Step 1: Understand the idiom.
Pie in the sky refers to something that sounds pleasant but is unlikely to be achieved - a fanciful, unrealistic idea or promise.
Step 2: Analyse the context.
The sentence says the politician's promise turned out to be a pie in the sky, implying it was never realised.
Step 3: Confirm the meaning.
An unrealistic hope perfectly matches the idiom. Partially implemented, practical, and hidden agenda do not fit.
\[ \boxed{\text{an unrealistic hope}} \]