Exceptions Debugging

Dealing with errors and exceptions

This chapter describes techniques to use when things go wrong:

• Exceptions: dealing with errors in your code

• Debugging: understanding or figuring out problems in other people's codes (debugging) The debugging techniques are also useful when you're trying to understand other people's R code, or R code that I've highlighted through out this book that you should be able to tease apart and figure out how it works.

• Getting help: what to do if you can't figure out what the problem is

As with many other parts of R, the approach to dealing with errors and exceptions comes from a LISP-heritage, and is quite different (although some of the terminology is the same) to that of languages like Java.

Interactive analysis vs. programming

There is a tension between interactive analysis and programming. When you a doing an analysis, you want R to do what you mean, and if it guesses wrong, then you'll discover it right away and can fix it. If you're creating a function, then you want to make it as robust as possible so that any problems become apparent right away (see fail fast below).

• Be explicit:

  • Be explicit about missings

  • Use TRUE and FALSE instead of T and F

  • Try to avoid relying on position matching or partial name matching when calling functions

• Avoid functions that can return different types of objects:

  • Always use drop = FALSE

  • Don't use sapply: use vapply, or lapply plus the appropriate transformation

Debugging

The key function for performing a post-mortem on an error is traceback, which shows all the calls leading up to the error. This can help you figure out where the error occurred, but often you'll need the interactive environment created by browser:

• c: leave interactive debugging and continue execution

• n: execute the next step. Be careful if you have a variable named n: to print it you'll need to be explicit print(n).

• return: the default behaviour is the same as c, but this is somewhat dangerous as it makes it very easy to accidentally continue during debugging. I recommend options(browserNLdisabled = TRUE) so that return is simply ignored.

• Q: stops debugging, terminate the function and return to the global workspace

• where: prints stack trace of active calls (the interactive equivalent of traceback)

You can add Don't forget that you can combine if statements with browser() to only debug when a certain situation occurs.

Browsing arbitrary R code

There are two ways to insert browser() statements in arbitrary R code:

• debug inserts a browser statement in the first line of the specified function. undebug will remove it, or you can use debugonce to insert a browser call for the next run, and have it automatically removed afterwards.

• utils::setBreakpoint does the same thing, but instead inserts browser in the function corresponding to the specified file name and line number.

These two functions are both special cases of trace(), which allows you to insert arbitrary code in any position in an existing function. The complement of trace is untrace. You can only perform one trace per function - subsequent traces will replace prior.

Locating warnings is a little trickier. The easiest way to turn it in an error with options(warn = 2) and then use the standard functions described above. Turn back to default behaviour with options(warn = 0).

Browsing on error

It's also possible to start browser automatically when an error occurs, by setting option(error = browser). This will start the interactive debugger in the environment in which the error occurred. Other functions that you can supply to error are:

• recover: a step up from browser, as it allows you to drill down into any of the calls in the call stack. This is useful because often the cause of the error is a number of calls back - you're just seeing the consequences. This is the result of "fail-slow" code

• dump.frames: an equivalent to recover for non-interactive code. Will save an rdata file containing the nested environments where the error occurred. This allows you to later use debugger to re-create the error as if you had called recover from where the error occurred

• NULL: the default. Prints an error message and stops function execution.

We can use withCallingHandlers to set up something similar for warnings. The following function will call the specified action when a warning is generated. The code is slightly tricky because we need to find the right environment to evaluate the action - it should be the function that calls warning.

on_warning <- function(action, code) {
  q_action <- substitute(action)
  
  withCallingHandlers(code, warning = function(c) {
    for(i in seq_len(sys.nframe())) {
      f <- as.character(sys.call(i)[[1]])
      if (f == "warning") break;
    }
    
    eval(q_action, sys.frame(i - 1))
  })
}

x <- 1
f <- function() {
  x <- 2
  g()
}
g <- function() {
  x <- 3
  warning("Leaving g")
}
on_warning(browser(), f())
on_warning(recover(), f())

Creative uses of trace

Trace is a useful debugging function that along with some of our computing on the language tools can be used to set up warnings on a large number of functions at a time. This is useful if you for automatically detecting some of the errors described above. The first step is to find all functions that have a na.rm argument. We'll do this by first building a list of all functions in base and stats, then inspecting their formals.

objs <- c(ls("package:base", "package:stats"))
has_missing_arg <- function(name) {
  x <- get(name)
  if (!is.function(x)) return(FALSE)

  args <- names(formals(x))
  "na.rm" %in% args
}
f_miss <- Filter(has_missing_arg, objs)

Next, we write a version of trace vectorised over the function name, and then use that function to add a warning to every function that we found above.

trace_all <- function(fs, tracer, ...) {
  lapply(fs, trace, tracer = tracer, print = FALSE, ...)
  invisible(return())
}

trace_all(f_miss, quote(if(missing(na.rm)) stop("na.rm not set")))
# But misses primitives

pmin(1:10, 1:10)
# Error in eval(expr, envir, enclos) : na.rm not set
pmin(1:10, 1:10, na.rm = T)
# [1]  1  2  3  4  5  6  7  8  9 10

One problem of this approach is that we don't automatically pick up any primitive functions, because for these functions formals returns NULL.

Exceptions

Defensive programming is the art of making code fail in a well-defined manner even when something unexpected occurs. There are two components of this art related to exceptions: raising exceptions as soon as you notice something has gone wrong, and responding to errors as cleanly as possible.

A general principle for errors is to "fail fast" - as soon as you figure out something as wrong, and your inputs are not as expected, you should raise an error.

Creating

There are a number of options for letting the user know when something has gone wrong:

• don't use cat() or print(), except for print methods, or for optional debugging information.

• use message() to inform the user about something expected - I often do this when filling in important missing arguments that have a non-trivial computation or impact. Two examples are reshape2::melt package which informs the user what melt and id variables where used if not specific, and plyr::join which informs which variables where used to join the two tables.

• use warning() for unexpected problems that aren't show stoppers. options(warning = 2) will turn errors into warnings. Warnings are often more appropriate for vectorised functions when a single value in the vector is incorrect, e.g. log(-1:2) and sqrt(-1:2).

• use stop() when the problem is so big you can't continue

• stopifnot() is a quick and dirty way of checking that pre-conditions for your function are met. The problem with stopifnot is that if they aren't met, it will display the test code as an error, not a more informative message. Checking pre-conditions with stopifnot is better than nothing, but it's better still to check the condition yourself and return an informative message with stop()

Handling

Error handling is performed with the try and tryCatch functions. try is a simpler version, so we'll start with that first. The try functions allows execution to continue even if an exception occurs, and is particularly useful when operating on elements in a loop. The silent argument controls whether or not the error is still printed.

elements <- list(1:10, c(-1, 10), c(T, F), letters)
results <- lapply(elements, log)
results <- lapply(elements, function(x) try(log(x)))

If code fails, try invisibly returns an object of class try-error. There isn't a built in function for testing for this class, so we'll define one. Then we can easily strip all errors out of a list with Filter:

is.error <- function(x) inherits(x, "try-error")
successful <- Filter(Negate(is.error), results)

Of if we want to know where the successes and failures were, when can use sapply or vapply:

vapply(results, is.error, logical(1))
sapply(results, is.error)

# (Aside: vapply is preferred when you're writing a function because
# it guarantees you'll always get a logical vector as a result, even if
# you list has length zero)
sapply(NULL, is.error)
vapply(NULL, is.error, logical(1))    

If we wanted to avoid the anonymous function call, we could create our own function to automatically wrap a call in a try:

try_to <- function(f, silent = FALSE) {
  function(...) try(f(...), silent = silent)
}
results <- lapply(elements, try_to(log))

tryCatch gives more control than try, but to understand how it works, we first need to learn a little about conditions, the S3 objects that represent errors, warnings and messages.

is.condition <- function(x) inherits(x, "condition")

There are three convenience methods for creating errors, warnings and messages. All take two arguments: the message to display, and an optional call indicating where the condition was created

e <- simpleError("My error", quote(f(x = 71)))
w <- simpleWarning("My warning")
m <- simpleMessage("My message")

There is one class of conditions that can't be directly: interrupts, which occur when the user presses Ctrl + Break, Escape, or Ctrl + C (depending on the platform) to terminate execution.

The components of a condition can be extracted with conditionMessage and conditionCall:

conditionMessage(e)
conditionCall(e)

Conditions can be signalled using signalCondition. By default, no one is listening, so this doesn't do anything.

signalCondition(e)
signalCondition(w)
signalCondition(m)

To listen to signals, we have two tools: tryCatch and withCallingHandlers.
tryCatch is an exiting handler: it catches the condition, but the rest of the code after the exception is not run. withCallingHandlers sets up calling handlers: it catches the condition, and then resumes execution of the code. We will focus first on tryCatch.

The tryCatch call has three arguments:

• expr: the code to run.

• ...: a set of named arguments setting up error handlers. If an error occurs, tryCatch will call the first handler whose name matches one of the classes of the condition. The only useful names for built-in conditions are interrupt, error, warning and message.

• finally: code to run regardless of whether expr succeeds or fails. This is useful for clean up, as described below. All handlers have been turned off by the time the finally code is run, so errors will propagate as usual.

The following examples illustrate the basic properties of tryCatch:

# Handlers are passed a single argument
tryCatch(stop("error"), 
  error = function(...) list(...)
)
# This argument is the signalled condition, so we'll call
# it c for short.

# If multiple handlers match, the first is used
tryCatch(stop("error"), 
  error = function(c) "a",
  error = function(c) "b"
)

# If multiple signals are nested, the the most internal is used first.
tryCatch(
  tryCatch(stop("error"), error = function(c) "a"),
  error = function(c) "b"
)

# Uncaught signals propagate outwards. 
tryCatch(
  tryCatch(stop("error")),
  error = function(c) "b"
)

# The first handler that matches a class of the condition is used, 
# not the "best" match:
a <- structure(list(message = "my error", call = quote(a)), 
  class = c("a", "error", "condition"))

tryCatch(stop(a), 
  error = function(c) "error",
  a = function(c) "a"
)
tryCatch(stop(a), 
  a = function(c) "a",
  error = function(c) "error"
)

# No matter what happens, finally is run:
tryCatch(stop("error"), 
  finally = print("Done."))
tryCatch(a <- 1, 
  finally = print("Done."))
  
# Any errors that occur in the finally block are handled normally
a <- 1
tryCatch(a <- 2, 
  finally = stop("Error!"))

What can handler functions do?

• Return a value.

• Pass the condition along, by re-signalling the error with stop(c), or signalCondition(c) for non-error conditions.

• Kill the function completely and return to the top-level with invokeRestart("abort")

A common use for try and tryCatch is to set a default value if a condition fails. The easiest way to do this is to assign within the try:

success <- FALSE
try({
  # Do something that might fail
  success <- TRUE
})

We can write a simple version of try using tryCatch. The real version of try is considerably more complicated to preserve the usual error behaviour.

try <- function(code, silent = FALSE) {
  tryCatch(code, error = function(c) {
    if (!silent) message("Error:", conditionMessage(c))
    invisible(structure(conditionMessage(c), class = "try-error"))
  })
} 
try(1)
try(stop("Hi"))
try(stop("Hi"), silent = TRUE)

rm(try)

withCallingHandlers({
  a <- 1
  stop("Error")
  a <- 2
}, error = function(c) {})

Using tryCatch

With the basics in place, we'll next develop some useful tools based the ideas we just learned about.

  browse_on_error <- function(f) {
    function(...) {
      tryCatch(f(...), error = function(c) browser())
    }
  }

The finally argument to tryCatch is very useful for clean up, because it is always called, regardless of whether the code executed successfully or not. This is useful when you have:

• modified options, par or locale
• opened connections, or created temporary files and directories
• opened graphics devices
• changed the working directory
• modified environment variables

The following function changes the working directory, executes some code, and always resets the working directory back to what it was before, even if the code raises an error.

in_dir <- function(path, code) {
  cur_dir <- getwd()
  tryCatch({
    setwd(path)
    force(code)
  }, finally = setwd(cur_dir))
}

getwd()
in_dir(R.home(), dir())
getwd()
in_dir(R.home(), stop("Error!"))
getwd()

Another more casual way of cleaning up is the on.exit function, which is called when the function terminates. It's not as fine grained as tryCatch, but it's a bit less typing.

Getting help

Currently, there are two main venues to get help when you are stuck and can't figure out what's causing the problem: stackoverflow and the R-help mailing list..

While you can get some excellent advice on R-help, fools are not tolerated gladly, so follow the tips below for getting the best help:

• Make sure you have the latest version of R, and the package (or packages) you are having problems with. It may be that your problem is the result of a bug that has been fixed recently.

• If it's not clear where the problem is, include the results of sessionInfo() so others can see your R setup.

• Spend some time creating a reproducible example. Make sure you have run your script in a vanilla R sessions (R --vanilla). This is often a useful example by itself, because in the course of making the problem reproducible you figure out what's causing the problem.

Exercises

1. Write a function that walks the code tree to find all functions that are missing an explicit drop argument that need them.

2. Write a function that takes code as an argument and runs that code with options(warn = 2) and returns options back to their previous values on exit (either from an exception or a normal exit).

3. Write a function that opens a graphics device, runs the supplied code, and closes the graphics device (always, regardless of whether or not the plotting code worked).