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5.5.1 Tail Recursion Exceptions

Although Python is claimed to be “properly” tail-recursive, some might dispute this, since there are situations where tail recursion is inhibited:

• When the call is enclosed by a special binding, or
• When the call is enclosed by a catch or unwind-protect, or
• When the call is enclosed by a block or tagbody and the block name or go tag has been closed over.

These dynamic extent binding forms inhibit tail recursion because they allocate stack space to represent the binding. Shallow-binding implementations of dynamic scoping also require cleanup code to be evaluated when the scope is exited.

In addition, optimization of tail-recursive calls is inhibited when the debug optimization quality is greater than 2 (see debugger-policy.)