PCEP Practice Question: Functions, Tuples, Dictionaries and Exceptions
What is the output of the following dictionary comprehension?
{x: x**2 for x in range(3)}⚠ Common exam trap
Python Institute often tests whether candidates remember that `range(3)` starts at 0, not 1, and that `0**2` equals 0, not an omitted or undefined value, causing many to drop the first key-value pair or miscalculate the square of 1.
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
✓
{0:0, 1:1, 2:4}
The dictionary comprehension `{x: x**2 for x in range(3)}` iterates over `x` values 0, 1, and 2 (from `range(3)`). For each `x`, it creates a key-value pair where the key is `x` and the value is `x**2` (x squared). This produces `{0: 0**2, 1: 1**2, 2: 2**2}`, which evaluates to `{0:0, 1:1, 2:4}`.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
{0:0, 1:1, 2:4}
Why this is correct
The comprehension iterates x over range(3), giving 0, 1 and 2, and maps each to its square. The resulting dictionary pairs 0 with 0, 1 with 1 and 2 with 4, producing exactly {0:0, 1:1, 2:4}.
- ✗
{0:1, 1:2, 2:3}
Why it's wrong here
The comprehension squares each value, producing {0:0, 1:1, 2:4}; this option lists the keys as values instead. It is tempting because it mirrors the key sequence, and it would be correct if the expression were the identity 'x' rather than 'x**2'.
- ✗
{0:0, 1:2, 2:4}
Why it's wrong here
Squaring 1 yields 1, not 2, so the middle pair is wrong; the correct mapping is {0:0, 1:1, 2:4}. It is tempting because doubling is a common operation, and it would be correct if the expression were 'x*2' rather than 'x**2'.
- ✗
{1:1, 2:4}
Why it's wrong here
range(3) yields 0, 1 and 2, so the key 0 must appear; omitting it drops a valid entry. It is tempting because starting at 1 feels natural, and it would be correct if the iterable were range(1,3).
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