Python Variables

Objects are Python's abstraction for data. In Python, data are represented by objects or by relations between objects. Use the type() function to get the class name of an object. For example, the following displays the class name of integer value.

>>> type(10)
<class 'int'>

The type of 10 is int. An object of int class contains a integer literal 10. The same thing for string value too.

>>> type('Hello World')
<class 'string'>

Thus, all values are actually an object of a class depending upon the value.

Variables in Python are names given to objects, so that it becomes easy to refer a value. In other words, a variable points to an object. A literal value is assigned to a variable using the = operator where the left side should be the name of a variable, and the right side should be a value. The following assigns a name to an integer value.

>>> num=10

Now, you can refer 10 using a variable name num, as shown below.

>>> print(num) #display value
>>> print(num * 2) # multiply and display result

Check the type of a variable using the type() function.

>>> type(num) # display type
<class 'int'>

In the same way, the following variable points to a string value.

>>> greet='Hello World'
>>> print(greet)
Hello World 
>>> type(greet)
<class 'string'>

Unlike other programming languages like C# or Java, Python is a dynamically-typed language, which means you don�t need to declare a type of a variable. The type will be assigned dynamically based on the assigned value.

>>> x=100
>>> type(x)
<class 'int'>

>>> x='Hello World'
>>> type(a)
<class 'string'>

Different operations can be performed on variables using various operators based on the type of variables. For example, the + operator sums up two int variables, whereas it concatenates two string type variables, as shown below.

>>> x=5
>>> y=5
>>> x+y
>>> x='Hello '
>>> y='World'
>>> x+y
'Hello World'

Object's Identity

Each object in Python has an id. It is the object's address in memory represented by an integer value. The id() function returns the id of the specified object where it is stored, as shown below.

>>> x=100
>>> id(x)
>>> greet='Hello'
>>> id(greet)

An id will be changed if a variable changed to different value.

>>> x=100
>>> id(x)
>>> x='Hello'
>>> id(x)

Multiple variables assigned to the same literal value will have the same id, for example:

>>> x=100
>>> id(x)
>>> y=x
>>> id(y)
>>> z=100
>>> id(z)

Thus, Python optimize memory usage by not creating separate objects if they point to same value.

Multiple Variables Assignment

You can declare multiple variables and assign values to each variable in a single statement, as shown below.

>>> x, y, z = 10, 20, 30

In the above example, the first int value 10 will be assigned to the first variable x, the second value to the second variable y, and the third value to the third variable z. Assignment of values to variables must be in the same order in they declared.

You can also declare different types of values to variables in a single statement, as shown below.

>>> x, y, z = 10, 'Hello', True

Assign a value to each individual variable separated by a comma will throw a syntax error, as shown below.

>>> x = 10, y = 'Hello', z = True
SyntaxError: can't assign to literal

The type of variables depends on the types of assigned value.

>>> x, y, z = 10, 'Hello', True
>>> type(x)
<class 'int'>
>>> type(y)
<class 'string'>
>>> type(z)
<class 'bool'>

Naming Conventions

Any suitable identifier can be used as a name of a variable, based on the following rules:

  1. The name of the variable should start with either an alphabet letter (lower or upper case) or an underscore (_), but it cannot start with a digit.
  2. More than one alpha-numeric characters or underscores may follow.
  3. The variable name can consist of alphabet letter(s), number(s) and underscore(s) only. For example, myVar, MyVar, _myVar, MyVar123 are valid variable names, but m*var, my-var, 1myVar are invalid variable names.
  4. Variable names in Python are case sensitive. So, NAME, name, nAME, and nAmE are treated as different variable names.
  5. Variable names cannot be a reserved keywords in Python.

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