The principle of optimism in the face of uncertainty is one of the most widely used and successful ideas in multi-armed bandits and reinforcement learning. However, existing optimistic algorithms (primarily UCB and its variants) are often unable to deal with large context spaces. Essentially all existing well performing algorithms for general contextual bandit problems rely on weighted action allocation schemes; and theoretical guarantees for optimism-based algorithms are only known for restricted formulations. In this paper we study general contextual bandits under the realizability condition, and propose a simple generic principle to design optimistic algorithms, dubbed Upper Counterfactual Confidence Bounds (UCCB). We show that these algorithms are provably optimal and efficient in the presence of large context spaces. Key components of UCCB include: 1) a systematic analysis of confidence bounds in policy space rather than in action space; and 2) the potential function perspective that is used to express the power of optimism in the contextual setting. We further show how the UCCB principle can be extended to infinite action spaces, by constructing confidence bounds via the newly introduced notion of counterfactual action divergence.
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