PCEP Computer Programming and Python Fundamentals Practice Question
What is the output of the following code? x = [1, 2, 3]; y = x; y.append(4); print(x)
⚠ Common exam trap
The PCEP exam often tests the distinction between reference assignment and copying for mutable objects, trapping candidates who assume `y = x` creates an independent copy of the list.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
[1,2,3,4]
In Python, variables hold references to objects, not copies. When `y = x` is executed, both `x` and `y` point to the same list object in memory. The `y.append(4)` method modifies that shared list in-place, so printing `x` reflects the change and outputs `[1, 2, 3, 4]`.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
[1,2,3,4]
Why this is correct
Assignment binds y to the same list object as x, not a copy. Calling append(4) mutates that shared object in place, so both names observe the change and printing x outputs [1, 2, 3, 4].
- ✗
Error
Why it's wrong here
Lists are mutable and y = x binds the same object, so append mutates the list both names reference; print(x) outputs [1, 2, 3, 4] without error. The option is tempting because appending to a list while iterating or indexing a non-sequence does raise exceptions, but neither occurs here.
- ✗
[4]
Why it's wrong here
Appending 4 adds one element to the existing list rather than replacing its contents, so x remains [1, 2, 3, 4]. A single-element [4] result would require rebinding x to a new list, for example x = [4], not mutation through an alias.
- ✗
[1,2,3]
Why it's wrong here
Because y = x binds the same list object, y.append(4) mutates that shared object, so x becomes [1, 2, 3, 4]. The [1,2,3] output would only occur if y were a copy, such as y = x.copy() or y = x[:].
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