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Constants, Datatypes, and Variables

Constants

Python uses four types of constants: integer, floating point, string, and boolean.

1. Integer
   
   1. Python integers are positive, negative, or zero numbers that do not have a fractional part:
      

      34  643938  -21  -38472874  0
      

   2. Python integers have an unlimited range.
2. Floating Point
   
   • Python floating point constants are positive or negative numbers with a fractional part, for example,
     

     46.1982    0.00441   -237.1273   23.00
     

   • The constant 23.0 is a floating point number, even though it could be converted to an integer without loss of data.
   • Floating point numbers can also be written in base 10 scientific notation with positive or negative exponents. Here are some examples:
     
     

     Python Constant	Scientific Notation	Meaning
     3.0e2	3.0 x 102	300
     5.98e24	5.98 x 1024	Mass of Earth in Kilos
     9.11e-31	9.11 x 10-31	Mass of Electron in Kilos

   • The range of float constants is machine dependent, but is usually at least -1.0 × 10³⁰⁰ to 1.0 × 10³⁰⁰.
   • Float numbers have at about 15 or 16 significant digits. Such Float numbers are called double precision values.
3. String
   
   • A Python string is a sequence of characters delimited by single or double quotes.
   • Examples of string literals:

     "abc"   'g32'   "*&^(%$#"   
     
     " " (space)   '' (null string)
     

   • Either single or double quotes can be used for a Python string.
4. Boolean   Python datatype bool
   • A boolean constant can take on either of the values True or False.

Datatypes

• A datatype is a Python representation of data.
• Python uses these datatypes to represent constants:
  
  

  Constant Type	Python Datatype
  Integer	int
  Floating Point	float
  String	str
  Boolean	bool

Variables

• A variable is a location in a computer that contains data, which might be changed during the execution of a Python script. There are three aspects to a variable: (1) name, (2) value, (3) address.
• The name of a Python variable must start with a letter (upper or lowercase) or underscore and consist entirely of letters, digits, or underscores.
• There is no limit to the length of a Python variable name. However, names longer than about 12 characters become unwieldy. Here are some examples of Python variable names:
  

  x   x123   customer3   number_of_customers
  

• Python is case sensitive, which means that capitalization matters. The names number_Of_Customers and number_of_customers represent two different variables.
• By convention, Python spells local variable names in lower case and uses underscores to separate words (underscore  notation). An example of a local variable name is number_of_letters.
• Although a single underscore is a legal variable name, it is not recommended.
• The address of a variable is the location in the computer's memory where the data is stored. The Python interpreter keeps a symbol table that maintains the correspondence between the variable's name and its address, so the programmer need not be explicity concerned about the address.
• The value of a variable is the data currently stored at its address. Every variable contains the data of an object. Objects can be from the int, float, str, or bool classes. They can also be from any other class defined by the user or from a Python module. A variable can only contain one value at a time.
• The datatype of a variable is inferred by Python by a process called duck typing. (If it looks like a duck and quacks like a duck, it's a duck!) For example, consider these Python statements
  

  n = 34
  x = 3.14
  s = "dog"
  f = true
  

  Python knows that n is int, x is float, s is str, and f is bool because of the datatypes of the constants on the right hand side of the equation.
• Explicit variable declaration is not used in Python.
• Each variable has a specific type. Although each of the following are allowed:
  

  # Following result is 12 because addition is performed
  s = 7 + 5           
  
  # Following result is "dogcat" because concatenation is performed
  t = "dog" + "cat"   
  

  However, the mixing variable types with the + operator is not allowed:
  

  s = "328" + 5
  

  Use either of the following instead:
  

  s = int("328") + 5
  
  t = "dog" + str(5)
  

The Assignment Operator

• The assignment operator is used to assign a value on the right side of an expression to a variable on the left side. For example,
  

  x = 5
  

  assigns the value 5 to the variable x.
• Question: why is the following not a legal assignment?
  

  5 = x