Python Append() vs. = on Lists: Why Do They Give Different Results?

Python Append() vs. = Operator on Lists: Understanding the Result Discrepancies

In Python, the append() method and the = operator can be used to modify lists. However, these operations sometimes produce unexpected results, particularly when dealing with nested lists. This article aims to explain why the two approaches behave differently.

The = Operator

The = operator adds the elements of the list on the right-hand side to the list on the left-hand side. The original list is modified, and the updated list is returned. For example:

<code class="python">c = [1, 2, 3]
c += c  # This is equivalent to c.extend(c)
print(c)  # Output: [1, 2, 3, 1, 2, 3]</code>

In this case, the = operator concatenates the elements of c with c, resulting in a list with double the original size.

The append() Method

The append() method inserts the specified object into the list at the end. Unlike the = operator, the append() method does not concatenate the elements of the object but rather adds it as a reference to the original list. For example:

<code class="python">c = [1, 2, 3]
c.append(c)  # This is equivalent to c.insert(len(c), c)
print(c)  # Output: [1, 2, 3, [...]]</code>

In this case, the append() method adds a reference to the list c at the end of c. This results in a circular reference, where c[-1] and c refer to the same object. When printing c, you will get the output [1, 2, 3, [...]], where [...] represents the infinite recursion.

Why the Difference?

The key difference between the = operator and the append() method is that the former acts on the elements of the specified object, while the latter treats the object as a whole. When adding a list using =, Python concatenates the elements, which effectively copies each element into the new list. In contrast, when adding a list using append(), Python simply adds a reference to the original list, which can lead to infinite recursion if the list contains a reference to itself.

Alternatives

If your intention is to concatenate two lists, you can use the extend() method instead of the = operator. The extend() method works similarly to = but does not modify the original list. For example:

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

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