It is conventional to use pass statement in python like the following piece of code.
try:
os.makedirs(dir)
except OSError:
pass
So, 'pass' bascially does not do anything here. In this case, why would we still put a few codes like this in the program? I am confused. Many thanks for your time and attention.
It's for the parser. If you wrote this:
try:
# Code
except Error:
And then put nothing in the except spot, the parser would signal an error because it would incorrectly identify the next indentation level. Imagine this code:
def f(x):
try:
# Something
except Error:
def g(x):
# More code
The parser was expecting a statement with a greater indentation than the except statement but got a new top-level definition. pass is simply filler to satisfy the parser.
This is in case you want the code to continue right after the lines in the try block. If you won't catch it - it either skips execution until it is caught elsewhere - or fails the program altogether.
Suppose you're creating a program that attempts to print to a printer, but also prints to the standard output - you may not want it to file if the printer is not available:
try:
print_to_printer('hello world')
except NoPrinterError:
pass # no printer - that's fine
print("hello world")
If you would not use a try-catch an error would stop execution until the exception is caught (or would fail the program) and nothing would be printed to standard output.
The pass is used to tell the program what to do when it catches an error. In this particular case you're pretty much ignoring it. So you're running your script and if you experience an error keep going without worrying as to why and how.
That particular case is when you are definite on what is expected. There are other cases where you can break and end the program, or even assign the error to a variable so you can debug your program by using except Error as e.
try:
os.makedirs(dir)
except OSError:
break
or:
try:
os.makedirs(dir)
except OSError as e:
print(str(e))
try:
# Do something
except:
# again some code
# few more code
There are two uses of pass. First, and most important use :- if exception arises for the code under try, the execution will jump to except block. And if you have nothing inside the except block, it will throw IndentationError at the first place. So, to avoid this error, even if you have nothing to do when exception arises, you need to put pass inside except block.
The second use, if you have some more code pieces after the try-except block (e.g. again some code and few more code), and you don't put pass inside except, then that code piece will not be executed (actually the whole code will not be executed since compiler will throw IndentationError). So, in order to gracefully handle the scenario and tell the interpreter to execute the lines after except block, we need to put pass inside the except block, even though we don't want to do anything in case of exception.
So, here pass as indicated from name, handles the except block and then transfers the execution to the next lines below the except block.
According to the Python documentation on idioms and anti-idioms in relation to exceptions: "You should try to use as few except clauses in your code as you can — the ones you do use will usually be inside calls which should always succeed, or a catch-all in a main function." Taking this sentence in sections...
"You should try to use as few except clauses in your code as you can"
A bit confusing for a newbie like myself, I thought it was good practise in Python to use the EAFP style -i.e. many try and except statements. Or am I missing the point?
"the ones you do use will usually be inside calls which should always succeed"
I don't understand the point that's being made here.
"or a catch-all in a main function."
So it's good style to any let code that throws an exception to simply pass it up the call stack until it reaches the top level where you have really generic exception handling?
"You should try to use as few except clauses in your code as you can"
It's easy to litter your code with exceptions:
def solve_linear(mat1, mat2):
det1 = determinant(mat1)
det2 = determinant(mat2)
try:
return det1 / det2
except ZeroDivisionError:
raise NoSolution
Here, it's probably fine to let the ZeroDivisionError propagate. You don't need to catch it.
"the ones you do use will usually be inside calls which should always succeed"
For example, look at this code which reads a file, or returns a cached value. It normally succeeds, in spite of the KeyError exception:
def read_file(path):
try:
return cache[path]
except KeyError:
fp = open(path, 'rb')
data = fp.read()
fp.close()
cache[path] = data
return data
"or a catch-all in a main function."
If your program is interactive, you'll probably want to catch almost everything at the top level. Here's the top loop of an interactive command-line program:
def main():
while True:
try:
text = raw_input('enter command: ')
cmd = parse_command(text)
except EOFError:
return
except KeyboardInterrupt:
return
except ValueError:
print 'Syntax error'
continue
try:
run_cmd(cmd)
except SystemExit:
raise
except Exception:
traceback.print_exc()
Concerning the first point, the whole point of using exceptions is that you don't have to wrap every line in one! E.g. in C, errors are usually determined by the return value of a function call. So you have to check those after every call if you want to catch all errors. With Python you can group a (possibly large) block of statements that go together in a try/except block and only deal with all errors once.
The second point is that (if possible) you want to solve failures close to the point where they occur. E.g. you are reading data from a network and get zero bytes. In that case it is usually perfectly allright to wait and try again.
The last point is that sometimes an error is so big that it cannot be handled at a low level. E.g. if you are trying to open a file that not exist, it wil fail. And your program cannot do whatever it was going to do with the contents of the file. It is best to handle that at the top level of the program and ask the user for another file name or maybe quit the program.
I think the point is that exceptions should be used only for 'exceptional' circumstances. That is the meaning behind the use "in calls which will usually succeed". An example of this might be some computation that under some really weird circumstances ends having to do a division by zero. Then you can enclose that in a try / except statement to deal with that possibility.
The catch-all in a main function would be applicable to the same scenario. Say your calculations end up with a division by zero somewhere deep in the call stack. From this point you can't continue, so there's no point in having a try/except statement right at the point of failure. It would make more sense to just have one at a higher level where you can reasonably recover from the error.
The example they give in the documentation is an example of this. When calling 'get_status' you would expect the file to be there. If it doesn't then you have the except statement to deal with that (although, as mentioned, in that particular case the 'with' statement is much better).
The Python philosophy is typically to "ask forgiveness, not permission." But the idea isn't to use a try-except clause to catch ALL possible errors. Ideally, each try-except will only catch the error(s) that are relevant.
BAD (doesn't specify a specific exception type):
a = [1, 2, 3]
i = 3
try:
print a[i]
except:
print "Not found!"
GOOD (only handles the exception we expect to get):
a = [1, 2, 3]
i = 3
try:
print a[i]
except IndexError:
print "Not found!"
The reason this is important is so we don't obscure other possible code errors. What if, in this example, i was 1.8? The first example would print Not Found!, obscuring the real issue, but the second example would return TypeError: list indices must be integers, not float letting us know that there's a logical flaw in our code.
I know try/except can handle errors in my program.
But, is there a way of making the error be displayed in the program execution, be ignored and let the execution go on?
In VBScript and other VB-derived languages, you can get this sort of behavior with "ON ERROR GOTO NEXT".
No such behavior exists in Python. Even if you wrap each top-level statement like:
try:
do_something()
except Exception as e:
print e
try:
do_something_else()
except Exception as e:
print e
you'd still have the result that statements within do_something are skipped at the point the exception is thrown.
Though perhaps if you have a particular use-case in mind, there may be other acceptable answers. For instance, in a top-level loop:
while True:
cmd = get_command()
if cmd == 'quit': break
try:
run_command(cmd)
except Exception as e:
print "Error running " + cmd + ":"
print e
import traceback
try:
# do whatever you want
except Exception:
traceback.print_exc()
Of course you should be more specific in a real scenario, i.e. you shouldn't catch and ignore all Exception instances, only the ones you are interested in and you know that they are safe to ignore.
Also note that if an exception happens somewhere within the try..except block, the execution will continue after the try..except block, not at the next statement. This is probably the closest to what you want to achieve.
I know using below code to ignore a certain exception, but how to let the code go back to where it got exception and keep executing? Say if the exception 'Exception' raises in do_something1, how to make the code ignore it and keep finishing do_something1 and process do_something2? My code just go to finally block after process pass in except block. Please advise, thanks.
try:
do_something1
do_something2
do_something3
do_something4
except Exception:
pass
finally:
clean_up
EDIT:
Thanks for the reply. Now I know what's the correct way to do it. But here's another question, can I just ignore a specific exception (say if I know the error number). Is below code possible?
try:
do_something1
except Exception.strerror == 10001:
pass
try:
do_something2
except Exception.strerror == 10002:
pass
finally:
clean_up
do_something3
do_something4
There's no direct way for the code to go back inside the try-except block. If, however, you're looking at trying to execute these different independant actions and keep executing when one fails (without copy/pasting the try/except block), you're going to have to write something like this:
actions = (
do_something1, do_something2, #...
)
for action in actions:
try:
action()
except Exception, error:
pass
update. The way to ignore specific exceptions is to catch the type of exception that you want, test it to see if you want to ignore it and re-raise it if you dont.
try:
do_something1
except TheExceptionTypeThatICanHandleError, e:
if e.strerror != 10001:
raise
finally:
clean_up
Note also, that each try statement needs its own finally clause if you want it to have one. It wont 'attach itself' to the previous try statement. A raise statement with nothing else is the correct way to re-raise the last exception. Don't let anybody tell you otherwise.
What you want are continuations which python doesn't natively provide. Beyond that, the answer to your question depends on exactly what you want to do. If you want do_something1 to continue regardless of exceptions, then it would have to catch the exceptions and ignore them itself.
if you just want do_something2 to happen regardless of if do_something1 completes, you need a separate try statement for each one.
try:
do_something1()
except:
pass
try:
do_something2()
except:
pass
etc. If you can provide a more detailed example of what it is that you want to do, then there is a good chance that myself or someone smarter than myself can either help you or (more likely) talk you out of it and suggest a more reasonable alternative.
This is pretty much missing the point of exceptions.
If the first statement has thrown an exception, the system is in an indeterminate state and you have to treat the following statement as unsafe to run.
If you know which statements might fail, and how they might fail, then you can use exception handling to specifically clean up the problems which might occur with a particular block of statements before moving on to the next section.
So, the only real answer is to handle exceptions around each set of statements that you want to treat as atomic
you could have all of the do_something's in a list, and iterate through them like this, so it's no so wordy. You can use lambda functions instead if you require arguments for the working functions
work = [lambda: dosomething1(args), dosomething2, lambda: dosomething3(*kw, **kwargs)]
for each in work:
try:
each()
except:
pass
cleanup()
Exceptions are usually raised when a performing task can not be completed in a manner intended by the code due to certain reasons. This is usually raised as exceptions. Exceptions should be handled and not ignored. The whole idea of exception is that the program can not continue in the normal execution flow without abnormal results.
What if you write a code to open a file and read it? What if this file does not exist?
It is much better to raise exception. You can not read a file where none exists. What you can do is handle the exception, let the user know that no such file exists. What advantage would be obtained for continuing to read the file when a file could not be opened at all.
In fact the above answers provided by Aaron works on the principle of handling your exceptions.
I posted this recently as an answer to another question. Here you have a function that returns a function that ignores ("traps") specified exceptions when calling any function. Then you invoke the desired function indirectly through the "trap."
def maketrap(*exceptions):
def trap(func, *args, **kwargs):
try:
return func(*args, **kwargs)
except exceptions:
return None
return trap
# create a trap that ignores all exceptions
trapall = maketrap(Exception)
# create a trap that ignores two exceptions
trapkeyattrerr = maketrap(KeyError, AttributeError)
# Now call some functions, ignoring specific exceptions
trapall(dosomething1, arg1, arg2)
trapkeyattrerr(dosomething2, arg1, arg2, arg3)
In general I'm with those who say that ignoring exceptions is a bad idea, but if you do it, you should be as specific as possible as to which exceptions you think your code can tolerate.
Python 3.4 added contextlib.suppress(), a context manager that takes a list of exceptions and suppresses them within the context:
with contextlib.suppress(IOError):
print('inside')
print(pathlib.Path('myfile').read_text()) # Boom
print('inside end')
print('outside')
Note that, just as with regular try/except, an exception within the context causes the rest of the context to be skipped. So, if an exception happens in the line commented with Boom, the output will be:
inside
outside
What is the intended use of the optional else clause of the try statement?
The statements in the else block are executed if execution falls off the bottom of the try - if there was no exception. Honestly, I've never found a need.
However, Handling Exceptions notes:
The use of the else clause is better
than adding additional code to the try
clause because it avoids accidentally
catching an exception that wasn’t
raised by the code being protected by
the try ... except statement.
So, if you have a method that could, for example, throw an IOError, and you want to catch exceptions it raises, but there's something else you want to do if the first operation succeeds, and you don't want to catch an IOError from that operation, you might write something like this:
try:
operation_that_can_throw_ioerror()
except IOError:
handle_the_exception_somehow()
else:
# we don't want to catch the IOError if it's raised
another_operation_that_can_throw_ioerror()
finally:
something_we_always_need_to_do()
If you just put another_operation_that_can_throw_ioerror() after operation_that_can_throw_ioerror, the except would catch the second call's errors. And if you put it after the whole try block, it'll always be run, and not until after the finally. The else lets you make sure
the second operation's only run if there's no exception,
it's run before the finally block, and
any IOErrors it raises aren't caught here
There is one big reason to use else - style and readability. It's generally a good idea to keep code that can cause exceptions near the code that deals with them. For example, compare these:
try:
from EasyDialogs import AskPassword
# 20 other lines
getpass = AskPassword
except ImportError:
getpass = default_getpass
and
try:
from EasyDialogs import AskPassword
except ImportError:
getpass = default_getpass
else:
# 20 other lines
getpass = AskPassword
The second one is good when the except can't return early, or re-throw the exception. If possible, I would have written:
try:
from EasyDialogs import AskPassword
except ImportError:
getpass = default_getpass
return False # or throw Exception('something more descriptive')
# 20 other lines
getpass = AskPassword
Note: Answer copied from recently-posted duplicate here, hence all this "AskPassword" stuff.
Python try-else
What is the intended use of the optional else clause of the try statement?
The intended use is to have a context for more code to run if there were no exceptions where it was expected to be handled.
This context avoids accidentally handling errors you did not expect.
But it's important to understand the precise conditions that cause the else clause to run, because return, continue, and break can interrupt the control flow to else.
In Summary
The else statement runs if there are no exceptions and if not interrupted by a return, continue, or break statement.
The other answers miss that last part.
From the docs:
The optional else clause is executed if and when control flows off the
end of the try clause.*
(Bolding added.) And the footnote reads:
*Currently, control “flows off the end” except in the case of an
exception or the execution of a return, continue, or break statement.
It does require at least one preceding except clause (see the grammar). So it really isn't "try-else," it's "try-except-else(-finally)," with the else (and finally) being optional.
The Python Tutorial elaborates on the intended usage:
The try ... except statement has an optional else clause, which, when
present, must follow all except clauses. It is useful for code that
must be executed if the try clause does not raise an exception. For
example:
for arg in sys.argv[1:]:
try:
f = open(arg, 'r')
except OSError:
print('cannot open', arg)
else:
print(arg, 'has', len(f.readlines()), 'lines')
f.close()
The use of the else clause is better than adding additional code to
the try clause because it avoids accidentally catching an exception
that wasn’t raised by the code being protected by the try ... except
statement.
Example differentiating else versus code following the try block
If you handle an error, the else block will not run. For example:
def handle_error():
try:
raise RuntimeError('oops!')
except RuntimeError as error:
print('handled a RuntimeError, no big deal.')
else:
print('if this prints, we had no error!') # won't print!
print('And now we have left the try block!') # will print!
And now,
>>> handle_error()
handled a RuntimeError, no big deal.
And now we have left the try block!
One use: test some code that should raise an exception.
try:
this_should_raise_TypeError()
except TypeError:
pass
except:
assert False, "Raised the wrong exception type"
else:
assert False, "Didn't raise any exception"
(This code should be abstracted into a more generic test in practice.)
Try-except-else is great for combining the EAFP pattern with duck-typing:
try:
cs = x.cleanupSet
except AttributeError:
pass
else:
for v in cs:
v.cleanup()
You might think this naïve code is fine:
try:
for v in x.cleanupSet:
v.clenaup()
except AttributeError:
pass
This is a great way of accidentally hiding severe bugs in your code. I typo-ed cleanup there, but the AttributeError that would let me know is being swallowed. Worse, what if I'd written it correctly, but the cleanup method was occasionally being passed a user type that had a misnamed attribute, causing it to silently fail half-way through and leave a file unclosed? Good luck debugging that one.
I find it really useful when you've got cleanup to do that has to be done even if there's an exception:
try:
data = something_that_can_go_wrong()
except Exception as e: # yes, I know that's a bad way to do it...
handle_exception(e)
else:
do_stuff(data)
finally:
clean_up()
Even though you can't think of a use of it right now, you can bet there has to be a use for it. Here is an unimaginative sample:
With else:
a = [1,2,3]
try:
something = a[2]
except IndexError:
print("out of bounds")
else:
print(something)
Without else:
try:
something = a[2]
except IndexError:
print("out of bounds")
if "something" in locals():
print(something)
Here you have the variable something defined if no error is thrown. You can remove this outside the try block, but then it requires some messy detection if a variable is defined.
There's a nice example of try-else in PEP 380. Basically, it comes down to doing different exception handling in different parts of the algorithm.
It's something like this:
try:
do_init_stuff()
except:
handle_init_suff_execption()
else:
try:
do_middle_stuff()
except:
handle_middle_stuff_exception()
This allows you to write the exception handling code nearer to where the exception occurs.
From Errors and Exceptions # Handling exceptions - docs.python.org
The try ... except statement has an optional else clause, which,
when present, must follow all except clauses. It is useful for code
that must be executed if the try clause does not raise an exception.
For example:
for arg in sys.argv[1:]:
try:
f = open(arg, 'r')
except IOError:
print 'cannot open', arg
else:
print arg, 'has', len(f.readlines()), 'lines'
f.close()
The use of the else clause is better than adding additional code to
the try clause because it avoids accidentally catching an exception
that wasn’t raised by the code being protected by the try ... except
statement.
try:
statements # statements that can raise exceptions
except:
statements # statements that will be executed to handle exceptions
else:
statements # statements that will be executed if there is no exception
Example :
try:
age=int(input('Enter your age: '))
except:
print ('You have entered an invalid value.')
else:
if age <= 21:
print('You are not allowed to enter, you are too young.')
else:
print('Welcome, you are old enough.')
The Output :
>>>
Enter your age: a
You have entered an invalid value.
>>> RESTART
>>>
Enter your age: 25
Welcome, you are old enough.
>>> RESTART
>>>
Enter your age: 13
You are not allowed to enter, you are too young.
>>>
Copied from : https://geek-university.com/python/the-try-except-else-statements/
Looking at Python reference it seems that else is executed after try when there's no exception.
The optional else clause is executed if and when control flows off the end of the try clause. 2 Exceptions in the else clause are not handled by the preceding except clauses.
Dive into python has an example where, if I understand correctly, in try block they try to import a module, when that fails you get exception and bind default but when it works you have an option to go into else block and bind what is required (see link for the example and explanation).
If you tried to do work in catch block it might throw another exception - I guess that's where the else block comes handy.
That's it. The 'else' block of a try-except clause exists for code that runs when (and only when) the tried operation succeeds. It can be used, and it can be abused.
try:
fp= open("configuration_file", "rb")
except EnvironmentError:
confdata= '' # it's ok if the file can't be opened
else:
confdata= fp.read()
fp.close()
# your code continues here
# working with (possibly empty) confdata
Personally, I like it and use it when appropriate. It semantically groups statements.
I would add another use case that seems straight forward when handling DB sessions:
# getting a DB connection
conn = db.engine.connect()
# and binding to a DB session
session = db.get_session(bind=conn)
try:
# we build the query to DB
q = session.query(MyTable).filter(MyTable.col1 == 'query_val')
# i.e retrieve one row
data_set = q.one_or_none()
# return results
return [{'col1': data_set.col1, 'col2': data_set.col2, ...}]
except:
# here we make sure to rollback the transaction,
# handy when we update stuff into DB
session.rollback()
raise
else:
# when no errors then we can commit DB changes
session.commit()
finally:
# and finally we can close the session
session.close()
Perhaps a use might be:
#debug = []
def debuglog(text, obj=None):
" Simple little logger. "
try:
debug # does global exist?
except NameError:
pass # if not, don't even bother displaying
except:
print('Unknown cause. Debug debuglog().')
else:
# debug does exist.
# Now test if you want to log this debug message
# from caller "obj"
try:
if obj in debug:
print(text) # stdout
except TypeError:
print('The global "debug" flag should be an iterable.')
except:
print('Unknown cause. Debug debuglog().')
def myfunc():
debuglog('Made it to myfunc()', myfunc)
debug = [myfunc,]
myfunc()
Maybe this will lead you too a use.
I have found the try: ... else: construct useful in the situation where you are running database queries and logging the results of those queries to a separate database of the same flavour/type. Let's say I have lots of worker threads all handling database queries submitted to a queue
#in a long running loop
try:
query = queue.get()
conn = connect_to_db(<main db>)
curs = conn.cursor()
try:
curs.execute("<some query on user input that may fail even if sanitized">)
except DBError:
logconn = connect_to_db(<logging db>)
logcurs = logconn.cursor()
logcurs.execute("<update in DB log with record of failed query")
logcurs.close()
logconn.close()
else:
#we can't put this in main try block because an error connecting
#to the logging DB would be indistinguishable from an error in
#the mainquery
#We can't put this after the whole try: except: finally: block
#because then we don't know if the query was successful or not
logconn = connect_to_db(<logging db>)
logcurs = logconn.cursor()
logcurs.execute("<update in DB log with record of successful query")
logcurs.close()
logconn.close()
#do something in response to successful query
except DBError:
#This DBError is because of a problem with the logging database, but
#we can't let that crash the whole thread over what might be a
#temporary network glitch
finally:
curs.close()
conn.close()
#other cleanup if necessary like telling the queue the task is finished
Of course if you can distinguish between the possible exceptions that might be thrown, you don't have to use this, but if code reacting to a successful piece of code might throw the same exception as the successful piece, and you can't just let the second possible exception go, or return immediately on success (which would kill the thread in my case), then this does come in handy.
Here is another place where I like to use this pattern:
while data in items:
try
data = json.loads(data)
except ValueError as e:
log error
else:
# work on the `data`
An else block can often exist to complement functionality that occurs in every except block.
try:
test_consistency(valuable_data)
except Except1:
inconsistency_type = 1
except Except2:
inconsistency_type = 2
except:
# Something else is wrong
raise
else:
inconsistency_type = 0
"""
Process each individual inconsistency down here instead of
inside the except blocks. Use 0 to mean no inconsistency.
"""
In this case, inconsistency_type is set in each except block, so that behaviour is complemented in the no-error case in else.
Of course, I'm describing this as a pattern that may turn up in your own code someday. In this specific case, you just set inconsistency_type to 0 before the try block anyway.
One of the use scenarios I can think of is unpredictable exceptions, which can be circumvented if you try again. For instance, when the operations in try block involves random numbers:
while True:
try:
r = random.random()
some_operation_that_fails_for_specific_r(r)
except Exception:
continue
else:
break
But if the exception can be predicted, you should always choose validation beforehand over an exception. However, not everything can be predicted, so this code pattern has its place.
I have found else useful for dealing with a possibly incorrect config file:
try:
value, unit = cfg['lock'].split()
except ValueError:
msg = 'lock monitoring config must consist of two words separated by white space'
self.log('warn', msg)
else:
# get on with lock monitoring if config is ok
An exception reading the lock config disables lock monitoring and ValueErrors log a helpful warning message.
Suppose your programming logic depends on whether a dictionary has an entry with a given key. You can test the result of dict.get(key) using if... else... construct, or you can do:
try:
val = dic[key]
except KeyError:
do_some_stuff()
else:
do_some_stuff_with_val(val)
One could use this construct for handling exceptions in a common way within the finally clause, while doing something else when there's no exception:
class TooManyRetries(RuntimeError):
pass
n_tries = 0
max_retries = 2
while True:
try:
n_tries += 1
if n_tries >= max_retries:
raise TooManyRetries
fail_prone_operation()
except Exception1 as ex:
# handle1
except Exception2 as ex:
# handle2
except Exception3 as ex:
# handle3
except TooManyRetries as ex:
raise
else: # No exception
n_tries = 0
finally:
common_restore_state()
continue
I may have missed it in the dozens of answers, but I prefer to minimize the LOC in a try: block, but I like how finally: can clean up a code segment. The else: block provides a clean way to accommodate that for file I/O, database work, etc., lots of examples above.
The else: block is confusing and (nearly) useless. It's also part of the for and while statements.
Actually, even on an if-statement, the else: can be abused in truly terrible ways creating bugs that are very hard to find.
Consider this.
if a < 10:
# condition stated explicitly
elif a > 10 and b < 10:
# condition confusing but at least explicit
else:
# Exactly what is true here?
# Can be hard to reason out what condition is true
Think twice about else:. It is generally a problem. Avoid it except in an if-statement and even then consider documenting the else- condition to make it explicit.