1.3. Python 2 and Python 3

Author: Pierre de Buyl

Python 2 / 3

Two major versions of Python exist, Python 2 and Python 3. Python 3 is the only supported version since january 2020 but the two versions coexisted for about a decade of transition from Python 2 to Python 3. The transition has come to and end as most software libraries drop Python 2 support.

1.3.1. A very short summary

  • Python 2 is not supported by the Python Software Foundation since January 1st 2020. There will be no more security patches for Python 2.7. See Sunsetting Python 2 and the Python 3 Q & A.
  • The default choice for everyone should be Python 3. Choosing Python 2 should remain motivated by specific circumstances such as the dependency on unported libraries, with the understanding of the lack of official and community support.
  • Python 2 and Python 3 share most of their syntax, enabling many programmers to port their programs. It is even possible to make many codes Python 2/3 compatible, even though there are limitations. This strategy was important in making the transition but is no longer recommended.
  • The division of integers, 1/2 for instance, returns 0 under Python 2 (integer division, preserving type) and 0.5 under Python 3 (real division, promoting the integer to a floating point value). A line of code can thus execute with no visible warning in both Python 2 and Python 3 but result in different outcomes.
  • Most scientific libraries have moved to Python 3. NumPy and many scientific software libraries dropped Python 2 support or will do so soon, see the Python 3 statement.

The SciPy Lecture Notes dropped Python 2 support in 2020. The release 2020.1 is almost entirely Python 2 compatible, so you may use it as a reference if necessary. Know that installing suitable packages will probably be challenging.

1.3.2. Breaking changes between Python 2 and Python 3

Python 3 differs from Python 2 in several ways. We list the most relevant ones for scientific users below.

1.3.2.2. Division

In Python 2, the division of two integers with a single slash character results in floor-based integer division:

>>> 1/2 

0

In Python 3, the default behavior is to use real-valued division:

>>> 1/2

0.5

Integer division is given by a double slash operator:

>>> 1//2

0

1.3.3. Some new features in Python 3

Changing print to a function and changing the result of the division operator were only two of the motivations for Python 3. An incomplete list of the changes follows (there are many more).

  • By default, strings are in unicode. Sequence of arbitrary bytes use the type bytes. This change leads to heavy porting for applications dealing with text.

  • Since Python 3.5 and NumPy 1.10, there is a matrix multiplication operator:

    >>> np.eye(2) @ np.array([3, 4])

    array([3., 4.])

  • Since Python 3.6, there is a new string formatting method, the “f-string”:

    >>> name = 'SciPy'

    >>> print(f"Hello, {name}!")

    Hello, SciPy!

  • In Python 2, range(N) return a list. For large value of N (for a loop iterating many times), this implies the creation of a large list in memory even though it is not necessary. Python 2 provided the alternative xrange, that you will find in many scientific programs.

    In Python 3, range() return a dedicated type and does not allocate the memory for the corresponding list.

    >>> type(range(8))

    <class 'range'>

    >>> range(8)

    range(0, 8)

    You can transform the output of range into a list if necessary:

    >>> list(range(8))

    [0, 1, 2, 3, 4, 5, 6, 7]