py_iterator | R Documentation |
Create a Python iterator from an R function
py_iterator(fn, completed = NULL, prefetch = 0L)
fn |
R function with no arguments. |
completed |
Special sentinel return value which indicates that
iteration is complete (defaults to |
prefetch |
Number items to prefetch. Set this to a positive integer to avoid a deadlock in situations where the generator values are consumed by python background threads while the main thread is blocked. |
Python generators are functions that implement the Python iterator
protocol. In Python, values are returned using the yield
keyword. In R,
values are simply returned from the function.
In Python, the yield
keyword enables successive iterations to use the state
of previous iterations. In R, this can be done by returning a function that
mutates its enclosing environment via the <<-
operator. For example:
sequence_generator <- function(start) { value <- start function() { value <<- value + 1 value } }
Then create an iterator using py_iterator()
:
g <- py_iterator(sequence_generator(10))
Python iterator which calls the R function for each iteration.
In Python, returning from a function without calling yield
indicates the
end of the iteration. In R however, return
is used to yield values, so
the end of iteration is indicated by a special return value (NULL
by
default, however this can be changed using the completed
parameter). For
example:
sequence_generator <-function(start) { value <- start function() { value <<- value + 1 if (value < 100) value else NULL } }
Some Python APIs use generators to parallellize operations by calling the
generator on a background thread and then consuming its results on
the foreground thread. The py_iterator()
function creates threadsafe
iterators by ensuring that the R function is always called on the main
thread (to be compatible with R's single-threaded runtime) even if the
generator is run on a background thread.
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