Multiple try codes in one block

Question

I have a problem with my code in the try block. To make it easy this is my code:

try:
    code a
    code b #if b fails, it should ignore, and go to c.
    code c #if c fails, go to d
    code d
except:
    pass

Is something like this possible?

Answer 1

assuming each code block returns non-null value. (where you should design it to be)

val = None

try:
    val = func_a()
except (AErr, ...):
    expt_a()  # e.g. print('bad a')

try:
    val = val or func_b()
except (BErr, ...):
    expt_b()

try:
    val = val or func_c()
except (CErr, ...):
    expt_c()

if there are plenty of these structures, further simplify as

func_errs_expt_list = [
    [func_a, (AErr,...), expt_a],
    [func_b, (BErr,...), expt_b],
    [func_c, (CErr,...), expt_c],
    ...
]

val = None

for func, errs, expt in func_errs_expt_list:
    try:
        val = val or func(*args, **kwargs)
    except errs:
        expt(*args, **kwargs)

introducing oo to prettify elements is possible

class BaseExceptionSafeBlock:
    @staticmethod
    def _try(*args, **kwargs):
        raise NotImplementedError('return non-null')

    @staticmethod
    def _expected_errors(*args, **kwargs):
        return tuple()  # overridable

    @staticmethod
    def _except(e, *args, **kwargs):
        pass  # overridable

    @classmethod
    def try(cls, *args, **kwargs):
        try:
            return cls._try(*args, **kwargs)
        except cls._expected_errors(*args, **kwargs) as e:
            cls._except(e, *args, **kwargs):
            return None


class ABlock(BaseExceptionSafeBlock):
    @staticmethod
    def _try(*args, **kwargs):
        # do something and return it

...

def execute(blocks, *args, **kwargs):
    for block in blocks:
        res = block.try(*args, **kwargs)
        if res is not None:
            return res
    return None


val = execute([ABlock, ...], *args, **kwargs)

Answer 2

Building on kxr's answer (not enough rep to comment) you can use For/Else (see docs) to avoid checking the i value. The else clause only executes when the for finishes normally, so it gets skipped when the break executes

for i in 2, 1, 0:
    try:
        if   i == 2: print(a / b)
        elif i == 1: print(a / (b + 1))
        elif i == 0: print(a / (b + 2))
        break        
    except Exception as ev:
        continue
else:
    print("it failed: %s" % ev)

Answer 3

Like Elazar suggested: "I think a decorator would fit here."

# decorator
def test(func):
    def inner(*args, **kwargs):
        try:
            func(*args, **kwargs)
        except: pass
    return inner

# code blocks as functions
@test
def code_a(x):
    print(1/x)

@test
def code_b(x):
    print(1/x)

@test
def code_c(x):
    print(1/x)

@test
def code_d(x):
    print(1/x)

# call functions
code_a(0)
code_b(1)
code_c(0)
code_c(4)

output:

1.0
0.25

Answer 4

I ran into this problem, but then it was doing the things in a loop which turned it into a simple case of issueing the continue command if successful. I think one could reuse that technique if not in a loop, at least in some cases:

while True:
    try:
        code_a
        break
    except:
        pass

    try:
        code_b
        break
    except:
        pass

    etc

    raise NothingSuccessfulError

Answer 5

I use a different way, with a new variable:

continue_execution = True
try:
    command1
    continue_execution = False
except:
    pass
if continue_execution:
    try:
        command2
    except:
        command3

to add more commands you just have to add more expressions like this:

try:
    commandn
    continue_execution = False
except:
    pass

Answer 6

You could try a for loop

for func,args,kwargs in zip([a,b,c,d], 
                            [args_a,args_b,args_c,args_d],
                            [kw_a,kw_b,kw_c,kw_d]):
    try:
       func(*args, **kwargs)
       break
    except:
       pass

This way you can loop as many functions as you want without making the code look ugly

Answer 7

Lets say each code is a function and its already written then the following can be used to iter through your coding list and exit the for-loop when a function is executed without error using the "break".

def a(): code a
def b(): code b
def c(): code c
def d(): code d

for func in [a, b, c, d]:  # change list order to change execution order.
   try:
       func()
       break
   except Exception as err:
       print (err)
       continue

I used "Exception " here so you can see any error printed. Turn-off the print if you know what to expect and you're not caring (e.g. in case the code returns two or three list items (i,j = msg.split('.')).

Answer 8

You can use fuckit module.
Wrap your code in a function with @fuckit decorator:

@fuckit
def func():
    code a
    code b #if b fails, it should ignore, and go to c.
    code c #if c fails, go to d
    code d

Answer 9

If you don't want to chain (a huge number of) try-except clauses, you may try your codes in a loop and break upon 1st success.

Example with codes which can be put into functions:

for code in (
    lambda: a / b,
    lambda: a / (b + 1),
    lambda: a / (b + 2),
    ):
    try: print(code())
    except Exception as ev: continue
    break
else:
    print("it failed: %s" % ev)

Example with arbitrary codes (statements) directly in the current scope:

for i in 2, 1, 0:
    try:
        if   i == 2: print(a / b)
        elif i == 1: print(a / (b + 1))
        elif i == 0: print(a / (b + 2))
        break        
    except Exception as ev:
        if i:
            continue
        print("it failed: %s" % ev)

Answer 10

You'll have to make this separate try blocks:

try:
    code a
except ExplicitException:
    pass

try:
    code b
except ExplicitException:
    try:
        code c
    except ExplicitException:
        try:
            code d
        except ExplicitException:
            pass

This assumes you want to run code c only if code b failed.

If you need to run code c regardless, you need to put the try blocks one after the other:

try:
    code a
except ExplicitException:
    pass

try:
    code b
except ExplicitException:
    pass

try:
    code c
except ExplicitException:
    pass

try:
    code d
except ExplicitException:
    pass

I'm using except ExplicitException here because it is never a good practice to blindly ignore all exceptions. You'll be ignoring MemoryError, KeyboardInterrupt and SystemExit as well otherwise, which you normally do not want to ignore or intercept without some kind of re-raise or conscious reason for handling those.

Answer 11

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