Lazy evaluation, also known as 'call by need ', is an evaluation strategy where values are produced only when needed. Lazy evaluation is the opposite of eager evaluation, Python's normal evaluation model, where functions are executed as seen and values are produced immediately. In this talk we will define lazy evaluation and contrast it with eager evaluation. We will discuss tools that exist in Python for using lazy evaluation and show how we can build on the primitives to better represent computations. We will introduce common vocabulary for discussing evaluation models, and compare different systems for implementing lazy evaluation. Finally, we will discuss optimizations that can be made to optimize lazily evaluated expressions.

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