A flux line in a Type-II superconductor with a single nonuniform columnardefect is studied by a perturbative diagrammatic expansion around an annealedapproximation. The system undergoes a finite temperature depinning transitionfor the (rather unphysical) on-the-average repulsive columnar defect, providedthat the fluctuations along the axis are sufficiently large to cause someportions of the column to become attractive. The perturbative expansion isconvergent throughout the weak pinning regime and becomes exact as thedepinning transition is approached, providing an exact determination of thedepinning temperature and the divergence of the localization length.
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