.. include:: <mmlalias.txt>

Variables
=========

.. contents::
   :local:

.. ot-topic:: python.basics.python_0140_variables
   :dependencies: python.basics.python_0130_syntax_etc

Variables (1)
-------------

**A variable is a name for ...** something

* *Something* has a type
* ... but its name hasn't

.. list-table::
   :width: 70%
   :align: left
   :widths: 30, 30

   * - ::

          >>> a = 42
	  >>> type(a)
	  <class 'int'>
	  >>> a = 1.5
	  >>> type(a)
	  <class 'float'>
	  >>> a = [42,'blah']
	  >>> type(a)
	  <class 'list'>
     - .. image:: 0140-variables-datatypes-basic.dia
          :width: 100%

Variables (2)
-------------

**Python is a "dynamic language"** (whatever that means)

* Names have no type
* Created when first assigned
* |longrightarrow| *Runtime error* when accessed but not yet there
* ... as opposed to *compiled languages* (whatever that means)

**Naming rules:** just like most other languages

* Start with Letters (Unicode since Python 3, ASCII in Python 2) or
  underscore
* Numbers in the following characters
* Case sensitive

Assignment Fun
--------------

.. code-block::
   :caption: Multiple assignments in one statement

   a, b, c = 1, "Eins", 1.0
   a, b = b, a  # "swap"

* *Tuple unpacking*
* Important concept throughout the entire language
* |longrightarrow| later
  
.. code-block::
   :caption: Assignment has a value

   a = b = c = 1

* Assignment is *right associative*
* |Longrightarrow| ``a, b, c`` are assigned ``1``

Assignment Details
------------------

**More than one ever wants to know ...**

* Day-to-day programming does not need to know
* Good to know when something goes wrong
* Only valid for *immutable* types (``int``, ``float``, ``str``)

.. list-table::
   :width: 70%
   :align: left
   :widths: 30, 20

   * - ::

          a = 42
	  b = a
	  b = 7

       Or equivalently ...::
      
	 a = 42
	 b = 42
	 b = 7

     - .. image:: 0140-variables-datatypes-assignment-details.dia