Why does Python\'s ` =` operator and the `append()` method produce different results when working with lists?

Python's append() vs. the = Operator for Lists: Why the Contrasting Results?

The apparent disparity between the append() method and the = operator when used with lists can be perplexing. To clarify this difference, let's delve into the underlying mechanisms of these operations.

The = Operator

When the = operator is applied to a list, it effectively appends the list on the right-hand side to the original list. However, this operation does not create a new list object; instead, it modifies the existing list in place.

The append() Method

In contrast, the append() method explicitly inserts the object provided as the argument to the end of the original list. This insertion results in a reference to the same list instance being stored in the last element of the original list. Consequently, recursively calling append() on the list leads to an infinite loop.

A Clear Illustration

Consider the following code snippet:

<code class="python">c = [1, 2, 3]
c += c
print(c)  # Output: [1, 2, 3, 1, 2, 3]</code>

In this example, the = operator appends the elements of c (i.e., [1, 2, 3]) to c, effectively doubling its length.

Now, let's modify the code slightly:

<code class="python">c = [1, 2, 3]
c.append(c)
print(c)  # Output: [1, 2, 3, [...]]</code>

Here, the append() method inserts the entire list c as the last element. This recursion results in an infinite loop as c[-1] becomes identical to c itself.

An Alternative Approach

If your intention is to concatenate lists, you can employ the extend() method instead of the = operator. The extend() method, like the , appends the elements of another list (or iterable) to the original list, thereby modifying it in place.

Conclusion

The key distinction between append() and the = operator lies in their respective behaviors. The = operator concatenates the elements of the right-hand list to the original list, while append() inserts the list itself as the last element, leading to a recursive loop.

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