The Python Interpreter
######################

.. note::

    Learning objectives:
    Install the Python interpreter.
    Use the interpreter as a calculator.
    Explain what "returning" a value means.

Installing Python
=================

* Go to the `Python Software Foundation`_ web site and download the latest release for your operating system.

* Different versions exist for `Windows`_, `Linux`_, `MacOS`_, and others.

* Your not very humble narrator even runs it on his cell phone.
    * (I need it for good reasons, like breaking into files I stole from *your* cell phone.)

.. _Windows: https://www.python.org/downloads/windows/
.. _Linux: https://www.python.org/downloads/source/
.. _MacOS: https://www.python.org/downloads/mac-osx/
.. _Python Software Foundation: https://www.python.org/

* You will see two flavors of Python, numbered Python 2 and Python 3.
    * They are very similar, but Python 3 is more advanced.
    * Think of Python 2 and Python 3 as separate programming languages.

Make it do math
===============

Let's start our Python interpreter and make it do stuff.  Let's make it do math.

.. code-block:: python

    >>> 7 + 5
    12
    >>> 12 - 5
    7
    >>> 12 - 5.0 # Now this is weird, right?  Where did that decimal point come from?
    7.0
    >>> 12 / 2
    6
    >>> 12 / 2.0
    6.0
    >>> 7 / 2 # I swear I am not making this up.  The computer isn't wrong.  You are.
    3

.. image:: images/masha_and_tongue.png

What happens with 7 / 2.0 or 7.0 / 2 ?


Why does it do that?
====================

.. note::  Computer programs read instructions from a disk into memory,
    then follow those instructions until they end or the computer runs out of resources.

- You are running something called the Python interpreter.  It's a program for running other programs.

- It takes Python instructions and runs them as soon as you press enter.

- Whenever it "thinks" of an answer to something, it shows you right away.  That's called **returning** a value.

- This is not how it will behave when you write a program and run it, but it is a great short-cut to figuring out
  how it works.

- No matter what you give the interpreter, Python will spit back its simplified value.


.. code-block:: python

    >>> 5
    5
    >>> 5.0
    5.0
    >>> '5'
    '5'

Notice how that last *\'5\'* is in quotes.  I'll be you thought it was a number.  Here's where the actual fun begins.

Arithmetic operators
====================

Here are the math operators for adding, multiplying, getting remainders, etc.

Use this for reference.

+--------+-----------------+-------------+------------+
| Symbol | Operation       | Result      | Example    |
+========+=================+=============+============+
| \+     | Add             | Sum         | 2 + 5 = 7  |
+--------+-----------------+-------------+------------+
| \-     | Subtract        | Difference  | 7 - 5 = 2  |
+--------+-----------------+-------------+------------+
| \*     | Multiply by     | Product     | 2 * 4 = 8  |
+--------+-----------------+-------------+------------+
| /      | Divide by       | Quotient    | 8 / 4 = 2  |
+--------+-----------------+-------------+------------+
| \*\*   | To the power of | Power value | 2 ** 3 = 8 |
+--------+-----------------+-------------+------------+
| %      | Modulus         | Remainder   | 14 % 6 = 2 |
+--------+-----------------+-------------+------------+

Try some order-of-operations experiments.  Go crazy.  Use some parentheses.

In fact, use all the parentheses!  No one is watching you.

.. code-block:: python

    >>> # What will this produce?
    >>> 5 - 2 * (16 / 2 - 4) + (2 + 3)**2

    >>> # What about this?
    >>> 5 - (2 * (16 / 2 - 4) + (2 + 3)**2)

    >>> # Unlike teenagers, computers always do what you say.
    >>> # ...unless it's after a pound sign.
    >>> # A pound sign tells Python to ignore everything after it.
    >>> print('I will do this.') # ...but I will not do that.
    I will do this.

* The "pound sign" (#) is also called a "number sign" or an "octothorpe."
* Before you even ask:  Nope, it is not also called a "hash tag" in computer science.
    * It might be soon, but there is a story to this.  It involves phones, the Olympics and some drama.
    * Some people are weirdly passionate about their pet octothorpes.
    * `You can read more about the phone part at this link.`_
    * `...and the Olympic part at this link.`_

.. _You can read more about the phone part at this link.: http://strowger-net.telefoniemuseum.nl/tel_tech_octothorpe.html
.. _...and the Olympic part at this link.: https://en.wikipedia.org/wiki/Jim_Thorpe