Better to 'Try' Something and Catch the Exception or Test If It's Possible First to Avoid an Exception

Better to 'try' something and catch the exception or test if it's possible first to avoid an exception?

You should prefer try/except over if/else if that results in

speed-ups (for example by preventing extra lookups)
cleaner code (fewer lines/easier to read)

Often, these go hand-in-hand.

speed-ups

In the case of trying to find an element in a long list by:

try:
    x = my_list[index]
except IndexError:
    x = 'NO_ABC'

the try, except is the best option when the index is probably in the list and the IndexError is usually not raised. This way you avoid the need for an extra lookup by if index < len(my_list).

Python encourages the use of exceptions, which you handle is a phrase from Dive Into Python. Your example not only handles the exception (gracefully), rather than letting it silently pass, also the exception occurs only in the exceptional case of index not being found (hence the word exception!).

cleaner code

The official Python Documentation mentions EAFP: Easier to ask for forgiveness than permission and Rob Knight notes that catching errors rather than avoiding them, can result in cleaner, easier to read code. His example says it like this:

Worse (LBYL 'look before you leap'):

#check whether int conversion will raise an error
if not isinstance(s, str) or not s.isdigit():
    return None
elif len(s) > 10:    #too many digits for int conversion
    return None
else:
    return int(s)

Better (EAFP: Easier to ask for forgiveness than permission):

try:
    return int(s)
except (TypeError, ValueError, OverflowError): #int conversion failed
    return None

Is it a known good practice to use a big try-catch per method in java?

For me, two try-catch blocks makes most methods too long. It obfuscates the intention if the method is doing many things.

With two try-catch blocks, it's doing at least four things, to be precise

two cases for main flow (two try blocks)
two cases for error handling
(catch blocks)

I would rather make short and clear methods out of each try-catch block- like

private String getHostNameFromConfigFile(String configFile, String defaultHostName) {
    try {
        BufferedReader reader = new BufferedReader(new FileReader(configFile));
        return reader.readLine();
    } catch (IOException e) {
        return defaultHostName;
    }
}
public Collection<String> readServerHostnames(File mainServerConfigFile, File  backupServerConfigFile) {
    String mainServerHostname=getHostNameFromConfigFile(mainServerConfigFile,"default- server.example.org");
    String backupServerHostName=getHostNameFromConfigFile(backupServerConfigFile,"default- server.example.ru")
    return Arrays.asList(mainServerHostname,backupServerHostName);
}

Robert C. Martin in 'Clean Code' takes it to next level, suggesting:

if the keyword 'try' exists in a function, it should be the very first word in the function and that there should be nothing after the catch/finally blocks.

I would definitely refactor the method with two separate try/catch blocks into smaller methods.

Which is better - Throwing an Exception or Checking for errors beforehand

You should do the "try-and-catch exception" method simply because you have to do it anyway.

If you check first, there's nothing to stop someone inserting a row for that user between your check and your insert, in which case the user will be in the table even though your check didn't find it.

Short of being able to run the check-and-insert within some sort of transaction (so that nobody else can insert that user in the interim). you can't be certain that the non-exception will work.

And although many DBMS' provide transactional support, I don't know of any that will lock a row you have yet to insert :-)

Of course, if your application is designed in such a way that only your process will be inserting users (and serialised), you can use the check-first method. But I'd be putting in copious comments to the effect that it will need to be revisited if ever you scale up.

How using try catch for exception handling is best practice

My exception-handling strategy is:

To catch all unhandled exceptions by hooking to the Application.ThreadException event, then decide:
- For a UI application: to pop it to the user with an apology message (WinForms)
- For a Service or a Console application: log it to a file (service or console)

Then I always enclose every piece of code that is run externally in try/catch :

All events fired by the WinForms infrastructure (Load, Click, SelectedChanged...)
All events fired by third party components

Then I enclose in 'try/catch'

All the operations that I know might not work all the time (IO operations, calculations with a potential zero division...). In such a case, I throw a new ApplicationException("custom message", innerException) to keep track of what really happened

Additionally, I try my best to sort exceptions correctly. There are exceptions which:

need to be shown to the user immediately
require some extra processing to put things together when they happen to avoid cascading problems (ie: put .EndUpdate in the finally section during a TreeView fill)
the user does not care, but it is important to know what happened. So I always log them:
In the event log
or in a .log file on the disk

It is a good practice to design some static methods to handle exceptions in the application top level error handlers.

I also force myself to try to:

Remember ALL exceptions are bubbled up to the top level. It is not necessary to put exception handlers everywhere.
Reusable or deep called functions does not need to display or log exceptions : they are either bubbled up automatically or rethrown with some custom messages in my exception handlers.

So finally:

Bad:

// DON'T DO THIS; ITS BAD
try
{
    ...
}
catch 
{
   // only air...
}

Useless:

// DON'T DO THIS; IT'S USELESS
try
{
    ...
}
catch(Exception ex)
{
    throw ex;
}

Having a try finally without a catch is perfectly valid:

try
{
    listView1.BeginUpdate();

    // If an exception occurs in the following code, then the finally will be executed
    // and the exception will be thrown
    ...
}
finally
{
    // I WANT THIS CODE TO RUN EVENTUALLY REGARDLESS AN EXCEPTION OCCURRED OR NOT
    listView1.EndUpdate();
}

What I do at the top level:

// i.e When the user clicks on a button
try
{
    ...
}
catch(Exception ex)
{
    ex.Log(); // Log exception

    -- OR --
    
    ex.Log().Display(); // Log exception, then show it to the user with apologies...
}

What I do in some called functions:

// Calculation module
try
{
    ...
}
catch(Exception ex)
{
    // Add useful information to the exception
    throw new ApplicationException("Something wrong happened in the calculation module:", ex);
}

// IO module
try
{
    ...
}
catch(Exception ex)
{
    throw new ApplicationException(string.Format("I cannot write the file {0} to {1}", fileName, directoryName), ex);
}

There is a lot to do with exception handling (Custom Exceptions) but those rules that I try to keep in mind are enough for the simple applications I do.

Here is an example of extensions methods to handle caught exceptions a comfortable way. They are implemented in a way they can be chained together, and it is very easy to add your own caught exception processing.

// Usage:

try
{
    // boom
}
catch(Exception ex)
{
    // Only log exception
    ex.Log();

    -- OR --

    // Only display exception
    ex.Display();

    -- OR --

    // Log, then display exception
    ex.Log().Display();

    -- OR --

    // Add some user-friendly message to an exception
    new ApplicationException("Unable to calculate !", ex).Log().Display();
}

// Extension methods

internal static Exception Log(this Exception ex)
{
    File.AppendAllText("CaughtExceptions" + DateTime.Now.ToString("yyyy-MM-dd") + ".log", DateTime.Now.ToString("HH:mm:ss") + ": " + ex.Message + "\n" + ex.ToString() + "\n");
    return ex;
}

internal static Exception Display(this Exception ex, string msg = null, MessageBoxImage img = MessageBoxImage.Error)
{
    MessageBox.Show(msg ?? ex.Message, "", MessageBoxButton.OK, img);
    return ex;
}

Is it a good practice to use try-except-else in Python?

"I do not know if it is out of ignorance, but I do not like that
kind of programming, as it is using exceptions to perform flow control."

In the Python world, using exceptions for flow control is common and normal.

Even the Python core developers use exceptions for flow-control and that style is heavily baked into the language (i.e. the iterator protocol uses StopIteration to signal loop termination).

In addition, the try-except-style is used to prevent the race-conditions inherent in some of the "look-before-you-leap" constructs. For example, testing os.path.exists results in information that may be out-of-date by the time you use it. Likewise, Queue.full returns information that may be stale. The try-except-else style will produce more reliable code in these cases.

"It my understanding that exceptions are not errors, they should only
be used for exceptional conditions"

In some other languages, that rule reflects their cultural norms as reflected in their libraries. The "rule" is also based in-part on performance considerations for those languages.

The Python cultural norm is somewhat different. In many cases, you must use exceptions for control-flow. Also, the use of exceptions in Python does not slow the surrounding code and calling code as it does in some compiled languages (i.e. CPython already implements code for exception checking at every step, regardless of whether you actually use exceptions or not).

In other words, your understanding that "exceptions are for the exceptional" is a rule that makes sense in some other languages, but not for Python.

"However, if it is included in the language itself, there must be a
good reason for it, isn't it?"

Besides helping to avoid race-conditions, exceptions are also very useful for pulling error-handling outside loops. This is a necessary optimization in interpreted languages which do not tend to have automatic loop invariant code motion.

Also, exceptions can simplify code quite a bit in common situations where the ability to handle an issue is far removed from where the issue arose. For example, it is common to have top level user-interface code calling code for business logic which in turn calls low-level routines. Situations arising in the low-level routines (such as duplicate records for unique keys in database accesses) can only be handled in top-level code (such as asking the user for a new key that doesn't conflict with existing keys). The use of exceptions for this kind of control-flow allows the mid-level routines to completely ignore the issue and be nicely decoupled from that aspect of flow-control.

There is a nice blog post on the indispensibility of exceptions here.

Also, see this Stack Overflow answer: Are exceptions really for exceptional errors?

"What is the reason for the try-except-else to exist?"

The else-clause itself is interesting. It runs when there is no exception but before the finally-clause. That is its primary purpose.

Without the else-clause, the only option to run additional code before finalization would be the clumsy practice of adding the code to the try-clause. That is clumsy because it risks
raising exceptions in code that wasn't intended to be protected by the try-block.

The use-case of running additional unprotected code prior to finalization doesn't arise very often. So, don't expect to see many examples in published code. It is somewhat rare.

Another use-case for the else-clause is to perform actions that must occur when no exception occurs and that do not occur when exceptions are handled. For example:

recip = float('Inf')
try:
    recip = 1 / f(x)
except ZeroDivisionError:
    logging.info('Infinite result')
else:
    logging.info('Finite result')

Another example occurs in unittest runners:

try:
    tests_run += 1
    run_testcase(case)
except Exception:
    tests_failed += 1
    logging.exception('Failing test case: %r', case)
    print('F', end='')
else:
    logging.info('Successful test case: %r', case)
    print('.', end='')

Lastly, the most common use of an else-clause in a try-block is for a bit of beautification (aligning the exceptional outcomes and non-exceptional outcomes at the same level of indentation). This use is always optional and isn't strictly necessary.

Try Except Statement to Test for Presence of Error Button, Throw Exception if Element Found

You can simply use find_elements_by_id and get the count.

errEleCount = len(driver.find_elements_by_id("errorShowMore"))
if errEleCount>0:
  # implement the logic here.

What is the advantage of using try {} catch {} versus if {} else {}

I'd use the try/catch block when the normal path through the code should proceed without error unless there are truly some exceptional conditions -- like the server being down, your credentials being expired or incorrect. I wouldn't necessarily use it to handle non-exceptional errors -- say like the current user not being in the correct role. That is, when you can reasonably expect and handle an error that is not an exceptional condition, I think you should do your checks.

In the case that you've described -- setting up and performing a query, a try/catch block is an excellent way to handle it as you normally expect the query to succeed. On the other hand, you'll probably want to check that the contents of result are what you expect with control flow logic rather than just attempting to use data that may not be valid for your purpose.

One thing that you want to look out for is sloppy use of try/catch. Try/catch shouldn't be used to protect yourself from bad programming -- the "I don't know what will happen if I do this so I'm going to wrap it in a try/catch and hope for the best" kind of programming. Typically you'll want to restrict the kinds of exceptions you catch to those that are not related to the code itself (server down, bad credentials, etc.) so that you can find and fix errors that are code related (null pointers, etc.).

Better to 'Try' Something and Catch the Exception or Test If It's Possible First to Avoid an Exception