1.2 Data Types and Variables

Most values in a computer program have a type. The type tells Python what the value represents and what operations make sense for it.

Common Python types include:

• Numeric: int, float, complex
• Text: str
• Boolean: bool
• Containers: list, tuple, set, dict

Variables

A variable is a name used to store or refer to data. The value that a variable points to has a type.

Use type() to inspect a value’s type.

num = 10
print(type(num))      # <class 'int'>

salary = 8232.56
print(type(salary))   # <class 'float'>

Here, num refers to an integer, so its type is int. salary refers to a decimal number, so its type is float.

Naming variables

Good variable names are clear and valid:

• Use letters, digits, and underscores, but do not start with a digit.
• Do not use Python reserved keywords.
• Prefer meaningful English words.

Names like age, student_name, and total_score are easier to read than a or x1.

Python keywords

Keywords are reserved by Python and cannot be used as ordinary variable names.

Common keywords include:

• False, None, True
• and, or, not
• if, elif, else
• for, while, break, continue
• def, return, class
• try, except, finally
• import, from, as

Using a keyword as a variable name would make Python unable to tell whether you mean a language feature or an ordinary value.