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Puzzle 27: Explanation

Explore how Python executes augmented assignments such as += by distinguishing between rebinding and in-place mutation. Understand special methods like __iadd__ and why functional programming avoids mutating passed objects. This lesson helps you clarify Python's behavior with lists and ranges and improve your coding approach.

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Try it yourself

Try executing the code below to verify the results:

Python 3.8
def add_n(items, n): 
  items += range(n)
items = [1] 
add_n(items, 3) 
print(items)

In Puzzle 16: Call Me Maybe, we talked about rebinding versus mutation. Most of the time, items += range(n) is translated to items = items + range(n), a process which is called rebinding.

In some cases, there is a special optimization for +=. Here’s a more detailed explanation from the Python ...