🐛 This Python Bug Looks Random, But It Isn't

Look at this:

functions = []

for i in range(3):
    functions.append(lambda: i)

for f in functions:
    print(f())

What would you expect?

0
1
2

Actual output:

2
2
2

Why? The lambdas don't store the current value of i. They remember the variable i.
By the time the functions are called, the loop has finished and i is 2.

This is called late binding.
If you actually want each function to capture the current value:

functions = []

for i in range(3):
    functions.append(lambda i=i: i)

Now:

0
1
2

This becomes particularly important when creating callbacks inside loops, especially in GUI code, asynchronous code, and event-driven applications.

The bug isn't in lambda. It's in understanding when the variable is looked up.

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Python Programming Notes

Python doesn't store variables the way you probably think it does. 🧠

When you write:

x = 10

Python doesn't really put 10 inside a box called x. 10 is an object, and x is simply a name that refers to that object.
👉 You can see this more clearly:

x = 10
y = x

print(x is y)

True

Both x and y are pointing to the same object. This is also why is and == are different:

a = [1, 2, 3]
b = [1, 2, 3]

print(a == b)   # True
print(a is b)   # False

The values are equal, but they are two different list objects. 📦
And this idea isn't limited to numbers and lists. In Python, almost everything you work with is an object - strings, functions, classes, dictionaries, and even modules.

Start with this mental model:
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Python Programming Roadmap

Stage 1: Python Basics (Syntax, Variables, Data Types) 
Stage 2: Control Flow (if/else, loops) 
Stage 3: Functions & Modules 
Stage 4: Data Structures (Lists, Tuples, Sets, Dicts) 
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