2.1.1 Python Introduction

What This Section Is About

This section is your entry point into learning Python. You do not need to master complex syntax right away. First, understand why Python is a good fit for AI, what it can do, and run your first program yourself so you can build the first impression that “code can solve real problems.”

Learning Objectives

• Understand what Python is and why it is so popular
• Understand Python’s central role in AI
• Write and run your first Python program
• Understand the basic structure of Python code

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Why Learn Python?

If a programming language is a tool, then Python is a Swiss Army knife—it can do almost anything, and it is easy to get started with.

First, let’s look at some data:

Dimension	Description
Popularity	Has ranked No. 1 on the TIOBE programming language leaderboard for many consecutive years
AI First Choice	Almost all AI / machine learning frameworks (PyTorch, TensorFlow) are Python-first
Job Market	A must-have skill for data science, AI engineering, and backend development roles
Learning Curve	Syntax is close to natural language, making it one of the easiest languages for beginners to pick up

In one sentence: If you want to do AI, Python is the starting point.

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What Exactly Is Python?

Python is a high-level programming language released by Guido van Rossum in 1991.

What does “high-level” mean? The farther a programming language is from hardware and the closer it is to human language, the more “high-level” it is. Compare these:

Level	What the same idea looks like
Machine language	10110000 01100001
C language	#include <stdio.h> plus a full main() function
Python	print("Hello World")

To print the same sentence, Python needs only 1 line, while C needs 5 lines. This is Python’s design philosophy: simple and elegant, so you can focus on solving problems instead of syntax details.

Core Features of Python

Feature	Description	Benefit to You
Simple syntax	Uses indentation instead of braces; code reads like English	Learn faster, write less
Interpreted language	Run directly after writing, no compilation needed	Easy to debug, see results immediately
Dynamic typing	No need to declare variable types	Code is shorter and more flexible
Rich ecosystem	More than 400,000 third-party libraries	Ready-made tools others have built, use them directly
Cross-platform	Runs on Windows, macOS, and Linux	One codebase, many environments

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What Can Python Do?

Python is used in many areas. Here are a few of the most important:

AI and Machine Learning (the core of this course)

Install scikit-learn before running this example

Before running the code below in Colab or Jupyter, install it first (only once):

!pip install scikit-learn

On your local terminal or in a Conda environment, use: pip install scikit-learn

# Train a simple linear regression model with a few lines of code (sample data, runnable as-is)
import numpy as np
from sklearn.linear_model import LinearRegression

X_train = np.array([[1], [2], [3], [4], [5]])   # features
y_train = np.array([2, 4, 6, 8, 10])             # labels (y ≈ 2*x)

model = LinearRegression()
model.fit(X_train, y_train)
# After training, you can use model.predict() for prediction

Mainstream frameworks: PyTorch, TensorFlow, scikit-learn, Hugging Face Transformers

Data Analysis and Visualization

import pandas as pd
import matplotlib.pyplot as plt

# Sample data (in real projects, you can use pd.read_csv("sales.csv") to load your own file)
data = pd.DataFrame({"month": ["January", "February", "March"], "revenue": [100, 150, 120]})

# Draw a chart in one line
data.plot(x="month", y="revenue", kind="bar")
plt.show()

Mainstream libraries: pandas, NumPy, Matplotlib, Seaborn

Web Backend Development

With Python, you can quickly build a website backend that provides an API, for example:

from fastapi import FastAPI

app = FastAPI()

@app.get("/hello")
def say_hello():
    return {"message": "Hello, world!"}

Run the service and visit it:

1. First save the code above into a file, such as main.py. Then open a terminal in that directory and run:

   pip install fastapi uvicorn
   uvicorn main:app --reload

2. After you see Uvicorn running on http://127.0.0.1:8000 in the terminal, open the following in your browser:
   • http://127.0.0.1:8000/hello → returns {"message":"Hello, world!"}
   • http://127.0.0.1:8000/docs → the auto-generated API documentation page, where you can click endpoints to test them

Mainstream frameworks: FastAPI, Django, Flask

Automation Scripts

import os

# Example: batch rename images in a folder (create a test directory first before running to avoid FileNotFoundError)
os.makedirs("photos", exist_ok=True)
for i in range(3):
    open(f"photos/old_{i}.jpg", "w").close()   # create 3 empty files as examples

for i, filename in enumerate(os.listdir("photos/")):
    new_name = f"photo_{i+1}.jpg"
    os.rename(f"photos/{filename}", f"photos/{new_name}")

# Check the result (in real projects, you can remove the test directory: os.removedirs, etc.)
print(os.listdir("photos/"))   # ['photo_1.jpg', 'photo_2.jpg', 'photo_3.jpg']

Web Scraping

# First install: !pip install beautifulsoup4
from bs4 import BeautifulSoup

# Use a sample HTML snippet to demonstrate parsing (no internet required, runnable as-is)
html = """
<html><body>
  <h1>Welcome to Learning Python</h1>
  <p>First paragraph</p>
  <p>Second paragraph</p>
</body></html>
"""
soup = BeautifulSoup(html, "html.parser")
title = soup.find("h1").text
paragraphs = soup.find_all("p")
print(f"Web page title: {title}")
print(f"Total {len(paragraphs)} paragraphs")

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Write Your First Python Program

Option 1: Use Python Interactive Mode in the Terminal

Open the terminal (you already learned this in Station 1) and type:

python

You will see a prompt like this:

Python 3.11.5 (main, Sep 11 2023, 08:31:25)
Type "help", "copyright", "credits" or "license" for more information.
>>>

>>> is Python’s interactive prompt, which means it is waiting for you to enter a command.

Try these:

>>> print("Hello, World!")
Hello, World!

>>> 1 + 1
2

>>> "AI" * 3
'AIAIAI'

>>> len("Python")
6

Type exit() or press Ctrl+D to exit interactive mode.

What interactive mode is for

Interactive mode is great for quick experiments—for example, if you are not sure how a function works, you can try it in interactive mode first, then write it into a file once you confirm it works.

Option 2: Write and Run in VS Code

1. Open VS Code (it has already been installed in Station 1)
2. Create a new file called hello.py (note the .py extension)
3. Enter the following code:

# This is my first Python program
print("Hello, World!")
print("I am learning Python!")
print("1 + 1 =", 1 + 1)

4. Save the file (Ctrl+S / Cmd+S)
5. Run it in the terminal:

python hello.py

Output:

Hello, World!
I am learning Python!
1 + 1 = 2

Congratulations, your first Python program is born!

Option 3: Run in Jupyter Notebook

You already installed Jupyter in Station 1. Open it:

jupyter notebook

Create a new Notebook, enter print("Hello from Jupyter!") in a code cell, and then press Shift+Enter to run it.

Which of the three should you choose?

• Interactive mode: quickly test a small piece of code
• VS Code + .py files: write formal project code
• Jupyter Notebook: data analysis and learning experiments (this course will mainly use this one)

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Basic Rules of Python Code

Before diving deeper, let’s first understand a few basic rules:

Indentation Matters

Python uses indentation (usually 4 spaces) to indicate code blocks, instead of braces {} like some other languages.

# Correct ✅
if True:
    print("Indented by 4 spaces")
    print("Same code block")

The following example is intentionally wrong and will raise IndentationError:

if True:
print("Not indented, Python will raise an error")

Note

Indentation errors are one of the most common mistakes for beginners. VS Code will help you indent automatically, but if you copy and paste code, make sure the indentation is still correct.

Comments Use #

# This is a comment line; Python will ignore it
print("This line will run")  # You can also write comments at the end of a line

# Multi-line comments are just multiple lines starting with #
# First comment line
# Second comment line

Comments are for humans to read and help you (and others) understand the code. Good comments explain why something is done, not what is done.

Python Is Case-Sensitive

service_name = "Login API"
Service_Name = "Search API"
SERVICE_NAME = "Worker"
# These are three different variables!

print(service_name)   # Login API
Print(service_name)   # Error! Python has no Print, only print

Files End with .py

Python script files use the .py extension, such as hello.py, train.py, and model.py.

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Python 2 or Python 3?

Short answer: Use Python 3, not Python 2.

Python 2 officially reached end of life on January 1, 2020. All new projects and all modern libraries support only Python 3. This course uses Python 3.10+.

Check your Python version:

python --version
# Should output Python 3.10.x or higher

If the output is Python 2.x, you need to use the python3 command, or check whether the Conda environment you configured in Station 1 is activated correctly.

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Hands-on Practice

Exercise 1: An Upgraded Hello World

Create a file called about_me.py and make it print your personal introduction:

print("=== Personal Introduction ===")
print("Name: [Your Name]")
print("Goal: Become an AI Engineer")
print("Currently learning: Python programming")
print("=" * 20)

Run it and see the output. Try changing the content and add more information.

Exercise 2: Use Python as a Calculator

In Python interactive mode, try the following operations:

>>> 100 + 200
>>> 10 * 3.14
>>> 2 ** 10        # ** is exponentiation, 2 to the 10th power
>>> 17 / 5         # division
>>> 17 // 5        # floor division (drop the decimal part)
>>> 17 % 5         # remainder

Write down the result of each operation and think about why.

Exercise 3: Explore print()

Try the following code and observe different uses of print():

print("Hello")
print("Hello", "World")           # multiple arguments separated by commas
print("Hello", "World", sep="-")  # connect with -
print("Hello", end=" ")           # no newline
print("World")
print("Price:", 99.9, "yuan")

Reference implementation and walkthrough

1. about_me.py should run from the terminal and print a clear multi-line introduction. Changing the content should not require changing Python syntax.
2. The calculator outputs should include 300, 31.400000000000002 or a close floating-point value, 1024, 3.4, 3, and 2.
3. print("Hello", "World") inserts a space. sep="-" changes the separator, and end=" " keeps the next print() on the same line.
4. If the script does not run, check the filename, current folder, Python 3 interpreter, and matching quotes or parentheses.
5. Keep the command and output as proof, not only the source code.

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Evidence to Keep

Keep this page’s proof of learning as a small evidence card:

Concept

variable, type, operator, input/output, branch, loop, structure, function, or module

Code

smallest runnable Python snippet for the concept

Output

printed value, type, branch result, loop trace, or returned value

Failure Check

type mismatch, indentation, off-by-one, mutable data, or import path issue

Expected Output

code plus printed result that proves the concept works

Summary

Key Point	Description
Python is the preferred language for AI development	Almost all AI frameworks are built on Python
Syntax is simple and close to natural language	Lowers the learning barrier and lets you focus on logic
Rich ecosystem	400,000+ third-party libraries, with ready-made solutions for most needs
Three ways to run code	Interactive mode, .py files, and Jupyter Notebook
Indentation is the soul of Python	Use 4-space indentation, not Tab

Learning Advice

Programming is a craft. You cannot learn it just by watching. For every exercise in this course, type the code yourself—do not copy and paste. Type it character by character. In the process of typing, you will make mistakes, debug, and understand more deeply.