What the
for
statement really
does
In the section called “Iterative Processing: The
for
Statement”, we defined a for
statement using the following summary:
for
variable
in
sequence
:
suite
This isn't completely accurate, it turns out. We aren't limited to
a sequence. The
for
statement actually requires an
iterator or generator. Given an object which is not an iterator or
generator, it will call the iter function to get an
iterator over the container.
A more correct syntax summary is the the folliowing:
for
variable
in
expression
:
suite
The Secret of
for
. Once we've looked at generator functions and iterators, we can
see what the
for
statement really does. The purpose
of the
for
statement is to visit each value yielded
by a generator, assigning each value to the
variable
. The
for
statement examines the
expression
to see if
it is a generator function, or an object. If it is an object, it must
respond to the iter function by providing a
generator function. All of the built-in collections provide the
necessary generator function.
Looking forward, we'll see many additional applications of this
feature of the
for
statement. As we look at designing
our own objects in Part III, “Data + Processing = Objects”, we'll want to assure that
our objects work well with the
for
statement,
also.
