Exception handling with method chaining - python-3.x

Question

class A:
def __init__(self, data):
self.data = data
def first():
# Some functionality
# raise Exception on some condition
return self
def second():
#Some functionality
a = A('test string').first().second()
In this case I want that if first() raises an error then the chaining breaks and that error is raised. Currently it just fails silently.
How can I fix this?

Answer 1

Probably the best you can do is localize the type of errors that are possible in the chain and then ladder the exception handlers:
class A:
def __init__(self, data):
self.data = data
def first(self):
# Some functionality
# raise Exception on some condition
if self.data=='A':
1/0
return self
def second(self):
next(0)
pass
#Some functionality
You would know that first is numerical, and second is an iterable function that might have a TypeError.
So try this:
try:
A('A').first().second()
except ZeroDivisionError as e:
print('Likely first:', e)
except TypeError as e:
print('Likely second:', e)
Would print Likely first: division by zero with A('A') and print Likely second: 'int' object is not an iterator if you use A('something_else'). You would use this construct if you wanted to stop the chain as executed.
You can also add raise to re-raise the error and get Python's diagnosis of when are where:
try:
A('B').first().second()
except ZeroDivisionError as e:
print('Likely first:', e)
raise
except TypeError as e:
print('Likely second:', e)
raise
Prints:
Likely second: 'int' object is not an iterator
Traceback (most recent call last):
File "Untitled 79.py", line 19, in <module>
A('B').first().second()
File "Untitled 79.py", line 14, in second
next(0)
TypeError: 'int' object is not an iterator
Or, better still, use the try and except in each method:
class A:
def __init__(self, data):
self.data = data
def first(self):
try:
1/0
except ZeroDivisionError as e:
print('DEFINITELY first:', e)
return self
def second(self):
try:
next(0)
except TypeError as e:
print('DEFINITELY second:', e)
return self
>>> A('').first().second()
DEFINITELY first: division by zero
DEFINITELY second: 'int' object is not an iterator

Answer 2

I don't think it could fail silently if its actually failing, as this would error out python. However, you might want something like this, maybe it will jog your thinking.
class A:
def __init__(self, data):
self.data = data
def first():
try:
# Some functionality
raise ERROR_YOU WANT_RAISE:
# raise Exception on some condition
except ERROR_TO_CATCH_FROM_RAISE:
# This is where you would handle the exception after its caught
# and break your chaining possibly. Set a flag for second()
# for example that handles the error.
return self
def second():
#Some functionality