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

Python doesn’t subscribe to the idea that exceptions should only be used for exceptional cases, in fact the idiom is ‘ask for forgiveness, not permission’. This means that using exceptions as a routine part of your flow control is perfectly acceptable, and in fact, encouraged.

This is generally a good thing, as working this way helps avoid some issues (as an obvious example, race conditions are often avoided), and it tends to make code a little more readable.

Imagine you have a situation where you take some user input which needs to be processed, but have a default which is already processed. The try: ... except: ... else: ... structure makes for very readable code:

try:
   raw_value = int(input())
except ValueError:
   value = some_processed_value
else: # no error occured
   value = process_value(raw_value)

Compare to how it might work in other languages:

raw_value = input()
if valid_number(raw_value):
    value = process_value(int(raw_value))
else:
    value = some_processed_value

Note the advantages. There is no need to check the value is valid and parse it separately, they are done once. The code also follows a more logical progression, the main code path is first, followed by ‘if it doesn’t work, do this’.

The example is naturally a little contrived, but it shows there are cases for this structure.

You should be careful about using the finally block, as it is not the same thing as using an else block in the try, except. The finally block will be run regardless of the outcome of the try except.

In [10]: dict_ = {"a": 1}

In [11]: try:
   ....:     dict_["b"]
   ....: except KeyError:
   ....:     pass
   ....: finally:
   ....:     print "something"
   ....:     
something

As everyone has noted using the else block causes your code to be more readable, and only runs when an exception is not thrown

In [14]: try:
             dict_["b"]
         except KeyError:
             pass
         else:
             print "something"
   ....:

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

The answer to this is that it is context dependent. If you do this:

d = dict()
try:
    item = d['item']
except KeyError:
    item = 'default'

It demonstrates that you don’t know Python very well. This functionality is encapsulated in the dict.get method:

item = d.get('item', 'default')

The try/except block is a much more visually cluttered and verbose way of writing what can be efficiently executing in a single line with an atomic method. There are other cases where this is true.

However, that does not mean that we should avoid all exception handling. In some cases it is preferred to avoid race conditions. Don’t check if a file exists, just attempt to open it, and catch the appropriate IOError. For the sake of simplicity and readability, try to encapsulate this or factor it out as apropos.

Read the Zen of Python, understanding that there are principles that are in tension, and be wary of dogma that relies too heavily on any one of the statements in it.

OP, YOU ARE CORRECT. The else after try/except in Python is ugly. it leads to another flow-control object where none is needed:

try:
    x = blah()
except:
    print "failed at blah()"
else:
    print "just succeeded with blah"

A totally clear equivalent is:

try:
    x = blah()
    print "just succeeded with blah"
except:
    print "failed at blah()"

This is far clearer than an else clause. The else after try/except is not frequently written, so it takes a moment to figure what the implications are.

Just because you CAN do a thing, doesn’t mean you SHOULD do a thing.

Lots of features have been added to languages because someone thought it might come in handy. Trouble is, the more features, the less clear and obvious things are because people don’t usually use those bells and whistles.

Just my 5 cents here. I have to come along behind and clean up a lot of code written by 1st-year out of college developers who think they’re smart and want to write code in some uber-tight, uber-efficient way when that just makes it a mess to try and read / modify later. I vote for readability every day and twice on Sundays.

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

A try block allows you to handle an expected error. The except block should only catch exceptions you are prepared to handle. If you handle an unexpected error, your code may do the wrong thing and hide bugs.

An else clause will execute if there were no errors, and by not executing that code in the try block, you avoid catching an unexpected error. Again, catching an unexpected error can hide bugs.

Example

For example:

try:
    try_this(whatever)
except SomeException as the_exception:
    handle(the_exception)
else:
    return something

The “try, except” suite has two optional clauses, else and finally. So it’s actually try-except-else-finally.

else will evaluate only if there is no exception from the try block. It allows us to simplify the more complicated code below:

no_error = None
try:
    try_this(whatever)
    no_error = True
except SomeException as the_exception:
    handle(the_exception)
if no_error:
    return something

so if we compare an else to the alternative (which might create bugs) we see that it reduces the lines of code and we can have a more readable, maintainable, and less buggy code-base.

finally

finally will execute no matter what, even if another line is being evaluated with a return statement.

Broken down with pseudo-code

It might help to break this down, in the smallest possible form that demonstrates all features, with comments. Assume this syntactically correct (but not runnable unless the names are defined) pseudo-code is in a function.

For example:

try:
    try_this(whatever)
except SomeException as the_exception:
    handle_SomeException(the_exception)
    # Handle a instance of SomeException or a subclass of it.
except Exception as the_exception:
    generic_handle(the_exception)
    # Handle any other exception that inherits from Exception
    # - doesn't include GeneratorExit, KeyboardInterrupt, SystemExit
    # Avoid bare `except:`
else: # there was no exception whatsoever
    return something()
    # if no exception, the "something()" gets evaluated,
    # but the return will not be executed due to the return in the
    # finally block below.
finally:
    # this block will execute no matter what, even if no exception,
    # after "something" is eval'd but before that value is returned
    # but even if there is an exception.
    # a return here will hijack the return functionality. e.g.:
    return True # hijacks the return in the else clause above

It is true that we could include the code in the else block in the try block instead, where it would run if there were no exceptions, but what if that code itself raises an exception of the kind we’re catching? Leaving it in the try block would hide that bug.

We want to minimize lines of code in the try block to avoid catching exceptions we did not expect, under the principle that if our code fails, we want it to fail loudly. This is a best practice.

It is my understanding that exceptions are not errors

In Python, most exceptions are errors.

We can view the exception hierarchy by using pydoc. For example, in Python 2:

$ python -m pydoc exceptions

or Python 3:

$ python -m pydoc builtins

Will give us the hierarchy. We can see that most kinds of Exception are errors, although Python uses some of them for things like ending for loops (StopIteration). This is Python 3’s hierarchy:

BaseException
    Exception
        ArithmeticError
            FloatingPointError
            OverflowError
            ZeroDivisionError
        AssertionError
        AttributeError
        BufferError
        EOFError
        ImportError
            ModuleNotFoundError
        LookupError
            IndexError
            KeyError
        MemoryError
        NameError
            UnboundLocalError
        OSError
            BlockingIOError
            ChildProcessError
            ConnectionError
                BrokenPipeError
                ConnectionAbortedError
                ConnectionRefusedError
                ConnectionResetError
            FileExistsError
            FileNotFoundError
            InterruptedError
            IsADirectoryError
            NotADirectoryError
            PermissionError
            ProcessLookupError
            TimeoutError
        ReferenceError
        RuntimeError
            NotImplementedError
            RecursionError
        StopAsyncIteration
        StopIteration
        SyntaxError
            IndentationError
                TabError
        SystemError
        TypeError
        ValueError
            UnicodeError
                UnicodeDecodeError
                UnicodeEncodeError
                UnicodeTranslateError
        Warning
            BytesWarning
            DeprecationWarning
            FutureWarning
            ImportWarning
            PendingDeprecationWarning
            ResourceWarning
            RuntimeWarning
            SyntaxWarning
            UnicodeWarning
            UserWarning
    GeneratorExit
    KeyboardInterrupt
    SystemExit

A commenter asked:

Say you have a method which pings an external API and you want to handle the exception at a class outside the API wrapper, do you simply return e from the method under the except clause where e is the exception object?

No, you don’t return the exception, just reraise it with a bare raise to preserve the stacktrace.

try:
    try_this(whatever)
except SomeException as the_exception:
    handle(the_exception)
    raise

Or, in Python 3, you can raise a new exception and preserve the backtrace with exception chaining:

try:
    try_this(whatever)
except SomeException as the_exception:
    handle(the_exception)
    raise DifferentException from the_exception

I elaborate in my answer here.

Whenever you see this:

try:
    y = 1 / x
except ZeroDivisionError:
    pass
else:
    return y

Or even this:

try:
    return 1 / x
except ZeroDivisionError:
    return None

Consider this instead:

import contextlib
with contextlib.suppress(ZeroDivisionError):
    return 1 / x

This is my simple snippet on howto understand try-except-else-finally block in Python:

def div(a, b):
    try:
        a/b
    except ZeroDivisionError:
        print("Zero Division Error detected")
    else:
        print("No Zero Division Error")
    finally:
        print("Finally the division of %d/%d is done" % (a, b))

Let’s try div 1/1:

div(1, 1)
No Zero Division Error
Finally the division of 1/1 is done

Let’s try div 1/0

div(1, 0)
Zero Division Error detected
Finally the division of 1/0 is done

See the following example which illustrate everything about try-except-else-finally:

for i in range(3):
    try:
        y = 1 / i
    except ZeroDivisionError:
        print(f"ti = {i}")
        print("tError report: ZeroDivisionError")
    else:
        print(f"ti = {i}")
        print(f"tNo error report and y equals {y}")
    finally:
        print("Try block is run.")

Implement it and come by:

    i = 0
    Error report: ZeroDivisionError
Try block is run.
    i = 1
    No error report and y equals 1.0
Try block is run.
    i = 2
    No error report and y equals 0.5
Try block is run.

Just because no-one else has posted this opinion, I would say

avoid else clauses in try/excepts because they’re unfamiliar to most people

Unlike the keywords try, except, and finally, the meaning of the else clause isn’t self-evident; it’s less readable. Because it’s not used very often, it’ll cause people that read your code to want to double-check the docs to be sure they understand what’s going on.

(I’m writing this answer precisely because I found a try/except/else in my codebase and it caused a wtf moment and forced me to do some googling).

So, wherever I see code like the OP example:

try:
    try_this(whatever)
except SomeException as the_exception:
    handle(the_exception)
else:
    # do some more processing in non-exception case
    return something

I would prefer to refactor to

try:
    try_this(whatever)
except SomeException as the_exception:
    handle(the_exception)
    return  # <1>
# do some more processing in non-exception case  <2>
return something

• <1> explicit return, clearly shows that, in the exception case, we are finished working

• <2> as a nice minor side-effect, the code that used to be in the else block is dedented by one level.

I’m attempting to answer this question in a slightly different angle.

There were 2 parts of the OP’s question, and I add the 3rd one, too.

1. What is the reason for the try-except-else to exist?
2. Does the try-except-else pattern, or the Python in general, encourage using exceptions for flow control?
3. When to use exceptions, anyway?

Question 1: What is the reason for the try-except-else to exist?

It can be answered from a tactical standpoint. There is of course reason for try...except... to exist. The only new addition here is the else... clause, whose usefulness boils down to its uniqueness:

• It runs an extra code block ONLY WHEN there was no exception happened in the try... block.

• It runs that extra code block, OUTSIDE of the try... block (meaning any potential exceptions happen inside the else... block would NOT be caught).

• It runs that extra code block BEFORE the final... finalization.

    db = open(...)
    try:
        db.insert(something)
    except Exception:
        db.rollback()
        logging.exception('Failing: %s, db is ROLLED BACK', something)
    else:
        db.commit()
        logging.info(
            'Successful: %d',  # <-- For the sake of demonstration,
                               # there is a typo %d here to trigger an exception.
                               # If you move this section into the try... block,
                               # the flow would unnecessarily go to the rollback path.
            something)
    finally:
        db.close()
  

  In the example above, you can’t move that successful log line into behind the finally... block. You can’t quite move it into inside the try... block, either, due to the potential exception inside the else... block.

Question 2: does Python encourage using exceptions for flow control?

I found no official written documentation to support that claim. (To readers who would disagree: please leave comments with links to evidences you found.) The only vaguely-relevant paragraph that I found, is this EAFP term:

EAFP

Easier to ask for forgiveness than permission. This common Python coding style assumes the existence of valid keys or attributes and catches exceptions if the assumption proves false. This clean and fast style is characterized by the presence of many try and except statements. The technique contrasts with the LBYL style common to many other languages such as C.

Such paragraph merely described that, rather than doing this:

def make_some_noise(speaker):
    if hasattr(speaker, "quack"):
        speaker.quack()

we would prefer this:

def make_some_noise(speaker):
    try:
        speaker.quack()
    except AttributeError:
        logger.warning("This speaker is not a duck")

make_some_noise(DonaldDuck())  # This would work
make_some_noise(DonaldTrump())  # This would trigger exception

or potentially even omitting the try…except:

def make_some_noise(duck):
    duck.quack()

So, the EAFP encourages duck-typing. But it does not encourage using exceptions for flow control.

Question 3: In what situation you should design your program to emit exceptions?

It is a moot conversation on whether it is anti-pattern to use exception as control flow. Because, once a design decision is made for a given function, its usage pattern would also be determined, and then the caller would have no choice but to use it that way.

So, let’s go back to the fundamentals to see when a function would better produce its outcome via returning a value or via emitting exception(s).

What is the difference between the return value and the exception?

1. Their "blast radius" are different. Return value is only available to the immediate caller; exception can be automatically relayed for unlimited distance until it is caught.

2. Their distribution patterns are different. Return value is by definition one piece of data (even though you could return a compound data type such as a dictionary or a container object, it is still technically one value).
   The exception mechanism, on the contrary, allows multiple values (one at a time) to be returned via their respective dedicate channel. Here, each except FooError: ... and except BarError: ... block is considered as its own dedicate channel.

Therefore, it is up to each different scenario to use one mechanism that fits well.

• All normal cases should better be returned via return value, because the callers would most likely need to use that return value immediately. The return-value approach also allows nesting layers of callers in a functional programming style. The exception mechanism’s long blast radius and multiple channels do not help here.
  For example, it would be unintuitive if any function named get_something(...) produces its happy path result as an exception. (This is not really a contrived example. There is one practice to implement BinaryTree.Search(value) to use exception to ship the value back in the middle of a deep recursion.)

• If the caller would likely forget to handle the error sentinel from the return value, it is probably a good idea to use exception’s characterist #2 to save caller from its hidden bug. A typical non-example would be the position = find_string(haystack, needle), unfortunately its return value of -1 or null would tend to cause a bug in the caller.

• If the error sentinel would collide with a normal value in the result namespace, it is almost certain to use an exception, because you’d have to use a different channel to convey that error.

• If the normal channel i.e. the return value is already used in the happy-path, AND the happy-path does NOT have sophisicated flow control, you have no choice but to use exception for flow control. People keep talking about how Python uses StopIteration exception for iteration termination, and use it to kind of justify "using exception for flow control". But IMHO this is only a practical choice in a particular situation, it does not generalize and glorify "using exception for flow control".

At this point, if you already make a sound decision on whether your function get_stock_price() would produce only return-value or also raise exceptions, or if that function is provided by an existing library so that its behavior has long be decided, you do not have much choice in writing its caller calculate_market_trend(). Whether to use get_stock_price()‘s exception to control the flow in your calculate_market_trend() is merely a matter of whether your business logic requires you to do so. If yes, do it; otherwise, let the exception bubble up to a higher level (this utilizes the characteristic #1 "long blast radius" of exception).

In particular, if you are implementing a middle-layer library Foo and you happen to be making a dependency on lower-level library Bar, you would probably want to hide your implementation detail, by catching all Bar.ThisError, Bar.ThatError, …, and map them into Foo.GenericError. In this case, the long blast radius is actually working against us, so you might hope "only if library Bar were returning its errors via return values". But then again, that decision has long been made in Bar, so you can just live with it.

All in all, I think whether to use exception as control flow is a moot point.