Constants and Variables

Table of Contents

Constants and variables are the most basic language elements (like words in English or characters in Chinese). Learning Python starts here.

Constants

Constants are predefined objects with fixed values in Python, such as True, False, and None:

True/False: Boolean values representing logical truth/falsity.

None: Represents an empty object (missing value). Constants cannot be modified during execution.

Some built-in or third-party modules also predefine constants. For example, the math module defines pi (π). Import the module first with import:

code.python 
>>> import math
>>> math.pi
3.141592653589793

Variables: Declaration, Assignment, and Deletion

In Python, variables do not need explicit declaration—declaration and assignment are done in one step: assigning a value to a variable creates it.

Use the assignment operator = to assign values. For example:

code.python 
>>> a = 1  # Create variable a and assign 1
>>> a      # Check the value of a
1
>>> print(a)  # Output the value of a
1

Data Types of Variables

Each variable has a data type. Common types include Boolean, numeric, string, list, tuple, etc. (Table 2-1). This chapter also covers structured types: NumPy arrays, pandas Series, and pandas DataFrame.

Table 2-1 Common Python Data Types

Type Name Type Character Description Example
Boolean bool Values: True or False >>> a=True; b=False
Integer int Integers (no size limit) >>> a=1; b=10000000
Float float Decimal numbers (supports scientific notation) >>> a=1.2; b=1.2e3
String str Immutable sequence of characters >>> a='A'; b='A'
List list Mutable, ordered, allows duplicates >>> a=[1, 'A', 3.14, []]
Tuple tuple Immutable (similar to lists) >>> a=(1, 'A', 3.14, ())
Dictionary dict Unordered key-value pairs (unique keys) >>> a={1:'A', 2:'B'}
Set set Unordered, mutable, no duplicates >>> a={1, 3.14, 'name'}
None NoneType Represents an empty object >>> a=None