I want to do:
try:
do()
except:
do2()
except:
do3()
except:
do4()
If do() fails, execute do2(), if do2() fails too, execute do3() and so on.
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4I have a feeling that describing your specific use case will lead to an answer with a more elegant solution...Kenan Banks– Kenan Banks2012-12-14 07:58:16 +00:00Commented Dec 14, 2012 at 7:58
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7 Answers
If you really don't care about the exceptions, you could loop over cases until you succeed:
for fn in (do, do2, do3, do4):
try:
fn()
break
except:
continue
This at least avoids having to indent once for every case. If the different functions need different arguments you can use functools.partial to 'prime' them before the loop.
Comments
I'd write a quick wrapper function first() for this.
usage: value = first([f1, f2, f3, ..., fn], default='All failed')
#!/usr/bin/env
def first(flist, default=None):
""" Try each function in `flist` until one does not throw an exception, and
return the return value of that function. If all functions throw exceptions,
return `default`
Args:
flist - list of functions to try
default - value to return if all functions fail
Returns:
return value of first function that does not throw exception, or
`default` if all throw exceptions.
TODO: Also accept a list of (f, (exceptions)) tuples, where f is the
function as above and (exceptions) is a tuple of exceptions that f should
expect. This allows you to still re-raise unexpected exceptions.
"""
for f in flist:
try:
return f()
except:
continue
else:
return default
# Testing.
def f():
raise TypeError
def g():
raise IndexError
def h():
return 1
# We skip two exception-throwing functions and return value of the last.
assert first([f, g, h]) == 1
assert first([f, g, f], default='monty') == 'monty'
Comments
It seems like a really odd thing to want to do, but I would probably loop over the functions and break out when there were no exception raised:
for func in [do, do2, do3]:
try:
func()
except Exception:
pass
else:
break
2 Comments
Wizard
Creat, but what's else here for? it seem redundant here.
gaqzi
The
else will only execute when there isn't an exception. If there's an exception the for will continue whereas if there wasn't an exception the break will execute. :) A lot of statements in Python takes an else with the intention of "if everything went well, then do this". See the documentation for the try statement: docs.python.org/3/reference/compound_stmts.html#tryHere is the simplest way I found, just embed the try under the previous except.
try:
do()
except:
try:
do2()
except:
do3()
1 Comment
Aaron John Sabu
I did the same thing but can't it be made more compact?
You should specify the type of the exception you are trying to catch each time.
try:
do()
except TypeError: #for example first one - TypeError
do_2()
except KeyError: #for example second one - KeyError
do_3()
and so on.
6 Comments
alwbtc
If I don't know the type of the exception?
BrenBarn
@alwbic: If you don't know the type of the exception, you don't know how to handle the exception.
alexvassel
@alwbic I am really sure that you must know what type(s) of exception(s) do you expect.
alwbtc
I don't agree with you. I know that my statement will fail, but i don't know in which order they will fail. But I know what to do if it fails.
alexvassel
@alwbic: It is just a bad practice "Do something if something fails", you don't now why it fails.
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if you want multiple try statments you can do it like this, including the except statement. Extract (refactor) your statements. And use the magic of and and or to decide when to short-circuit.
def a():
try: # a code
except: pass # or raise
else: return True
def b():
try: # b code
except: pass # or raise
else: return True
def c():
try: # c code
except: pass # or raise
else: return True
def d():
try: # d code
except: pass # or raise
else: return True
def main():
try:
a() and b() or c() or d()
except:
pass
Comments
import sys
try:
f = open('myfile.txt')
s = f.readline()
i = int(s.strip())
except OSError as err:
print("OS error: {0}".format(err))
except ValueError:
print("Could not convert data to an integer.")
except:
print("Unexpected error:", sys.exc_info()[0])
raise
1 Comment
Thierry Lathuille
This doesn't answer the question at all!