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Generator Methods

Generators behave as though they have three methods. In the following summaries, g is the generator (or iterator) object.

g (.next)

The next method resumes execution after the yield statement. This is called automatically by the for statement. The next method does one of two things:

  • It raises StopIteration. An iterator created from a built-in container (sequence, set, dict, file) does this at the end of the sequence. A generator does this when it returns; either because it finished the suite of statements, or it executed an explicit return statement. This exception is handled automatically by the for statement.

  • It changes its internal state and yields the next value. An iterator updates its positions in the given container. A generator, hopefully, is designed properly to update its internal state and make progress toward completion. It is possible to mis-design a generator so that it never terminates.

g (.close)

The close method will force the generator to stop prematurely. This raises a GeneratorExit within the generator. If the generator has acquired resources (like a file, socket, lock or database connection), it should use a try statement. When the close method raises an exception, the event will force the execution of any finally clause in the try statement, allowing the generator to release the resources it acquired.

g , (.throw type , value , traceback )

The throw method will force the generator to handle an exception when it reaches the next yield statement. This allows a client function to send exception events into the generator.

g , (.send arg ) → next value

This is a variation on the next method; it also resumes execution after the yield statement. The argument values will become the return value from the yield statement. The generator can then use these values to change it's internal state and yield another value or raise StopIteration.

  Published under the terms of the Open Publication License Design by Interspire