When Does Python\'s `append()` and ` =` Operator Produce Different Results with Lists?

Why Python's append() and = Operators Produce Different Results with Lists

In Python, the append() method and the = operator behave differently when applied to lists. While both may seem to append elements to an existing list, they actually produce distinct outcomes due to their underlying operations.

append()

The append() method directly appends an object, whether an element or another list, to the end of the original list. This process results in a reference to the original list being added to the list.

= Operator

On the other hand, the = operator performs element-wise addition of the two operands. When used with lists, it effectively concatenates the elements of the second operand to the end of the first operand, creating a new list.

Example

Consider the following code:

<code class="python">>>> c = [1, 2, 3]

# Appending a list to itself using append() leads to infinite recursion
c.append(c)

# Element-wise addition using += concatenates the lists
c += c</code>

Output

In the first case, appending the list c to itself using c.append(c) creates an infinite recursion. This is because the last element of c is actually a reference to c itself, and this reference is appended to the list, resulting in an infinite loop.

In the second case, using c = c performs element-wise addition. This means that the elements of c are added to themselves, resulting in a new list [1, 2, 3, 1, 2, 3].

Alternative: extend()

If the desired behavior is to append the elements of one list to another, the extend() method can be used instead of =. It modifies the original list in place without creating a new list.

<code class="python">c.extend([4, 5, 6])</code>

Conclusion

In summary, Python's append() method appends objects directly to the end of a list, while the = operator with lists performs element-wise addition and creates a new list. The choice of method depends on the intended operation and whether modification of the original list is desired.

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