Python Programming Fundamentals for Class 11 and 12 – Basics Of Python

The previous chapter has given a brief introduction about Python’s user interface. This chapter will scratch the surface of Python programming and expose the basic elements which a programmer needs to know beforehand.

Variable, identifier and literal
A variable is a storage location that has an associated symbolic name (called “identifier”), which contains some value (can be literal or other data) that can change. An identifier is a name used to identify a variable, function, class, module or other object. Literal is a notation for constant values of some built-in type. Literal can be string, plain integer, long integer, floating point number, imaginary number. For e.g., in the expressions

var1=5
 var2='Tom'

varl and var2 are identifiers, while 5 and ‘ Tom’ are integer and string literals, respectively.
Consider a scenario where a variable is referenced by the identifier a and the variable contains a list. If the same variable is referenced by the identifier b as well, and if an element in the list is changed, the change will be reflected in both identifiers of same variable.

>>> a=[1,2,3]
 >>> b=a
 >>> b
 [1, 2, 3]
 >>> a[1]=10
 >>> a
 [1, 10, 3]
 >>> b
 [1, 10,3]

Now, the above scenario can be modified a bit, where a and b are two different variables.

>>> a=[1, 2, 3]
 >>> b=a[:] # Copying data from a to b.
 >>> b [1, 2, 3]
 >>> a[1]=10
 >>> a
 [1, 10, 3]
 >>> b
 [1, 2, 3]

There are some rules that need to be followed for valid identifier naming:

The first character of the identifier must be a letter of the alphabet (uppercase or lowercase) or an underscore
The rest of the identifier name can consist of letters (uppercase or lowercase character), underscores (‘_’) or digits (0-9).
Identifier names are case-sensitive. For example, myname and myName are not the same.
Identifiers can be of unlimited length.

Token
A token is a string of one or more characters that is significant as a group. Consider an expression:

sum=6+2

The tokens in the above expression are given in table 2-1:
Table 2-1: Tokens

Token	Token type
sum	Identifier
=	Assignment operator
6	Integer literal
+	Addition operator
2	Integer literal

The process of converting a sequence of characters into a sequence of tokens is called “lexical analysis”. A program or function that performs lexical analysis is called a lexical analyzer, lexer, or tokenizer. A lexer is generally combined with a parser (beyond the scope of this book), which together analyze the syntax of computer language. Python supports the following categories of tokens: NEWLINE, INDENT, DEDENT, identifiers, keywords, literals, operators, and delimiters.

Keywords
The following identifiers (as shown as output in the following code) are used as reserved words (or “keywords”) of the language, and cannot be used as ordinary identifiers.

>>> import keyword
 >>> for kwd in keyword.kwlist:
 ... print kwd
 ...
 and
 as
 assert
 break
 class
 continue
 def
 del
 elif
 else
 except
 exec
 finally
 for
 from
 global
 if
 import
 in
 is
 lambda
 not
 or
 pass
 print
 raise
 return
 try
 while
 with
 yield

One can also check if an identifier is keyword or not using iskeyword () function.

>>> import keyword
 >>> keyword.iskeyword('hi')
 False
 >>> keyword.iskeyword('print')
 True

Operators and operands
An operator is a symbol (such as +, x, etc.) that represents an operation. An operation is an action or procedure which produces a new value from one or more input values^ called operands. There are two types of operators: unary and binary. Unary operator operates only on one operand, such as negation. On the other hand, binary operator operates on two operands, which includes addition, subtraction, multiplication, division, exponentiation operators etc. Consider an expression 3 + 8, here 3 and 8 are called operands, while V is called operator. The operators can also be categorized into:

Arithmetic operators.
Comparison (or Relational) operators.
Assignment operators.
Logical operators.
Bitwise operators.
Membership operators.
Identity operators.

Arithematics operators
Table 2-2 enlists the arithematic operators with a short note on the operators.
Table 2-2: Arithematic operators

Operator	Description
+	Addition operator- Add operands on either side of the operator.
–	Subtraction operator – Subtract right hand operand from left hand operand.
*	Multiplication operator – Multiply operands on either side of the operator.
/	Division operator – Divide left hand operand by right hand operand.
%	Modulus operator – Divide left hand operand by right hand operand and return remainder.
* *	Exponent operator – Perform exponential (power) calculation on operands.
//	Floor Division operator – The division of operands where the result is the quotient in which the digits after the decimal point are removed.