
>>> round(15.5)
16

%python2
>>> round(16.5)
17.0
>>> round(17.5)
18.0
>>> round(15.5)
16.0

How to set up Python 2.7 and Python 3 at the same time on your Mac OSX?

You can follow the steps in thisLink to set up both of them.

Install modules in Python3

If you want to install modules for Python3, please use pip3 (not pip). To install a module in user mode, you can use

pip3 install --user {name-of-the-module}

NOTE: Some online tutorials instruct you to install python module with sudo prefix, e.g. sudo pip3 install pandas. Don't do that unless other methods do not work. It is better practice to enter virtual env and run pip install ... (or pip3 install) in shell or !pip install xxx in notebook. In this way, the modules and other environment updates only happen within venv. Using sudo xxx is usually a quick solution but not recommended. sudo basically gives the followup command super user privilege which makes the system vulnerable to malicious software. One convention is to refrain from sudo whenever possible because -- with great power comes great responsibility -- sudo has too much power that is not always needed. If you are not in virtualenv, or if you want to install a module in larger scope, not only for the current project, you can use the --user mode as shown above. This installs the module in a sub foder under your user home directory (echo $HOME).