In Python, I would do this:
try:
some_func()
except Exception:
handle_error()
else:
print("some_func was successful")
do_something_else() # exceptions not handled here, deliberately
finally:
print("this will be printed in any case")
I find this very elegant to read; the else block will only be reached if no exception was thrown.
How does one do this in Kotlin? Am I supposed to declare a local variable and check that below the block?
try {
some_func()
// do_something_else() cannot be put here, because I don't want exceptions
// to be handled the same as for the statement above.
} catch (e: Exception) {
handle_error()
} finally {
// reached in any case
}
// how to handle 'else' elegantly?
I found Kotlin docs | Migrating from Python | Exceptions, but this does not cover the else block functionality as found in Python.
Another way to use runCatching is to use the Result's extension functions
runCatching {
someFunc()
}.onFailure { error ->
handleError(error)
}.onSuccess { someFuncReturnValue ->
handleSuccess(someFuncReturnValue)
}.getOrDefault(defaultValue)
.also { finalValue ->
doFinalStuff(finalValue)
}
Take a look at the docs for Result: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-result/index.html
If you do not care about the default value, for example, you want just to hide the loading you could use this:
runCatching {
show_loading(true) //show loading indicator
some_func() //this could throw an exception
}.onFailure {
handle_error(it.message)
}.getOrNull().run {
show_loading(false) //hide loading indicator regardless error or success
}
Related
I have following code
IAsyncOperation<bool> trythiswork()
{
bool contentFound{ false };
try
{
auto result = co_await someAsyncFunc();
winrt::check_bool(result)
if (result)
{
contentFound = true;
}
}
catch (...)
{
LOG_CAUGHT_EXCEPTION();
}
co_return contentFound;
}
When the result is false, it fails and throws but catch goes to fail fast and program terminates. How does log function terminate the program? Isn't it supposed to only log the exception? I assumed that I am handling this exception so program won't crash but it is crashing.
So how to throw and catch so that program does not terminate? I do want to throw. And also catch and preferably log the exception as well.
Thanks
The issue can be reproduced using the following code:
IAsyncOperation<bool> someAsyncFunc() { co_return false; }
IAsyncOperation<bool> trythiswork()
{
auto contentFound { false };
try
{
auto result = co_await someAsyncFunc();
winrt::check_bool(result);
// throw std::bad_alloc {};
contentFound = true;
}
catch (...)
{
LOG_CAUGHT_EXCEPTION();
}
co_return contentFound;
}
int main()
{
init_apartment();
auto result = trythiswork().get();
}
As it turns out, everything works as advertised, even if not as intended. When running the code with a debugger attached you will see the following debug output:
The exception %s (0x [trythiswork]
Not very helpful, but it shows that logging itself works. This is followed up by something like
FailFast(1) tid(b230) 8007023E {Application Error}
causing the process to terminate. The WIL only recognizes exceptions of type std::exception, wil::ResultException, and Platform::Exception^. When it handles an unrecognized exception type it will terminate the process by default. This can be verified by commenting out the call to check_bool and instead throwing a standard exception (such as std::bad_alloc). This produces a program that will log exception details, but continue to execute.
The behavior can be customized by registering a callback for custom exception types, giving clients control over translating between custom exception types and HRESULT values. This is useful in cases where WIL needs to interoperate with external library code that uses its own exception types.
For C++/WinRT exception types (based on hresult_error) the WIL already provides error handling helpers that can be enabled (see Integrating with C++/WinRT). To opt into this all you need to do is to #include <wil/cppwinrt.h> before any C++/WinRT headers. When using precompiled headers that's where the #include directive should go.
With that change, the program now works as desired: It logs exception information for exceptions that originate from C++/WinRT, and continues to execute after the exception has been handled.
After I installed Xcode 7 beta and convert my swift code to Swift 2, I got some issue with the code that I can't figure out. I know Swift 2 is new so I search and figure out since there is nothing about it, I should write a question.
Here is the error:
Call can throw, but it is not marked with 'try' and the error is not
handled
Code:
func deleteAccountDetail(){
let entityDescription = NSEntityDescription.entityForName("AccountDetail", inManagedObjectContext: Context!)
let request = NSFetchRequest()
request.entity = entityDescription
//The Line Below is where i expect the error
let fetchedEntities = self.Context!.executeFetchRequest(request) as! [AccountDetail]
for entity in fetchedEntities {
self.Context!.deleteObject(entity)
}
do {
try self.Context!.save()
} catch _ {
}
}
Snapshot:
You have to catch the error just as you're already doing for your save() call and since you're handling multiple errors here, you can try multiple calls sequentially in a single do-catch block, like so:
func deleteAccountDetail() {
let entityDescription = NSEntityDescription.entityForName("AccountDetail", inManagedObjectContext: Context!)
let request = NSFetchRequest()
request.entity = entityDescription
do {
let fetchedEntities = try self.Context!.executeFetchRequest(request) as! [AccountDetail]
for entity in fetchedEntities {
self.Context!.deleteObject(entity)
}
try self.Context!.save()
} catch {
print(error)
}
}
Or as #bames53 pointed out in the comments below, it is often better practice not to catch the error where it was thrown. You can mark the method as throws then try to call the method. For example:
func deleteAccountDetail() throws {
let entityDescription = NSEntityDescription.entityForName("AccountDetail", inManagedObjectContext: Context!)
let request = NSFetchRequest()
request.entity = entityDescription
let fetchedEntities = try Context.executeFetchRequest(request) as! [AccountDetail]
for entity in fetchedEntities {
self.Context!.deleteObject(entity)
}
try self.Context!.save()
}
When calling a function that is declared with throws in Swift, you must annotate the function call site with try or try!. For example, given a throwing function:
func willOnlyThrowIfTrue(value: Bool) throws {
if value { throw someError }
}
this function can be called like:
func foo(value: Bool) throws {
try willOnlyThrowIfTrue(value)
}
Here we annotate the call with try, which calls out to the reader that this function may throw an exception, and any following lines of code might not be executed. We also have to annotate this function with throws, because this function could throw an exception (i.e., when willOnlyThrowIfTrue() throws, then foo will automatically rethrow the exception upwards.
If you want to call a function that is declared as possibly throwing, but which you know will not throw in your case because you're giving it correct input, you can use try!.
func bar() {
try! willOnlyThrowIfTrue(false)
}
This way, when you guarantee that code won't throw, you don't have to put in extra boilerplate code to disable exception propagation.
try! is enforced at runtime: if you use try! and the function does end up throwing, then your program's execution will be terminated with a runtime error.
Most exception handling code should look like the above: either you simply propagate exceptions upward when they occur, or you set up conditions such that otherwise possible exceptions are ruled out. Any clean up of other resources in your code should occur via object destruction (i.e. deinit()), or sometimes via defered code.
func baz(value: Bool) throws {
var filePath = NSBundle.mainBundle().pathForResource("theFile", ofType:"txt")
var data = NSData(contentsOfFile:filePath)
try willOnlyThrowIfTrue(value)
// data and filePath automatically cleaned up, even when an exception occurs.
}
If for whatever reason you have clean up code that needs to run but isn't in a deinit() function, you can use defer.
func qux(value: Bool) throws {
defer {
print("this code runs when the function exits, even when it exits by an exception")
}
try willOnlyThrowIfTrue(value)
}
Most code that deals with exceptions simply has them propagate upward to callers, doing cleanup on the way via deinit() or defer. This is because most code doesn't know what to do with errors; it knows what went wrong, but it doesn't have enough information about what some higher level code is trying to do in order to know what to do about the error. It doesn't know if presenting a dialog to the user is appropriate, or if it should retry, or if something else is appropriate.
Higher level code, however, should know exactly what to do in the event of any error. So exceptions allow specific errors to bubble up from where they initially occur to the where they can be handled.
Handling exceptions is done via catch statements.
func quux(value: Bool) {
do {
try willOnlyThrowIfTrue(value)
} catch {
// handle error
}
}
You can have multiple catch statements, each catching a different kind of exception.
do {
try someFunctionThatThowsDifferentExceptions()
} catch MyErrorType.errorA {
// handle errorA
} catch MyErrorType.errorB {
// handle errorB
} catch {
// handle other errors
}
For more details on best practices with exceptions, see http://exceptionsafecode.com/. It's specifically aimed at C++, but after examining the Swift exception model, I believe the basics apply to Swift as well.
For details on the Swift syntax and error handling model, see the book The Swift Programming Language (Swift 2 Prerelease).
I am trying my first features with Behat and I am facing the problem I don't know how to implement expected exceptions.
I found the issue https://github.com/Behat/Behat/issues/140 and robocoder is talking about one possible way, which is used by Behat, too. But it seems that they aren't really handling exceptions.
My point is to achieve forced exception handling. I don't want any construct catching all exceptions and forget them.
One possible way would be:
When <player> transfers <transfer> from his account it should fail with <error>
Implementation
try {
...
} catch (\Exception $ex) {
assertEquals($error, $ex->getMessage());
}
I don't like the scenario description. I want to use the then keyword, e.g.
When <player> transfers <transfer> from his account
Then it should fail with error <error>
This description has the disadvantage I need two methods:
method1($arg1, $arg2) {
// Test the transfer
}
method2($arg1, $arg2) {
// Check if the exception is the right one
}
To be able to check in method2 the exception needs to be stored.
The only possible way I see is to use a try/catch and store it to a variable.
Someone else would catch it and do nothing with it. Nobody will notice, when running the tests.
How can I prevent that exceptions are discarded?
Has anybody else a similar scenario implemented?
Thanks for any hints.
EDIT:
Behat context:
playerTransfer($player, $amount) {
$player->transfer($amount);
}
Method from entity class:
transfer($amount) {
if ($this->getWealth() < $amount) {
throw NotEnoughMoney();
}
...
}
Always try to catch method outcome to context class field, for example:
//inside Behat context class method
try {
$this->outcome = $func();
}
catch(\Exception $ex) {
$this->outcome = $ex;
}
Now when expecting exception at next step just check if $this->outcome is instanceof desired exception with message/code.
I think the problem is in your implementation. Do you check if transfer is successful in "When transfers from his account" ? Do you need to check it ?
Failure test:
When <player> transfers <transfer> from his account
Then I should see error <error>
Successful step:
When <player> transfers <transfer> from his account
Then I should see "transfer successful"
Here's how I successfully did it in a project of mine where I had to repeat a few steps till the condition held true:
/**
* #Given /^I execute some conditions$/
*/
public function executeConditions()
{
$flag = 1;
do {
try {
<steps to be executed till the condition holds true>
$flag=1;
} catch (\Exception $ex) {
$flag = 0;
}
}while ($flag>0);
}
I have an if condition which checks for value and the it throws new NumberFormatException
Is there any other way to code this
if (foo)
{
throw new NumberFormatException
}
// ..
catch (NumberFormatException exc)
{
// some msg...
}
If you are doing something such as this:
try
{
// some stuff
if (foo)
{
throw new NumberFormatException();
}
}
catch (NumberFormatException exc)
{
do something;
}
Then sure, you could avoid the exception completely and do the 'do something' part inside the conditional block.
If your aim is to avoid to throw a new exception:
if(foo)
{
//some msg...
} else
{
//do something else
}
Don't throw exceptions if you can handle them in another, more elegant manner. Exceptions are expensive and should only be used for cases where there is something going on beyond your control (e.g. a database server is not responding).
If you are trying to ensure that a value is set, and formatted correctly, you should try to handle failure of these conditions in a more graceful manner. For example...
if(myObject.value != null && Checkformat(myObject.Value)
{
// good to go
}
else
{
// not a good place to be. Prompt the user rather than raise an exception?
}
In Java, you can try parsing a string with regular expressions before trying to convert it to a number.
If you're trying to catch your own exception (why???) you could do this:
try { if (foo) throw new NumberFormatException(); }
catch(NumberFormatexception) {/* ... */}
if you are trying to replace the throwing of an exception with some other error handling mechanism your only option is to return or set an error code - the problem is that you then have to go and ensure it is checked elsewhere.
the exception is best.
If you know the flow that will cause you to throw a NumberFormatException, code to handle that case. You shouldn't use Exception hierarchies as a program flow mechanism.
I want to return from a configbody but cannot do so explicitly without causing the variable not to be set.
I'd like help understanding the behavior I'm seeing. Please consider the following code (using Itcl 3.4):
package require Itcl
catch {itcl::delete class Model}
itcl::class Model {
public variable filename "orig"
}
itcl::configbody Model::filename {
if 1 {
return ""
} else {
}
}
Model my_model
my_model configure -filename "newbie"
puts "I expect the result to be 'newbie:' [my_model cget -filename]"
When I return empty string, filename is not set to the new value. If I do not return but just allow the proc to fall through, filename does change. You can see this by changing the 1 to a 0 in the above code.
I suspect its related to the following statement:
When there is no return in a script, its value is the value of the last command evaluated in the script.
If someone would explain this behavior and how I should be returning, I'd appreciate the help.
Tcl handles return by throwing an exception (of type TCL_RETURN). Normally, the outer part of a procedure or method handler intercepts that exception and converts it into a normal result of the procedure/method, but you can intercept things with catch and see beneath the covers a bit.
However, configbody does not use that mechanism. It just runs the script in some context (not sure what!) and that context treats TCL_RETURN as an indication to fail the update.
Workaround:
itcl::configbody Model::filename {
catch {
if 1 {
return ""
} else {
}
} msg; set msg
# Yes, that's the single argument form of [set], which READS the variable...
}
Or call a real method in the configbody, passing in any information that's required.