R: how to find what S3 method will be called on an object?

Question

I know about methods(), which returns all methods for a given class. Suppose I have x and I want to know what method will be called when I call

Answer 1

findMethod defined below is not a one-liner but its body has only 4 lines of code (and if we required that the generic be passed as a character string it could be reduced to 3 lines of code). It will return a character string representing the name of the method that would be dispatched by the input generic given that generic and its arguments. (Replace the last line of the body of findMethod with get(X(...)) if you want to return the method itself instead.) Internally it creates a generic X and an X method corresponding to each method of the input generic such that each X method returns the name of the method of the input generic that would be run. The X generic and its methods are all created within the findMethod function so they disappear when findMethod exits. To get the result we just run X with the input argument(s) as the final line of the findMethod function body.

findMethod <- function(generic, ...) {
  ch <- deparse(substitute(generic))
  f <- X <- function(x, ...) UseMethod("X")
  for(m in methods(ch)) assign(sub(ch, "X", m, fixed = TRUE), "body<-"(f, value = m))
  X(...)
}

Now test it. (Note that the one-liner in the question fails with an error in several of these tests but findMethod gives the expected result.)

findMethod(as.ts, iris)
## [1] "as.ts.default"

findMethod(print, iris)
## [1] "print.data.frame"

findMethod(print, Sys.time())
## [1] "print.POSIXct"

findMethod(print, 22)
## [1] "print.default"

# in this example it looks at 2nd component of class vector as no print.ordered exists
class(ordered(3))
## [1] "ordered" "factor" 
findMethod(print, ordered(3))
## [1] "print.factor"

findMethod(`[`, BOD, 1:2, "Time")
## [1] "[.data.frame"