LECTURE 2.2

Hello, Python

Your first lines of code: numbers, variables and text

Introduction to Data Science and Machine Learning · Md Mahbub E Noor · University of Barishal

Today's key line of code

print("Hello, Data Science!")

Where we are: Lecture 2, part 2 of 3

Part 2 of 3L2Python basics

L3Decisions and loops

L4Counting words

L5Real data

L6Build the model

Today you will

1Show results with print()
2Use Python as a calculator
3Store values in variables
4Work with text, the raw material of spam
5Read error messages without panic

Md Mahbub E NoorLecture 2.2: Hello, PythonUniversity of Barishal2 / 16

Why Python?

Reads like English

print("hello") does exactly what it says. Beginners get results on day one.

Free, everywhere

No license to buy. Runs offline in Jupyter, or online in Colab.

Built for data

Tools like pandas and scikit-learn, which we use to build the spam detector, are Python libraries.

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print() shows a result

print() displays whatever is inside the brackets
Text goes inside quotes: "Hello"
Numbers do not need quotes
Each print() starts a new line

Code

print("Hello, Data Science!")
print(2 + 3)
print("2 + 3")

Output

Hello, Data Science!
5
2 + 3

Md Mahbub E NoorLecture 2.2: Hello, PythonUniversity of Barishal4 / 16

Python as a calculator

+ - * / work as you expect
** means power: 7 ** 2 is 7 squared
/ always gives a decimal: 7 / 2 is 3.5
# starts a comment: a note Python ignores

Code

# Python as a calculator
print(7 + 3)
print(7 * 3)
print(7 / 2)
print(7 ** 2)

Output

Md Mahbub E NoorLecture 2.2: Hello, PythonUniversity of Barishal5 / 16

Variables store values

= means "store this", not "is equal to"
The name goes on the left, the value on the right
Use clear names: spam_found, not x
Names use lowercase and underscores, no spaces

Code

messages_today = 12
spam_found = 3
share = spam_found / messages_today
print(share)

Output

0.25

Md Mahbub E NoorLecture 2.2: Hello, PythonUniversity of Barishal6 / 16

Watch it run: variables changing

WATCH FOR

Each variable appears in Memory when it is first stored
x = x + 1 uses the old value
share does not update by itself
The highlighted line is the next one to run

Silent video: press play, pause any time.

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Every value has a type

int: whole numbers like 12
float: decimals like 0.25
str (string): text like "free prize"
bool: True or False, answers to yes/no questions

Code

print(type(12))
print(type(0.25))
print(type("free prize"))
print(type(True))

Output

<class 'int'>
<class 'float'>
<class 'str'>
<class 'bool'>

Md Mahbub E NoorLecture 2.2: Hello, PythonUniversity of Barishal8 / 16

Text is data too

len() counts characters, spaces included
.lower() makes all letters small, so FREE and free match
in asks: does this text appear inside?
This is the first building block of a spam filter

Code

message = "Claim your FREE prize now"
print(len(message))
print(message.lower())
print("free" in message)
print("free" in message.lower())

Output

25
claim your free prize now
False
True

Md Mahbub E NoorLecture 2.2: Hello, PythonUniversity of Barishal9 / 16

f-strings mix text and values

Put f before the quotes
Anything inside { } is calculated and inserted
{share:.0%} shows a number as a percentage
Perfect for clear, readable reports

Code

spam_found = 3
total = 12
print(f"{spam_found} of {total} messages were spam")
share = spam_found / total
print(f"That is {share:.0%} of the inbox")

Output

3 of 12 messages were spam
That is 25% of the inbox

Md Mahbub E NoorLecture 2.2: Hello, PythonUniversity of Barishal10 / 16

Your turn 5 minutes

1Store your first name in a variable called name
2Store any text message in message
3Print a sentence like: Nadia's message has 42 characters
4Print whether the message contains "offer" (ignore capitals)

Code

name = "..."
message = "..."
print(f"...")
print(...)

Starter code: replace the ... with your own code

Md Mahbub E NoorLecture 2.2: Hello, PythonUniversity of Barishal11 / 16

A first look at errors (more in Lecture 2.3)

Read the last line of an error first. It names the problem.

What you typed	What Python says	What it means
total = 12 print(totl)	NameError: name 'totl' is not defined	Typo in a name. Python only knows names you created. Check spelling.
print("hello)	SyntaxError: unterminated string literal (detected at line 1)	A quote or bracket was opened but not closed. Close it.
print("Total: " + 12)	TypeError: can only concatenate str (not "int") to str	Cannot glue text and a number with +. Use an f-string.

Md Mahbub E NoorLecture 2.2: Hello, PythonUniversity of Barishal12 / 16

Interactive lab: Variables Lab

In the lab you can

Memory boxesRun code line by line and watch each box change.
OperatorsChange a and b: every result and type updates live.
Text toolsType a message: indexes, len, in and .lower() update.
f-stringsMove sliders and format numbers with :.2f and :.0%.

Open the lab ►︎

Opens in a new tab and works offline. All labs: labs/START_HERE.html

Md Mahbub E NoorLecture 2.2: Hello, PythonUniversity of Barishal13 / 16

Today's toolkit

`print(x)`	Show a value
`# note`	Comment, ignored by Python
`name = value`	Store a value in a variable
`+ - * / **`	Arithmetic, ** is power
`type(x)`	Which kind of value is this?

`len(text)`	Number of characters
`text.lower()`	All letters to lowercase
`"a" in text`	Does the text contain "a"? True/False
`f"{x} items"`	Insert values into text

Md Mahbub E NoorLecture 2.2: Hello, PythonUniversity of Barishal14 / 16

Homework 2.2

Due before Lecture 2.3 · [date]

Set up

Create a notebook (Jupyter or Colab) named HW2_2_<your ID>. Put your name in a text cell at the top.

About me

Store your name, department and favourite number in variables. Print one f-string sentence using all three.

Predict, then run

For each line, write your prediction as a comment first, then run it: print(10 / 4), print(10 // 4), print("ab" * 3), print(len("data science")).

Message inspector

Store a real SMS you received (remove names and numbers). Print its length, its lowercase version, and whether it contains "free".

Break it on purpose

Create one error deliberately. Keep the error message and explain in one sentence what it means.

Submit on [LMS / Google Classroom]: your notebook file HW2_2_<your ID>.ipynb, or a Colab link set to “Anyone with the link can view”.

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Recap: Lecture 2.2 in 90 seconds

Watch it again at home to revise.

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