How Can String Concatenation Seem to Mutate Immutable Strings in Python?

Python Strings: Immutable Entities and Assignment Confusion

Python prides itself on immutability in its strings, implying that string values once created cannot be altered. However, the perplexing behavior encountered in an attempted string concatenation raises questions about this immutability.

The Puzzle: Mutating Immutable Strings with Concatenation?

Consider the following code snippet:

<code class="python">a = "Dog"
b = "eats"
c = "treats"

print(a, b, c)  # Dog eats treats

print(a + " " + b + " " + c)  # Dog eats treats

print(a)  # Dog

a = a + " " + b + " " + c
print(a)  # Dog eats treats  # Surprise!</code>

The code initially initializes three strings, then concatenates them with spaces. To our astonishment, the value of a changes after reassignment to the concatenated result.

The Unveiling: Variable Reassignment, Not String Mutation

The key to understanding this behavior lies in recognizing that Python strings are indeed immutable, while their references are mutable. When we reassign a in the last line, we are not modifying the existing string "Dog". Instead, we are creating a new string "Dog eats treats" and assigning its reference to a.

The Immutable Truth Remains

In essence, a string variable stores the address (or reference) to a string value, not the value itself. We can reassign the variable to different addresses, but the string values at those addresses remain unalterable.

Therefore, while it may appear as though we are modifying strings with concatenations, we are merely creating new strings and reassigning the references. The immutability of strings remains intact.

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