Variables and expressions
=========================

Variables
---------

The most commonly used command in Python is assignment. The Python code to
create a variable called ``x`` that is to be given the value ``π`` is:

.. code-block:: pycon

   >>> pi = 3.14159

In Python, unlike many other programming languages, neither a variable
declaration nor an end-of-line delimiter is necessary. The line is terminated by
the end of the line. Variables are created automatically when they are assigned
for the first time.

.. note::
   In Python, variables are labels that refer to objects. Any number of labels
   can refer to the same object, and if that object changes, so does the value
   to which all those variables refer. To better understand what this means, see
   the following example:

   .. code-block:: pycon

      >>> x = [1, 2, 3]
      >>> y = x
      >>> y[0] = 4
      >>> print(x)
      [4, 2, 3]

   However, variables can also refer to constants:

   .. code-block:: pycon

      >>> x = 1
      >>> y = x
      >>> z = y
      >>> y = 4
      >>> print(x, y, z)
      1 4 1

   In this case, after the third line, ``x``, ``y`` and ``z`` all refer to the
   same immutable integer object with the value ``1``. The next line, ``y = 4``,
   causes ``y`` to refer to the integer object ``4``, but this does not change
   the references of ``x`` or ``z``.

Python variables can be set to any object, whereas in many other languages
variables can only be stored in the declared type.

Variable names are case-sensitive and can contain any alphanumeric character as
well as underscores, but must begin with a letter or underscore.

Keywords
--------

If you receive a ``SyntaxError``, check whether the variable name is a keyword.
Keywords are reserved for use in Python language constructs, so you cannot turn
them into variables. After calling :ref:`help` you can enter ``keywords`` to get
the keywords:

.. code-block:: pycon

   >>> help()
   help> keywords
   Here is a list of the Python keywords.  Enter any keyword to get more help.
   False               class               from                or
   None                continue            global              pass
   True                def                 if                  raise
   and                 del                 import              return
   as                  elif                in                  try
   assert              else                is                  while
   async               except              lambda              with
   await               finally             nonlocal            yield
   break               for                 not

Built-in functions
------------------

You can use a variable name to overwrite built-in functions, types and other
objects so that they can then only be accessed via the :doc:`builtins
<python3:library/builtins>` module. These variable names should therefore never
be used. You can obtain a list of the :mod:`__builtins__` objects with:

.. code-block:: pycon

   >>> dir(__builtins__)
   ['ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException', 'BaseExceptionGroup', 'BlockingIOError', 'BrokenPipeError', 'BufferError', 'BytesWarning', 'ChildProcessError', 'ConnectionAbortedError', 'ConnectionError', 'ConnectionRefusedError', 'ConnectionResetError', 'DeprecationWarning', 'EOFError', 'Ellipsis', 'EncodingWarning', 'EnvironmentError', 'Exception', 'ExceptionGroup', 'False', 'FileExistsError', 'FileNotFoundError', 'FloatingPointError', 'FutureWarning', 'GeneratorExit', 'IOError', 'ImportError', 'ImportWarning', 'IndentationError', 'IndexError', 'InterruptedError', 'IsADirectoryError', 'KeyError', 'KeyboardInterrupt', 'LookupError', 'MemoryError', 'ModuleNotFoundError', 'NameError', 'None', 'NotADirectoryError', 'NotImplemented', 'NotImplementedError', 'OSError', 'OverflowError', 'PendingDeprecationWarning', 'PermissionError', 'ProcessLookupError', 'RecursionError', 'ReferenceError', 'ResourceWarning', 'RuntimeError', 'RuntimeWarning', 'StopAsyncIteration', 'StopIteration', 'SyntaxError', 'SyntaxWarning', 'SystemError', 'SystemExit', 'TabError', 'TimeoutError', 'True', 'TypeError', 'UnboundLocalError', 'UnicodeDecodeError', 'UnicodeEncodeError', 'UnicodeError', 'UnicodeTranslateError', 'UnicodeWarning', 'UserWarning', 'ValueError', 'Warning', 'ZeroDivisionError', '__build_class__', '__debug__', '__doc__', '__import__', '__loader__', '__name__', '__package__', '__spec__', 'abs', 'aiter', 'all', 'anext', 'any', 'ascii', 'bin', 'bool', 'breakpoint', 'bytearray', 'bytes', 'callable', 'chr', 'classmethod', 'compile', 'complex', 'copyright', 'credits', 'delattr', 'dict', 'dir', 'divmod', 'enumerate', 'eval', 'exec', 'exit', 'filter', 'float', 'format', 'frozenset', 'getattr', 'globals', 'hasattr', 'hash', 'help', 'hex', 'id', 'input', 'int', 'isinstance', 'issubclass', 'iter', 'len', 'license', 'list', 'locals', 'map', 'max', 'memoryview', 'min', 'next', 'object', 'oct', 'open', 'ord', 'pow', 'print', 'property', 'quit', 'range', 'repr', 'reversed', 'round', 'set', 'setattr', 'slice', 'sorted', 'staticmethod', 'str', 'sum', 'super', 'tuple', 'type', 'vars', 'zip']

Expressions
-----------

Python supports arithmetic and similar expressions. The following code
calculates the average of ``x`` and ``y`` and stores the result in the variable
``z``:

.. code-block:: pycon

    >>> x = 1
    >>> y = 2
    >>> z = (x + y) / 2

.. note::
   Arithmetic operators that use only integers do not always return an integer.
   As of Python 3, division returns a floating point number. If you want the
   traditional integer division to return an integer, you can use ``//``
   instead.

Checks
------

* Create some variables in the Python shell. What happens if you add spaces,
  hyphens or other characters to the variable names?

* Do the results change if you use brackets to group numbers in different ways?

* Which of the following variable and function names do you think are not good
  Python style, and why?

  ``var*``, ``varname``, ``func_name()``, ``varName``, ``VARNAME``,r
  ``very_very_long_var_name``