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6.8 The `yield` statement

  `yield_stmt` | ::= | `"yield" expression_list`

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The `yield` statement is only used when defining a generator function, and is only used in the body of the generator function. Using a `yield` statement in a function definition is sufficient to cause that definition to create a generator function instead of a normal function.

When a generator function is called, it returns an iterator known as a generator iterator, or more commonly, a generator. The body of the generator function is executed by calling the generator's `next()` method repeatedly until it raises an exception.

When a `yield` statement is executed, the state of the generator is frozen and the value of expression_list is returned to `next()`'s caller. By ``frozen'' we mean that all local state is retained, including the current bindings of local variables, the instruction pointer, and the internal evaluation stack: enough information is saved so that the next time `next()` is invoked, the function can proceed exactly as if the `yield` statement were just another external call.

The `yield` statement is not allowed in the `try` clause of a `try` ... `finally` construct. The difficulty is that there's no guarantee the generator will ever be resumed, hence no guarantee that the `finally` block will ever get executed.

Note: In Python 2.2, the `yield` statement is only allowed when the `generators` feature has been enabled. It will always be enabled in Python 2.3. This `__future__` import statment can be used to enable the feature:

      from __future__ import generators

See Also:

PEP 0255, Simple Generators
  The proposal for adding generators and the `yield` statement to Python.