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Keywords
              Keywords have special meaning which are reserved by Python for special purposes and are not allowed to be
              used as identifiers. They are case sensitive. We can see the list of the reserved keywords by giving the following
              command at the Python command prompt:

                  >>> import keyword
                  >>> keyword.kwlist
              Following is the list of keywords displayed by giving the above commands:

                    False                class                finally              is                   return
                    None                 continue             for                  lambda               try

                    True                 def                  from                 nonlocal             while
                    and                  del                  global               not                  with

                    as                   elif                 if                   or                   yield
                    assert               else                 import               pass

                    break                except               in                   raise


              Identifiers
              Identifiers are the user-defined names of variables, lists, classes, functions, dictionaries, tuples, etc. which are
              used to identify an element in Python.

              Naming the identifiers follows rules as given below:
                 • An identifier can have letters in uppercase or lowercase, numbers or underscore(_).

                 • It cannot begin with a number or an underscore.
                 • No special characters like $, period(.), space, etc. are allowed.
                 • It should not be a Python keyword.

                 • They are case sensitive. Uppercase and lowercase letters are different.
                 • Identifiers can be of any length.
              Some of the examples of valid identifiers are:  Num1, First_Name, Value, Scores_FirstTerm.

              Some of the examples of invalid identifiers are: 1stsubject, My name, #mobile, City&Code, and Pin Code.

              Variables

              Variable is a name given to a memory location to hold a specific value. It is just like a container that can hold any
              type of data that can be changed later in the program. Variable has:
                 • A name that follows the rules of identifier naming conventions and is case sensitive.

                 • A value that can be integer, float, string, boolean, etc.
              Unlike other programming languages, variables in Python are not created before. They are created automatically
              when a value is assigned to it using the assignment operator (=). A variable assigned with a value of one type
              can be re-assigned a new value of a different type as shown below:

                  >>> a=9
                  >>>a


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