Identifiers
===========

Identifiers are short pieces of text that are **used** as:

  * variables names
  * function names
  * function argument keywords
  * class names
  * module names

Unlike string literals they are *not* enclosed in quotation marks.

.. interactive_code_block::
   :caption: Identifiers are like string literals pieces of text but can be easily distinguished from each other

   # A variable is an identifier that acts as a placeholder for a value
   a_variable_name = 'is not enclosed in quotation marks like string literals'
   print(a_variable_name)

The variable name to the left of the equal sign ``=`` is not enclosed in quotation marks.

To get a valid, working identifier a small set of **rules** has to be followed: 

  * It must not be one of the `keywords <keywords_and_built_in_functions.html#keywords>`__
  * It may contain

    * lowercase and uppercase *letters* ``a`` .. ``z`` and ``A``  .. ``Z``
    * *digits* ``0`` .. ``9`` except as the first character
    * *underscores* ``_``
   
Actually Python 3 allows some `additional characters <https://docs.python.org/3/reference/lexical_analysis.html#identifiers>`__, so letters like ä, ö, ü, ß *could* also be used.

.. interactive_code_block::
   :caption: Valid identifiers (used as a variable names)
   
   # Identifiers with letters only are hard to read
   numberofdaysperweek = 7

   # Identifiers with uppercase letters only even harder to read
   NUMBEROFMONTHSPERYEAR = 12

   # Mixing upper- and lowercase letters (= camel case because of the 'humps') improves readablity a bit
   firstDayOfTheWeek = 'Monday'

   # Seprating words with underscores improves readablity much more (and uses only slightly more space)
   second_day_of_the_week = 'Tuesday'

   # Numbers can also been used (but not as first character!)
   python_3 = 'an easy to learn programming language'

   # Underscores can also be used as the first character
   _do_overscores_exist = False

   # Keywords can only be used as identifiers if the case is changed
   Finally = 'the sun is shining! :^)'

   print('If You read this, the code above contained no errors.')

.. interactive_code_block::
   :caption: Invalid identifiers
   
   # Identifiers must not start with a number
   2nd_day_of_the_week = 'Tuesday'

   # Predefined names like keywords or built-in functions must not be used as identifier
   continue = 'don\\'t stop'

   # Special characters within identifiers are not allowed
   us$ = 'a currency'
   
   # Spaces within identifiers are also not allowed
   the final frontier = 'space'

   print('If You read this, the code above contained no errors.')

Each of the variables in the statements above throw an error (and aborts running the program). |br|
► Try to correct all of them, one statment after the other, and watch the error messages.