Python Introduction

• In order to provide an input, process it and to receive output, we need to write a program.
• Program, is a group of instructions which controls processing.
• In other words, base for processing is ‘the Program’.
• In this chapter we will come to know about various element of processing like – character set, token, expressions, statements, input.

Python Character Set

• Character Set-is a group of letters or signs which are specific to a language.
• Character set includes letter, sign, number, symbol.
  –Letters: A-Z, a-z
  –Digits: 0-9
  –Special Symbols: _, +, -, *, /, (, ), {, } . . . Etc.
  –White Spaces: blank space, tab, carriage return, newline, formfeed etc.
  –Other characters: Python can process all characters of ASCII and UNICODE.

Tokens

Token- is the smallest unit of any programming language. It is also known as Lexical Unit.

Types of token are-
i. Keywords
ii. Identifiers (Names)
iii. Literals
iv. Operators
v. Punctuators

Keywords

• Keywords are those words which provides a special meaning to interpreter.
• These are reserved for specific functioning.
• These can not be used as identifiers, variable name or any other purpose.
• Available keywords in Python are-

Identifiers

• These are building blocks of a program and are used to give names to different parts/blocks of a program like – variable, objects, classes, functions.
• An identifier may be a combination of letters and numbers.
• An identifier must begin with an alphabet or an underscore( _ ). Subsequent letters may be numbers(0-9).
• Python is case sensitive. Uppercase characters are distinct from lowercase characters (P and p are different for interpreter).
• Length of an Identifier is unlimited.
• Keywords can not be used as an identifier.
• Space and special symbols are not permitted in an identifier name except an underscore( _ ) sign.
• Some valid identifiers are –
  –Myfile, Date9_7_17, Z2T0Z9, _DS, _CHK FILE13.
• Some invald identifiers are –
  –DATA-REC, 29COLOR, break, My.File.

Literals / Values

• Literals are often called Constant Values.
• Python permits following types of literals –
  –String literals – “Pankaj”
  –Numeric literals – 10, 13.5, 3+5i
  –Boolean literals – True or False
  –Special Literal None
  –Literal collections

String Literals

• String Literal is a sequence of characters that can be a combination of letters, numbers and special symbols, enclosed in quotation marks, single, double or triple(“ “ or ‘ ‘ or “’ ‘”).
• In python, string is of 2 types-
  –Single line string
      * Text = “Hello World” or Text = ‘Hello World’
  –Multi line string
      * Text = ‘hello\         or         Text = ‘’’hello
              world’                                 word ‘’’

Numeric Literals

Numeric values can be of three types –

–int (signed integers)
      * Decimal Integer Literals – 10, 17, 210 etc.
      * Octal Integer Literals – 0o17, 0o217 etc.
      * Hexadecimal Integer Literals – 0x14, 0x2A4, 0xABD etc.

–float ( floating point real value)
      * Fractional Form – 2.0, 17.5 -13.5, -.00015 etc.
      * Exponent Form – -1.7E+8, .25E-4 etc.

–complex (complex numbers)
      * 3+5i etc.

Boolean Literals

• It can contain either of only two values – True or False
       * A= True
       * B=False

Special Literals

• None, which means nothing (no value).
       * X = None

Operators

• An Operator is a symbol that trigger some action when applied to identifier (s)/ operand (s)
• Therefore, an operator requires operand (s) to compute upon.
  example :
  c = a + b
  Here, a, b, c are operands and operators are = and + which are performing differently.

Types of Operators

Python supports following types of operators –

–Unary Operator
      * Unary plus (+)
      * Unary Minus (-)
      * Bitwise complement (~)
      * Logical Negation (not)

–Binary Operator
      * Arithmetic operator (+, -, *, /, %, **, //)
      * Relational Operator(<, >, <=, >=, ==, != )
      * Logical Operator (and, or)
      * Assigment Operator (=, /=, +=, -=, *=, %=, **=, //=)
      * Bitwise Operator (& bitwise and, ^ bitwise xor, | bitwise or)
      * Shift operator (<< shift left, >> shift right)
      * Identity Operator (is, is not)
      * Membership Operator (in, not in)

Punctuators

• In Python, punctuators are used to construct the program and to make balance between instructions and statements. Punctuators have their own syntactic and semantic significance.
• Python has following Punctuators –
  ‘, ”, #, \, (, ), [, ], {, }, @. ,, :, .. `, =

A Python Program Structure

A Python Program Structure

• As we have seen above, a program contains following compnents-
       –Expressions like a+b, a>b etc.
       –Statements like a=10, c=a+b etc.
       –Comments, lines starting with #.
       –Function, block starting with def keyword
       –Blocks and indentation like if and else blocks