6.8 The yield statement
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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
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