String formatting using f-strings in Python - Step-by-Step Execution

Concept Flow - String formatting using f-strings

Start

↓

Define variables

↓

Write f-string with placeholders

↓

Python replaces placeholders with variable values

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Print or use formatted string

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End

This flow shows how Python takes variables and inserts their values into a string using f-strings, then outputs the final formatted string.

Execution Sample

Python

name = "Anna"
age = 30
message = f"Hello, {name}. You are {age} years old."
print(message)

This code creates a greeting message by inserting variables into a string using an f-string.

Execution Table

Step	Action	Expression Evaluated	Result	Output
1	Assign name	name = "Anna"	name = "Anna"
2	Assign age	age = 30	age = 30
3	Create f-string	f"Hello, {name}. You are {age} years old."	"Hello, Anna. You are 30 years old."
4	Print message	print(message)		Hello, Anna. You are 30 years old.

💡

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
name	undefined	"Anna"	"Anna"	"Anna"	"Anna"
age	undefined	undefined	30	30	30
message	undefined	undefined	undefined	"Hello, Anna. You are 30 years old."	"Hello, Anna. You are 30 years old."

Key Moments - 3 Insights

Why do we put variables inside curly braces {} in the f-string?

Curly braces tell Python where to insert the value of the variable inside the string, as shown in step 3 of the execution table.

What happens if we forget the 'f' before the string?

Without the 'f', Python treats the string as normal text and does not replace variables, so placeholders like {name} stay as is, not shown in this example but important to remember.

Can we use expressions inside the curly braces?

Yes, you can put expressions like {age + 5} inside the braces, and Python will calculate and insert the result when creating the string.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'message' after step 3?

A "Hello, Anna. You are 30 years old."

B "Hello, {name}. You are {age} years old."

C undefined

D "Hello, Anna. You are {age} years old."

💡 Hint

Check the 'Result' column in row for step 3 in the execution_table.

At which step is the final formatted string printed?

A Step 1

B Step 4

C Step 2

D Step 3

💡 Hint

Look at the 'Output' column in the execution_table to find when the string appears.

If we change age to 35, how does the message change in the variable_tracker?

A The message stays the same.

B The message becomes undefined.

C The message updates to "Hello, Anna. You are 35 years old."

D The message shows "Hello, Anna. You are {age} years old."

💡 Hint

Refer to the 'message' row in variable_tracker and imagine age changing before step 3.

Concept Snapshot

f-strings format strings by placing variables inside curly braces {}.
Start the string with f"..." to enable this.
Python replaces placeholders with variable values.
You can include expressions inside braces.
This is a clean, readable way to build strings.

Full Transcript

This lesson shows how Python uses f-strings to insert variable values into strings. We start by defining variables 'name' and 'age'. Then we write an f-string with placeholders {name} and {age}. Python replaces these placeholders with the actual values. Finally, the formatted string is printed. The execution table traces each step, showing how variables change and when the string is created and printed. Key points include the use of curly braces to mark variables, the need for the 'f' prefix, and the ability to use expressions inside the braces. The visual quiz tests understanding of these steps and variable changes.