Python Learning on Telegram: “🐛 This Python Bug Looks Random, But It Isn't Lo…”

🐛 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.