John Calvin Commentary Psalms 31:7

"I will be glad and rejoice in thy lovingkindness; For thou hast seen my affliction: Thou hast known my soul in adversities;" — Psalms 31:7 (ASV)

I will be glad and rejoice in your goodness. Here a thanksgiving is inserted, although many are rather of the opinion that David’s prayer is suspended and that he makes a vow, to be fulfilled when he is delivered from present danger. But as no condition is attached, I am more inclined to think that, stopping suddenly in the middle of his prayer, he promises himself a deliverance for which he will have abundant cause for thanksgiving.

Nor is it surprising that different feelings are mingled in the psalms in which David described his own temptations, as well as the resistance his faith offered to them; considering also that when he sang the praises of God, after having already obtained deliverance from Him, he includes different periods in his song. As he says here, that God had regarded his affliction, intimating by this the effect of the assistance God had given him.

And to confirm this better, he adds, that he had not been delivered into the hands of his enemies: words which contain an implied antithesis—namely, that when he was surrounded on every side by severe afflictions, he was wonderfully delivered by God. This is also further intimated by the following sentence: You have set my feet in a large place, which signifies a sudden and unexpected change.