List Slicing and Copying

One Statement, Two Results

A list method call can have two effects that must be tracked separately: it can change the list itself, and it can produce a return value. For append, extend, and sort, Python changes the existing list in place and the method call returns None. This combination is the source of a common mistake: the list changes successfully, but assigning the method call back to the list variable replaces the variable's list value with None.

In-Place Change and Return Value

Think of append, extend, and sort as instructions applied to an existing list. The instruction changes that list rather than producing a replacement list for you to store. At the same time, the method call has a result: None. Therefore, after a call such as items.append(value), inspect items to see the changed list, but do not expect the call itself to be a new list.

items = [3, 1] result = items.append(5) print(items) print(result)

Three List Operations

append, extend, and sort all modify a list in place, but they do different jobs. append adds an element. extend adds elements from another iterable. sort changes the order of the list's contents. The difference between append and extend is especially important when combining lists or working with nested data because the argument is treated differently by the two methods.

with_append = [1, 2]
with_append.append([3, 4])

with_extend = [1, 2]
with_extend.extend([3, 4])

print(with_append)
print(with_extend)

[1, 2, [3, 4]]
[1, 2, 3, 4]

Tracing a Sort Call

scores = [5, 3, 1] returned = scores.sort() print(scores) print(returned)

The Assignment Trap

• Assigning append back to the list variable.

  append changes items in place but returns None. The assignment therefore replaces the list stored in items with None.

  Fix: Call items.append(3) without assigning its result.

• Assigning extend back to the list variable.

  extend changes the existing list and returns None.

  Fix: Use items.extend([3, 4]) by itself, then use items to inspect the changed list.

• Assigning sort back to the list variable.

  sort reorders the existing list and returns None, so the assignment discards the list value held by scores.

  Fix: Use scores.sort() without assignment.

• Treating append and extend as identical operations.

  append adds an element, while extend adds elements from another iterable. Their arguments are treated differently.

  Fix: Choose append when adding one element and extend when adding elements from another iterable.

Practice the Trace

Practical Boundary

The source material for this lesson establishes the behavior of append, extend, and sort: each operates in place and returns None. It does not specify slice syntax or separate copying behavior, so those details should not be inferred from these three methods. When analyzing a list statement, identify the exact method being called first, then trace its in-place change and its return value.

1. The central distinction is between changing a list and returning a value. append adds one element, extend adds elements from another iterable, and sort changes the list's order. All three modify the existing list in place and return None. Use these methods without assigning their results back to the list variable. When debugging, trace the list state and the method return value as two separate results.