.. _what is python:

---------------
What is Python?
---------------

This section will give a very brief introduction to the Python
language.

.. seealso::

   * The Python_ home page.
   * Python Recipes_.
   * Try a `Python quick reference card`_ or a `different reference card`_.


.. _Recipes: http://code.activestate.com/recipes/langs/python
.. _Python quick reference card: https://perso.limsi.fr/pointal/python/pqrc
.. _different reference card: http://rgruet.free.fr/
.. _Python: https://www.python.org/


Executing Python code
---------------------

You can execute Python code interactively by starting the interpreter
like this::

  $ python3
  >>> print('hello')
  hello

You can also put the ``print('hello')`` line in a file (``hello.py``)
and execute it as a Python script::

  $ python3 hello.py
  hello

Or like this::

  $ python3 -i hello.py
  hello
  >>> print('hi!')
  hi!

Finally, you can put ``#!/usr/bin/env python3`` in the first line of
the ``hello.py`` file, make it executable (``chmod +x hello.py``) and
execute it like any other executable.


.. tip::

   For a better interactive experience, consider ipython_.

   .. _ipython: http://ipython.scipy.org



Types
-----

Python has the following predefined types:

===========  =====================  ==========================
type         description            example
===========  =====================  ==========================
``bool``     boolean                ``False``
``int``      integer                ``117``
``float``    floating point number  ``1.78``
``complex``  complex number         ``0.5 + 2.0j``
``str``      string                 ``'abc'``
``tuple``    tuple                  ``(1, 'hmm', 2.0)``
``list``     list                   ``[1, 'hmm', 2.0]``
``dict``     dictionary             ``{'a': 7.0, 23: True}``
===========  =====================  ==========================

A ``dict`` object is mapping from keys to values:

>>> d = {'s': 0, 'p': 1}
>>> d['d'] = 2
>>> d
{'p': 1, 'd': 2, 's': 0}
>>> d['p']
1

In this example all keys are strings and all values are integers.
Types can be freely mixed in the same dictionary; any type can be used
as a value and most types can be used as keys (mutable objects cannot
be keys).

A ``list`` object is an ordered collection of arbitrary objects:

>>> l = [1, ('gg', 7), 'hmm', 1.2]
>>> l[1]
('gg', 7)
>>>
>>> l
[1, ('gg', 7), 'hmm', 1.2]
>>> l[-2]
'hmm'

Indexing a list with negative numbers counts from the end of the list,
so element -2 is the second last.

A ``tuple`` behaves like a ``list`` - except that it can't be modified
in place.  Objects of types ``list`` and ``dict`` are *mutable* - all
the other types listed in the table are *immutable*, which means that
once an object has been created, it can not change.  Tuples can
therefore be used as dictionary keys, lists cannot.

.. note::

   List and dictionary objects *can* change.  Variables in
   Python are references to objects - think of the = operator as a
   "naming operator", *not* as an assignment operator.  This is demonstrated here:

   >>> a = ['q', 'w']
   >>> b = a
   >>> a.append('e')
   >>> a
   ['q', 'w', 'e']
   >>> b
   ['q', 'w', 'e']

   The line b = a gives a new name to the array, and both names now
   refer to the same list.

   However, often a new object is created and
   named at the same time, in this example the number 42 is *not*
   modified, a new number 47 is created and given the name ``d``.  And
   later, ``e`` is a name for the number 47, but then a *new*
   number 48 is created, and ``e`` now refers to that number:

   >>> c = 42
   >>> d = c + 5
   >>> c
   42
   >>> d
   47
   >>> e = d
   >>> e += 1
   >>> (d, e)
   (47, 48)

.. note::

   Another very important type is the ``ndarray`` type described
   here: :ref:`numpy`.  It is an array type for efficient numerics,
   and is heavily used in ASE.



Loops
-----

A loop in Python can be done like this:

>>> things = ['a', 7]
>>> for x in things:
...     print(x)
...
a
7

The ``things`` object could be any sequence.  Strings, tuples, lists,
dictionaries, ndarrays and files are sequences.  Try looping over some
of these types.

Often you need to loop over a range of numbers:

>>> for i in range(5):
...     print(i, i*i)
...
0 0
1 1
2 4
3 9
4 16


Functions and classes
---------------------

A function is defined like this:

>>> def f(x, m=2, n=1):
...     y =  x + n
...     return y**m
...
>>> f(5)
36
>>> f(5, n=8)
169

Here ``f`` is a function, ``x`` is an argument, ``m`` and ``n`` are keywords with default values ``2`` and ``1`` and ``y`` is a variable.

A *class* is defined like this:

>>> class A:
...     def __init__(self, b):
...         self.c = b
...     def m(self, x):
...         return self.c * x
...     def get_c(self):
...         return self.c

You can think of a class as a template for creating user defined
objects.  The ``__init__()`` function is called a *constructor*,
it is being called when objects of this type are being created.

In the class ``A`` ``__init__`` is a constructor, ``c`` is an
attribute and ``m`` and ``get_c`` are methods.

>>> a = A(7)
>>> a.c
7
>>> a.get_c()
7
>>> a.m(3)
21

Here we make an instance (or object) ``a`` of type ``A``.


Importing modules
-----------------

If you put the definitions of the function ``f`` and the class ``C``
in a file ``stuff.py``, then you can use that code from another piece
of code::

  from stuff import f, C
  print(f(1, 2))
  print(C(1).m(2))

or::

  import stuff
  print(stuff.f(1, 2))
  print(stuff.C(1).m(2))

or::

  import stuff as st
  print(st.f(1, 2))
  print(st.C(1).m(2))


Python will look for ``stuff.py`` in these directories:

1) current working directory
2) directories listed in your :envvar:`PYTHONPATH`
3) Python's own system directory (typically :file:`/usr/lib/pythonX.Y`)

and import the first one found.
