Displaying error locations with options(show.error.locations = TRUE) doesn't seem to work when handling exceptions with tryCatch. I am trying to display location of the error but I don't know how:
options(show.error.locations = TRUE)
tryCatch({
some_function(...)
}, error = function (e, f, g) {
e <<- e
cat("ERROR: ", e$message, "\nin ")
print(e$call)
})
If I then look at the variable e, the location doesn't seem to be there:
> str(e)
List of 2
$ message: chr "missing value where TRUE/FALSE needed"
$ call : language if (index_smooth == "INDEX") { rescale <- 100/meanMSI[plotbaseyear] ...
- attr(*, "class")= chr [1:3] "simpleError" "error" "condition"
If I don't trap the error, it is printed on the console along with source file and line number. How to do it with tryCatch?
Context
As noted by Willem van Doesburg, it is not possible to use the traceback() function to display where the error occured with tryCatch(), and to my knowledge there is currently no practical way to store the position of the error with base functions in R while using tryCatch .
The idea of a separate error handler
The possible solution I found consists of two parts, the main one is writing an error handler similar to that of Chrispy from "printing stack trace and continuing after error occurs in R" which produces a log with the position of the error.
The second part is capturing this output into a variable, similarly to what was suggested by Ben Bolker in "is it possible to redirect console output to a variable".
The call stack in R seems to be purged when an error is raised and then handled (I might be wrong so any information is welcomed), hence we need to capture the error while it is occuring.
Script with an error
I used an example from one of your previous questions regarding where and R error occured with the following function stored in a file called "TestError.R" which I call in my example bellow:
# TestError.R
f2 <- function(x)
{
if (is.null(x)) "x is Null"
if (x==1) "foo"
}
f <- function(x)
{
f2(x)
}
# The following line will raise an error if executed
f(NULL)
Error tracing function
This is the function I adapted form Chrispy's code as I mentionned above.
Upon execution, if an error is raised, the code underneath will print where the error occured, in the case of the above function, it will print :
"Error occuring: Test.R#9: f2(x)" and "Error occuring: Test.R#14: f(NULL)" meaning the error result from a trouble with the f(NULL) function at line 14 which references the f2() function at line 9
# Error tracing function
withErrorTracing = function(expr, silentSuccess=FALSE) {
hasFailed = FALSE
messages = list()
warnings = list()
errorTracer = function(obj) {
# Storing the call stack
calls = sys.calls()
calls = calls[1:length(calls)-1]
# Keeping the calls only
trace = limitedLabels(c(calls, attr(obj, "calls")))
# Printing the 2nd and 3rd traces that contain the line where the error occured
# This is the part you might want to edit to suit your needs
print(paste0("Error occuring: ", trace[length(trace):1][2:3]))
# Muffle any redundant output of the same message
optionalRestart = function(r) { res = findRestart(r); if (!is.null(res)) invokeRestart(res) }
optionalRestart("muffleMessage")
optionalRestart("muffleWarning")
}
vexpr = withCallingHandlers(withVisible(expr), error=errorTracer)
if (silentSuccess && !hasFailed) {
cat(paste(warnings, collapse=""))
}
if (vexpr$visible) vexpr$value else invisible(vexpr$value)
}
Storing the error position and the message
We call the script TestError.R above and capture the printed output in a variable, here called errorStorage with which we can deal later on or simply display.
errorStorage <- capture.output(tryCatch({
withErrorTracing({source("TestError.R")})
}, error = function(e){
e <<- e
cat("ERROR: ", e$message, "\nin ")
print(e$call)
}))
Hence we keep the value of e with the call and message as well as the position of the error location.
The errorStorage output should be as follow:
[1] "[1] \"Error occuring: Test.R#9: f2(x)\" \"Error occuring: Test.R#14: f(NULL)\""
[2] "ERROR: argument is of length zero "
[3] "in if (x == 1) \"foo\""
Hoping this might help.
You can use traceback() in the error handler to show the call stack. Errors in a tryCatch don't produce line numbers. See also the help on traceback. If you use your tryCatch statements defensively, this will help you narrow down the location of the error.
Here is a working example:
## Example of Showing line-number in Try Catch
# set this variable to "error", "warning" or empty ('') to see the different scenarios
case <- "error"
result <- "init value"
tryCatch({
if( case == "error") {
stop( simpleError("Whoops: error") )
}
if( case == "warning") {
stop( simpleWarning("Whoops: warning") )
}
result <- "My result"
},
warning = function (e) {
print(sprintf("caught Warning: %s", e))
traceback(1, max.lines = 1)
},
error = function(e) {
print(sprintf("caught Error: %s", e))
traceback(1, max.lines = 1)
},
finally = {
print(sprintf("And the result is: %s", result))
})
Related
I've read a few other SO questions about tryCatch and cuzzins, as well as the documentation:
Exception handling in R
catching an error and then branching logic
How can I check whether a function call results in a warning?
Problems with Plots in Loop
but I still don't understand.
I'm running a loop and want to skip to next if any of a few kinds of errors occur:
for (i in 1:39487) {
# EXCEPTION HANDLING
this.could.go.wrong <- tryCatch(
attemptsomething(),
error=function(e) next
)
so.could.this <- tryCatch(
doesthisfail(),
error=function(e) next
)
catch.all.errors <- function() { this.could.go.wrong; so.could.this; }
catch.all.errors;
#REAL WORK
useful(i); fun(i); good(i);
} #end for
(by the way, there is no documentation for next that I can find)
When I run this, R honks:
Error in value[[3L]](cond) : no loop for break/next, jumping to top level
What basic point am I missing here? The tryCatch's are clearly within the for loop, so why doesn't R know that?
The key to using tryCatch is realising that it returns an object. If there was an error inside the tryCatch then this object will inherit from class error. You can test for class inheritance with the function inherit.
x <- tryCatch(stop("Error"), error = function(e) e)
class(x)
"simpleError" "error" "condition"
Edit:
What is the meaning of the argument error = function(e) e? This baffled me, and I don't think it's well explained in the documentation. What happens is that this argument catches any error messages that originate in the expression that you are tryCatching. If an error is caught, it gets returned as the value of tryCatch. In the help documentation this is described as a calling handler. The argument e inside error=function(e) is the error message originating in your code.
I come from the old school of procedural programming where using next was a bad thing. So I would rewrite your code something like this. (Note that I removed the next statement inside the tryCatch.):
for (i in 1:39487) {
#ERROR HANDLING
possibleError <- tryCatch(
thing(),
error=function(e) e
)
if(!inherits(possibleError, "error")){
#REAL WORK
useful(i); fun(i); good(i);
}
} #end for
The function next is documented inside ?for`.
If you want to use that instead of having your main working routine inside an if, your code should look something like this:
for (i in 1:39487) {
#ERROR HANDLING
possibleError <- tryCatch(
thing(),
error=function(e) e
)
if(inherits(possibleError, "error")) next
#REAL WORK
useful(i); fun(i); good(i);
} #end for
I found other answers very confusing. Here is an extremely simple implementation for anyone who wants to simply skip to the next loop iteration in the event of an error
for (i in 1:10) {
skip_to_next <- FALSE
# Note that print(b) fails since b doesn't exist
tryCatch(print(b), error = function(e) { skip_to_next <<- TRUE})
if(skip_to_next) { next }
}
for (i in -3:3) {
#ERROR HANDLING
possibleError <- tryCatch({
print(paste("Start Loop ", i ,sep=""))
if(i==0){
stop()
}
}
,
error=function(e) {
e
print(paste("Oops! --> Error in Loop ",i,sep = ""))
}
)
if(inherits(possibleError, "error")) next
print(paste(" End Loop ",i,sep = ""))
}
The only really detailed explanation I have seen can be found here: http://mazamascience.com/WorkingWithData/?p=912
Here is a code clip from that blog post showing how tryCatch works
#!/usr/bin/env Rscript
# tryCatch.r -- experiments with tryCatch
# Get any arguments
arguments <- commandArgs(trailingOnly=TRUE)
a <- arguments[1]
# Define a division function that can issue warnings and errors
myDivide <- function(d, a) {
if (a == 'warning') {
return_value <- 'myDivide warning result'
warning("myDivide warning message")
} else if (a == 'error') {
return_value <- 'myDivide error result'
stop("myDivide error message")
} else {
return_value = d / as.numeric(a)
}
return(return_value)
}
# Evalute the desired series of expressions inside of tryCatch
result <- tryCatch({
b <- 2
c <- b^2
d <- c+2
if (a == 'suppress-warnings') {
e <- suppressWarnings(myDivide(d,a))
} else {
e <- myDivide(d,a) # 6/a
}
f <- e + 100
}, warning = function(war) {
# warning handler picks up where error was generated
print(paste("MY_WARNING: ",war))
b <- "changing 'b' inside the warning handler has no effect"
e <- myDivide(d,0.1) # =60
f <- e + 100
return(f)
}, error = function(err) {
# warning handler picks up where error was generated
print(paste("MY_ERROR: ",err))
b <- "changing 'b' inside the error handler has no effect"
e <- myDivide(d,0.01) # =600
f <- e + 100
return(f)
}, finally = {
print(paste("a =",a))
print(paste("b =",b))
print(paste("c =",c))
print(paste("d =",d))
# NOTE: Finally is evaluated in the context of of the inital
# NOTE: tryCatch block and 'e' will not exist if a warning
# NOTE: or error occurred.
#print(paste("e =",e))
}) # END tryCatch
print(paste("result =",result))
One thing I was missing, which breaking out of for loop when running a function inside a for loop in R makes clear, is this:
next doesn't work inside a function.
You need to send some signal or flag (e.g., Voldemort = TRUE) from inside your function (in my case tryCatch) to the outside.
(this is like modifying a global, public variable inside a local, private function)
Then outside the function, you check to see if the flag was waved (does Voldemort == TRUE). If so you call break or next outside the function.
To handle warnings or errors one can use
result = tryCatch({
expr
}, warning = function(w) {
warning-handler-code
}, error = function(e) {
error-handler-code
}, finally = {
cleanup-code
}
but if expr gives a message through simpleMessage, how can I get it? Is there something like?:
message = function(m) {message-handler-code}
Or another function which allows me to capture the message?
Thank you!
Yes, you can use message = just as you can with warning and error:
result = tryCatch({
message("Hello world")
1 + 1
}, message = function(m) {m}
)
result
#> <simpleMessage in message("Hello world"): Hello world
>
It's more likely however that you would want to capture your result and message separately:
result <- withCallingHandlers({
message("Hello world")
1 + 1
}, message = function(m) {
lastMessage <<- m
invokeRestart("muffleMessage")
})
result
#> [1] 2
if(exists("lastMessage")) message(lastMessage)
#> Hello world
tryCatch is the most commonly useful solution for handling conditions.
However, tryCatch aborts execution and returns the value of the handler, rather than resuming execution of the code. This may not always be what you want; sometimes you want to handle a condition and carry on.
R allows this thanks to its incredibly powerful condition system.
For example, you can choose to silence all messages:
suppressMessages(expr)
The nice thing here is that the suppressMessages isn’t magic — it’s a plain old R function and you could write it yourself (but you do need to understand the condition system, and the price for its versatility is that it’s fairly complicated).
To illustrate this, here’s another way of handling messages — using withCallingHandlers — which suppresses the messages, carries on executing the code it’s called with, but at the same time logs the message in a list:
messages = list()
withCallingHandlers({
message('Hello world')
1 + 1
}, message = function (msg) {
messages <<- c(messages, msg)
tryInvokeRestart('muffleMessage')
})
tryInvokeRestart('muffleMessage') is the same method by which suppressMessages works. The only difference is that we added code to store the message.
As a last step, the above can even be wrapped inside a function:
with_logging = function (expr) {
messages = list()
log_message = function (msg) {
messages <<- c(messages, conditionMessage(msg))
tryInvokeRestart('muffleMessage')
}
result = withCallingHandlers(expr, message = log_message)
list(result = result, messages = messages)
}
And to use it:
with_logging({
message('this is a test')
1 + 1
})
$result
[1] 2
$messages
$messages[[1]]
[1] "this is a test\n"
I'm working on an R package where the same input-checking functions are called by multiple "actual" functions that are exported to users. If I use a simple stop() call, the error message is going to say that an error occurred in the input-checking function, which is not that useful...
I thought I'd get around this by wrapping the call to the input-checking function inside a tryCatch(), and then handling the error in the outer function. This does mostly what I want, but doesn't quite give the output that I'd like. The closest I've come is the following:
f <- function(i) {
tryCatch({
check_input(i)
}, error = function(e) stop("in f: ", e$message, call. = FALSE)
)
}
check_input <- function(i) {
if(i < 0)
stop("i is negative, value given was ", i)
}
f(-1)
# Error: in f: i is negative, value given was -1
Ideally, I'd like the error message to be
Error in f: i is negative, value given was -1
, which would be the case if stop were called within f() instead of check_input().
Here's how you can grab the name of the function from the call stack and paste it in to the error message
f <- function(i) {
check_input(i)
}
g <- function(i) {
check_input(i)
}
check_input <- function(i, from=deparse(sys.calls()[[sys.nframe()-1]][[1]])) {
getmsg <- function(m) ifelse(!is.null(from), paste0("in ", from, ": ", m), m)
if(i < 0)
stop(getmsg(paste0("i is negative, value given was ", i)), call. = FALSE)
}
f(-1)
# Error: in f: i is negative, value given was -1
g(-1)
# Error: in g: i is negative, value given was -1
You could also call check_input(i, from="otherfunction") to show whatever function name you want or check_input(i, from=NULL) to suppress the function name.
I intend to record the errors in my R code while calling functions in a dataframe (ERR_LOG, say). I want to use 'try' to identify errors while calling a function,if any.The dataframe(ERR_LOG) will have the following columns :
Time : The time at which the function was called (Sys.time)
Loc : For which function call was this error recorded (name of the
function)
Desc : Description of the error which R throws at us (Error message
in R)
Example :
First I would like to initialize a blank dataframe 'ERR_LOG' with these columns
Then write the function
f <- function(a){
x <- a*100
return(x)
}
Now I put the output of the call to 'f' in 'chk'
chk <- try(f())
The above call gives the error 'Error in a * 100 : 'a' is missing' (description of the error)
Check
if(inherits(chk,'try-error'))
{then I want to populate ERR_LOG and stop the code execution}
How can this be done in R?
use tryCatch instead of try
Then inside tryCatch(), use the argument error=function(e){}
e will have an element named message, which is what you would like
Use the following call with browser to explore e$message:
x <- tryCatch(stop("This is your error message"), error=function(e) {browser()})
Note that your function need not be anonymous.
MyErrorParser <- function(e) {
m <- e$message
if (grepl("something", m))
do something
return (something_else)
}
## THEN
tryCatch(stop("This is a test"), error=MyErrorParser)
When a call exists of multiple lines, a potential error only includes the first line of match.call() resulting in some lost information and an incomplete sentence. A simple example:
#proper error message:
runif(n=1, k=5)
#incomplete error message:
runif(n=1, k={5})
What would be a way to get R to include the full call to the error message (maybe by collapsing the multiple lines or so)? I am mostly interested in using this in a tryCatch setting.
I had a go at investigating the error object in a tryCatch setting via:
tryCatch( runif(n=1,k={5}),
error = function(e) recover() )
And then selected the 4th environment (value[[3]](cond)) to examine e.
I noticed that e$call was:
Browse[1]> e$call
runif(n = 1, k = {
5
})
So it seems that the error message just uses that first line.
You can collapse all the lines together with:
Browse[1]> paste(deparse(e$call),collapse='')
[1] "runif(n = 1, k = { 5})"
So you could try something like:
tryCatch( runif(n=1,k={5}),
error = function(e) {
cat(sprintf('Error in %s: %s\n',
paste(deparse(e$call),collapse=''),
e$message))
} )
But this doesn't fix up the error message itself, just the call leading up to it:
Error in runif(n = 1, k = { 5}): unused argument(s) (k = {
So the 'Error in xxx' is complete, but the 'unused argument(s) xxx' is still not. It's a start, but not all the way there.
I'm not sure how to improve on this (and am also interested to know if it's possible).