Suthahar Jegatheesan MSDEVBUILD / blog Multi-Stack Dev Build · by Suthahar
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Programming

In Python, a name is a sticker, not a box

Two names can sit on one object. Whether it can change is what makes lists and strings behave differently.

What you'll learn

  • Why b = a is a second name, not a copy
  • Which Python types change in place and which are replaced
  • How the mutable default argument bug gets written
Python 8 steps 24 seconds —views

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Understand it one step at a time

The short runs these in order in about 24 seconds. Here they are written out — pick any step to jump the short straight to it.

1 Step 1 of 8

You changed b, a changed too

A list assigned to a second name with b = a is not a copy at all. Both variable names simply point to the exact same object in memory.

Frequently asked questions

When you write b = a in Python, does it copy the object?
No — it gives the same object a second name. Both a and b point at the same underlying object, so if that object is mutable, a change made through either name is visible through both.
What is the classic mutable-default-argument bug?
Defining a function with a mutable default, like def f(items=[]), creates that list object once, at function-definition time — not on every call. Every call that doesn't pass its own list shares and mutates the same default list across calls.

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