Understanding Code: Python Basics for Conditions, Loops, Variables, Functions, Modules, and Classes

Code is a language that lets humans communicate precise instructions to a computer, which executes commands without making its own decisions.

Python is used as the teaching language. It has a small set of keywords and simple syntax, making it suitable for illustrating basic programming concepts.

1. Conditional Statements (Judgment)

Conditional logic uses if , else , elif (or elseif ) together with comparison operators ( < , > , = , <= , >= , == ) and logical operators and , or . Example:

if 水果店打折：
    买西瓜
else:
    不买了

A more complex decision can be nested:

if 水果店打折：
    买西瓜
else:
    与老板讲价
    if 讲价成功：
        买西瓜
    else:
        不吃了。

Logical operators determine how multiple conditions are combined; parentheses can control precedence.

2. Loops

Python loops use for…in… , while , break , and continue . Example of iterating over three fruit stores to find a discount:

for 水果店 in (鲜丰水果、四季水果和路边摊):
    if 水果店 打折：
        买西瓜
        break  # exit loop after buying
    else:
        什么都不做
带西瓜回家

The break statement stops the loop once the desired condition is met.

3. Variables

Variables act as memory cells that store values. They can be assigned with = and compared with == . Example of tracking the best discount:

之前所有水果店的最大折扣 = None
之前给最大折扣的水果店 = None
for 水果店 in (鲜丰水果、四季水果和路边摊):
    当前水果店折扣 = 老板给的折扣
    if 当前水果店折扣 < 之前所有水果店的最大折扣:  # 4折比5折力度大
        之前所有水果店的最大折扣 = 当前水果店折扣
        之前给最大折扣的水果店 = 水果店
去 之前给最大折扣的水果店 买一个西瓜

Variable scope follows indentation: a variable is visible from its first appearance down through deeper‑indented blocks.

4. Functions

Functions group reusable code. They are defined with def function_name(): and called by writing the function name followed by parentheses. Example of a watermelon‑buying function:

def 买西瓜_函数():
    for 水果店 in (鲜丰水果、四季水果和路边摊):
        if 水果店 打折：
            买西瓜
        else:
            与老板讲价
            if 打折成功 and 打折力度 <= 5:
                买两个西瓜
            elif 打折成功 and 打折力度 > 5:
                买一个西瓜
            else:
                不吃了。
买西瓜_函数()
休息一天。
买西瓜_函数()

Function parameters allow the same logic to work for different fruits. The following function takes a fruit name as an argument:

def 买水果函数(水果名):
    for 水果店 in (鲜丰水果、四季水果和路边摊):
        if 水果店 打折：
            买[水果名]
        else:
            与老板讲价
            if 打折成功 and 打折力度 <= 5:
                买两个[水果名]
            elif 打折成功 and 打折力度 > 5:
                买一个[水果名]
            else:
                不吃了。

买水果函数(西瓜)  # 买西瓜
买水果函数(柚子)  # 买柚子

Python also supports default and keyword arguments, e.g.:

def sum(a, b = 0, c = 1):
    return a + b + c

func(1, 0, 1)
func(1)
func(1, c=1)

5. Modules

Modules are files (e.g., a.py ) that contain functions, classes, or variables. They can be imported using different styles:

import a
a.buy_xigua()
a.buy_youzi()

from a import *
buy_xigua()
buy_youzi()

from a import buy_xigua, buy_youzi
buy_xigua()
buy_youzi()

Importing only what is needed keeps programs lightweight and clear.

6. Classes and Objects

Classes define a blueprint with attributes (state) and methods (behavior). Objects are instances created from a class at runtime. They allow grouping related variables and functions together, enabling more complex modeling than functions alone.

For example, a "Person" class could store a name, balance, and the number of watermelons owned, and provide a method to buy a watermelon.

These concepts together form the foundation for writing organized, maintainable Python programs.