Merge pull request #475 from pdebuyl/add_python_2_3

add python 2/3 section in intro

Author: pdebuyl

intro/index.rst

@@ -18,6 +18,7 @@ itself, to numerical computing or plotting.
 
    intro.rst
    language/python_language.rst
+   python_2_python_3.rst
    numpy/index.rst 
    matplotlib/index.rst 
    scipy.rst

intro/python_2_python_3.rst

@@ -0,0 +1,140 @@
+.. _python_2_python_3:
+
+..
+    >>> import numpy as np
+
+Python 2 and Python 3
+=====================
+
+**Author**: *Pierre de Buyl*
+
+.. topic:: 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.
+
+
+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
+  <https://www.python.org/doc/sunset-python-2/>`_ and the `Python 3 Q & A
+  <http://python-notes.curiousefficiency.org/en/latest/python3/questions_and_answers.html>`_.
+
+- **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
+  <https://python3statement.org/>`_.
+
+
+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.
+
+
+
+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.
+
+
+Print function
+..............
+
+The most visible change is that ``print`` is not a "statement" anymore but a
+function.
+
+Whereas in Python 2 you could write ::
+
+    >>> print 'hello, world' # doctest: +SKIP
+    hello, world
+
+in Python 3 you must write
+
+    >>> print('hello, world')
+    hello, world
+
+By making :func:`print` a function, one can pass arguments such a file identifier where the
+output will be sent.
+
+
+Division
+........
+
+In Python 2, the division of two integers with a single slash character results in
+floor-based integer division::
+
+    >>> 1/2 # doctest: +SKIP
+    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
+
+
+
+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"
+  <https://docs.python.org/3/reference/lexical_analysis.html#f-strings>`_::
+
+     >>> 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, :func:`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]
+