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Function Nesting Use Cases (mostly functional idioms, almost certainly incomplete since it’s off the top of my head):

• closures
• function factory (programmatic function creation based on parameters)
• creating functions by calling functool.partial
• creating functions by using lambda
• any other reasons you need to create functions during call time

Trade-offs:

• functions are strongly coupled
• the code is always called (unless it’s in an if block)
• additional code complexity
• additional runtime cost (potentially, because the inner function get’s re-defined with every call to the outer function)
• much harder to extend
• much harder to introspect on the inner function defintion

Nguồn:: Stack Overflow, When to use python function nesting?