First steps
-------------


Start the **Ipython** shell (an enhanced interactive Python shell):

* by typing "ipython" from a Linux/Mac terminal, or from the Windows cmd shell,
* **or** by starting the program from a menu, e.g. in the `Python(x,y)`_ or
  `EPD`_ menu if you have installed one of these scientific-Python suites.
* **or** by starting the program from a menu, e.g. the `Anaconda Navigator`_,
  the `Python(x,y)`_ menu or the `EPD`_ menu if you have installed one of these
  scientific-Python suites.

.. _`Python(x,y)`: https://python-xy.github.io/
.. _`Anaconda Navigator`: https://anaconda.org/anaconda/anaconda-navigator
.. _`EPD`: http://store.enthought.com/

.. tip::

    If you don't have Ipython installed on your computer, other Python
    shells are available, such as the plain Python shell started by
    typing "python" in a terminal, or the Idle interpreter. However, we
    advise to use the Ipython shell because of its enhanced features,
    especially for interactive scientific computing.

Once you have started the interpreter, type ::

    >>> print("Hello, world!")
    Hello, world!

.. tip::

    The message "Hello, world!" is then displayed. You just executed your
    first Python instruction, congratulations!

To get yourself started, type the following stack of instructions ::

    >>> a = 3
    >>> b = 2*a
    >>> type(b)     # doctest: +SKIP
    <type 'int'>
    >>> print(b)
    6
    >>> a*b 
    18
    >>> b = 'hello' 
    >>> type(b)    # doctest: +SKIP
    <type 'str'>
    >>> b + b
    'hellohello'
    >>> 2*b
    'hellohello'

.. We need to skip the call to 'type' because in Python3 is prints as
   'type', but in Python2 as 'class'

.. tip::

  Two variables ``a`` and ``b`` have been defined above. Note that one does
  not declare the type of a variable before assigning its value. In C,
  conversely, one should write:

  .. sourcecode:: c

      int a = 3;

  In addition, the type of a variable may change, in the sense that at
  one point in time it can be equal to a value of a certain type, and a
  second point in time, it can be equal to a value of a different
  type. `b` was first equal to an integer, but it became equal to a
  string when it was assigned the value `'hello'`. Operations on
  integers (``b=2*a``) are coded natively in Python, and so are some
  operations on strings such as additions and multiplications, which
  amount respectively to concatenation and repetition.