🔁 Why [1] * 3 gives [1, 1, 1] instead of [1][1][1] in Python?

When I saw the code below for the first time:

[self.constant] * len(X)

I thought it would return something like this:

[1][1][1]

But instead, the actual result was:

[1, 1, 1]

So I wanted to figure out: Why is the output a flat list, and not nested or separate elements like [1][1][1]?

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✅ What’s really happening?

Let’s walk through a real example.

self.constant = 1
X = [10, 20, 30]

Then:

[self.constant] * len(X)
→ [1] * 3
→ [1, 1, 1]

It creates a single list with the value 1 repeated three times.

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❓Why not [1][1][1]?

That would be invalid Python syntax.

• [1][1] means: "Take list [1] and access index 1"
• But [1] only has one element at index 0
• So [1][1] → IndexError

And [1][1][1] would continue that and raise another error.

So the format [1][1][1] is not meaningful or valid in Python.

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🔄 So what does * do with lists?

The * operator repeats the elements inside the list.

Expression	Result	Description
[1] * 3	[1, 1, 1]	Repeats value 1 three times
["hi"] * 2	["hi", "hi"]	Repeats string "hi" twice
[[1]] * 3	[[1], [1], [1]]	Repeats the inner list

✅ Want a list of lists?

Then use a list comprehension like this:

[[self.constant] for _ in range(len(X))]

That gives:

[[1], [1], [1]]

Each [1] is a new list, so they are not shared references (important for avoiding bugs later).

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🧠 Summary

Expression	Output	Meaning
[1] * 3	[1, 1, 1]	One list, repeating values
[[1]] * 3	[[1], [1], [1]]	One list, repeating sublists (but be careful: they point to the same object)
[[1] for _ in range(3)]	[[1], [1], [1]]	Three new lists, each with one element

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Hope this clears things up!
Let me know if you want to dive into shared references or memory issues with list multiplication. That’s a fun next step 🧪