Main caveat from writing Python 3 Sagemath code when you are used to Python 2

We list the most visible changes that any user would certainly run into the first time she uses a Python 3 based version when she used to run a Python 2 based one. This page is not intended to be a complete manual of the differences but a short page of what you need to know to get started. The What's new page of the Python official website provides more technical details.

1. print

In Python 2, print is a keyword (as for, if, etc)

   1 sage-8.9: print "hello", 1, 2
   2 hello 1 2

In Python 3, print becomes a function {{{#highlight python sage-9.0: print("hello", 1, 2) hello 1 2 }}} Writing a print statement without the parenthesis results in a SyntaxError {{{#highlight python sage: print "hello", 1, 2

• File "<ipython-input-9-e91077222f2e>", line 1

  • print "hello", Integer(1), Integer(2)
    • ^

SyntaxError: invalid syntax }}} One cool thing about this print function is that it takes argument {{{#highlight python

}}}

2. range and xrange

In Python range is a function that returns a list.

   1 sage-8.9: range(5)
   2 [0, 1, 2, 3, 4]
   3 sage-8.9: type(range(5))
   4 <type 'list'>

In Python 3, range is an object that somehow behave as a list (ie elements can still be acessed with square bracket, it has a length, etc) but it is not a list.

   1 sage-9.0: range(5)
   2 range(0, 5)
   3 sage-9.0: range(5)[2]
   4 2
   5 sage-9.0: range(5)[1::2]
   6 range(1, 5, 2)
   7 sage-9.0: type(range(5))
   8 <class 'range'>

The iterator xrange is no longer valid in Python 3 simply use range instead.

3. strings, unicode and bytes

In Python 2 a chain of characters between simple, double or triple quotes creates an ascii string. To create a unicode string one has to use the prefix u (which remains valid in Python 3).

In Python 3, a chain of characters between quote will gives rise to a unicode string. To create specifically an ascii string one has to use the prefix b (which is already valid in Python 2).