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Variables

Introduction

Variables are containers used to store values. In Python, variables are defined using the assignment operator =. For example:

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x = 1
y = 100.0
z = "good"

Variables can also be updated using the assignment operator:

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x = 1
print('The initial value of x is', x)
x = 2
print('The value after updating x is', x)

The output is:

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The initial value of x is 1
The value after updating x is 2

Assignment Operator

The syntax of the assignment statement is:

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<variable-name> = <expression>

The assignment operator works from right to left. For example:

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x = 1 + 2 * 3 / 2
print(x)

The output is:

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4.0

Having a literal to the left of the assignment operator will result in an error:

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3 = x

This will throw the following error:

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SyntaxError: cannot assign to literal

Dynamic Typing

Python supports dynamic typing. In a dynamically typed language, a variable is simply a value bound to a name; the value has a type — like int or str — but the variable itself doesn’t. For example:

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a = 1
print(type(a))
a = 1 / 2
print(type(a))

The output is:

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<class 'int'>
<class 'float'>

Referencing vs Defining

When a variable that has already been defined is used in an expression, we say that the variable is being referenced.

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x = 2
print(x * x, 'is the square of', x)

If a variable is referenced before it has been assigned a value, Python throws a NameError:

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print(someVar)

The output is:

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NameError: name 'someVar' is not defined

Keywords and Naming Rules

Certain words in Python, called keywords, have special meanings. Examples include:

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not, and, or, if, for, while, in, is, def, class

Keywords cannot be used as variable names:

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and = 2

In addition to this restriction, variable names in Python must follow these rules:

  1. A variable name can only contain alphanumeric characters and underscores: a-z, A-Z, 0-9, _
  2. A variable name must start with a letter or an underscore.
  3. Variable names are case-sensitive (age, Age, and AGE are three different variables).

Reusing Variables

Variables can be used in computing the values of other variables. For example:

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x = 10
y = x ** 2
z = (x + 1) * (y + 1)

Multiple Assignment

You can assign values to multiple variables in one line:

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x, y = 1, 2

Another way is to assign the same value to multiple variables:

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x = y = z = 10

Even though x, y, and z start off equal, the equality is broken if one of them is updated:

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x = x * 1
y = y * 2
z = z * 3
print(x, y, z)

The output is:

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10 20 30

Assignment Shortcuts

Python provides shorthand notations for arithmetic operations combined with assignment:

Shortcut Meaning
x += a x = x + a
x -= a x = x - a
x *= a x = x * a
x /= a x = x / a
x %= a x = x % a
x **= a x = x ** a

For example:

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x = 1
x += 1
print(x)

The output is:

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2

Deleting Variables

You can delete variables using the del keyword:

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x = 100
print('x is a variable whose value is', x)
print('we are now going to delete x')
del x
print(x)

After the deletion, trying to access x will throw a NameError:

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NameError: name 'x' is not defined