What is the most idiomatic alternative to Python's try…catch…else in Dart?

Question

Coming from Python, I really miss the else clause in the try-except chains in Dart.

What's the most idiomatic to simulate the else clause in Dart?

---

Here is an example that benefits from an else block.

This:

var didFail = false;
try {
    startDownload()
} catch (e) {
    didFail = true;
    downloadFailed()
}   
if (!didFail) {
    downloadSuccess()
}    
afterDownload()

vs:

try {
    startDownload()
} catch (e) {
    downloadFailed()
} else {
    downloadSuccess()    
}
afterDownload()

Answer 1

Full disclosure: my entire experience with Dart is the 2 minutes I just spent reviewing the syntax of its try statement. This is based solely on a observation about Python's semantics.

---

What does else do in Python?

Skip to the end of the answer for the suggested Dart code.

The following two pieces of code are very similar in Python:

try:
    ...
    <last code>
except SomeError:
    ...
finally:
    ...

and

try:
    ...
except SomeError:
    ...
else:
    <last code>
finally:
    ...

<last code> will be executed in the same circumstances in both. The difference is that any exceptions raised by <last statement> will be caught in the first, but not in the second.

Simulating else in Python

To simulate else's semantics in Python, you would use an additional try statement and a flag to indicate if an exception should be rethrown.

else_exception = False
try:
    ...
    try:
        <last code>
    except Exception as e:
        else_exception = True
except SomeError:
    ...
finally:
    if else_exception:
        raise e
    ...

We check if the nested try caught an exception in the finally clause, since the else clause would have executed before anything else in finally. If there was an exception, reraise it now that it won't be immediately caught, just as in the else. Then you can proceed with the rest of finally.

---

Simulating else in Dart

As far as I can tell, the same logic would be necessary in Dart.

bool else_exception = false;
try {
  ...
  try {
     <last code>
  } catch (e) {
    else_exception = true;
  }
} on SomeError catch (e) {
  ...
} finally {
  if (else_exception) {
    throw e;
  }
  ...
}

Note that if <last code> throws an exception, the above code will not properly preserve the stacktrace. To do that, some more care would be needed:

bool else_exception = false;
try {
  ...
  try {
     <last code>
  } catch (e) {
    else_exception = true;
    rethrow;
  }
} on SomeError catch (e) {
  if (else_exception) {
     rethrow;
  }
  ...
}

Answer 2

In most cases, you should just be able to write whatever goes into the else block directly at the very end of the try block. There may be some cases where the else block is useful, and can provide somewhat clearer or more expressive code, but you can, e.g., write your code much more compact than what you did in your "no else" example as

try {
    start_download() // exception?
    // yay, no exception
    download_success()
} catch (e) { // preferrably "on KindOfException catch (e)"
    download_failed()        
}
afterDownload()

It may not be as explicit that download_success only executes if there was no exception, but implicitly it is clear, because if there were an exception, the try block would have been aborted and the execution would have gone into the catch block.

Of course, this also means that exceptions raised in download_success() will also go to the catch block. This may be prevented by using a more specific exception, e.g. on VeryBadDownloadException catch (e), assuming that start_download and download_success will not raise exactly the same kinds of exceptions.