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variables.md

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Variables

  • Variables play a very important role in most programming languages, and Python is no exception.
  • A variable allows you to store a value by assigning it to a name, which can be used to refer to the value later in the program.
  • To assign a variable, use one equals sign. Unlike most lines of code we've looked at so far, it doesn't produce any output at the Python console.

Example

>>> x = 7
>>> print(x)
7
>>> print(x + 3)
10
>>> print(x)
7

NOTE : You can use variables to perform corresponding operations, just as you did with numbers and strings. As you can see, the variable stores its value throughout the program.

  • Variables can be reassigned as many times as you want, in order to change their value. In Python, variables don't have specific types, so you can assign a string to a variable, and later assign an integer to the same variable.

Example

>>> x = 123.456
>>> print(x)
123.456
>>> x = "This is a string"
>>> print(x + "!")
This is a string!

NOTE : However, it is not good practice. To avoid mistakes, try to avoid overwriting the same variable with different data types.

Variable Names

  • Certain restrictions apply in regard to the characters that may be used in Python variable names.
  • The only characters that are allowed are letters, numbers, and underscores. Also, they can't start with numbers.
  • Not following these rules results in errors.
>>> this_is_a_normal_name = 7

>>> 123abc = 7
SyntaxError: invalid syntax

>>> spaces are not allowed
SyntaxError: invalid syntax

Variable Assignment

  • Trying to reference a variable you haven't assigned to causes an error.
  • You can use the del statement to remove a variable, which means the reference from the name to the value is deleted, and trying to use the variable causes an error.
  • Deleted variables can be reassigned to later as normal.

Example

>>> foo = "a string"
>>> foo
'a string'
>>> bar
NameError: name 'bar' is not defined
>>> del foo
>>> foo
NameError: name 'foo' is not defined
  • You can also take the value of the variable from the user input.
>>> foo = input("Enter a number: ")
Enter a number: 7
>>> print(foo)
7