catching my own exception

[nick]

I have a problem with catching my own exception. Here is the code:
http://fly.srk.fer.hr/~nick/travapi/blame.php?repname=Travian+API&path=/village.py&

Line 252 calls a method, which on line 207 raises a
SomethingBeingBuiltError exception. On line 253 I catch that
exception, but when I run that code, I get:

Traceback (most recent call last):
File "village.py", line 252, in <module>
v.upgrade_cheapest()
File "/media/data/home/code/travapi/village.py", line 207, in upgrade_cheapest
raise SomethingBeingBuiltError()
village.SomethingBeingBuiltError: 'Unable to upgrade, check if something is being built.'

If I change line 253 to only "except:", everything works as
expected. How come? What am I missing? Any help would be appreciated.

P.S.: I'm aware I'm exposing my login/pass in the code. Don't care
really, those are throwaway accounts.

 

[Jon Clements]

I have a problem with catching my own exception. Here is the code:http://fly.srk.fer.hr/~nick/travapi/blame.php?repname=Travian+API&pat...

Line 252 calls a method, which on line 207 raises a
SomethingBeingBuiltError exception. On line 253 I catch that
exception, but when I run that code, I get:

Traceback (most recent call last):
  File "village.py", line 252, in <module>
    v.upgrade_cheapest()
  File "/media/data/home/code/travapi/village.py", line 207, in upgrade_cheapest
    raise SomethingBeingBuiltError()
village.SomethingBeingBuiltError: 'Unable to upgrade, check if something is being built.'

If I change line 253 to only "except:", everything works as
expected. How come? What am I missing? Any help would be appreciated.

P.S.: I'm aware I'm exposing my login/pass in the code. Don't care
really, those are throwaway accounts.

--
"Now the storm has passed over me
I'm left to drift on a dead calm sea
And watch her forever through the cracks in the beams
Nailed across the doorways of the bedrooms of my dreams"

http://www.travian.com/spielregeln.php -- rule 3???

 

[Steven D'Aprano]

I have a problem with catching my own exception. Here is the code:
http://fly.srk.fer.hr/~nick/travapi/blame.php?repname=Travian+API&path=% 2Fvillage.py&

Line 252 calls a method, which on line 207 raises a
SomethingBeingBuiltError exception. On line 253 I catch that exception,
but when I run that code, I get:

Traceback (most recent call last):
File "village.py", line 252, in <module>
v.upgrade_cheapest()
File "/media/data/home/code/travapi/village.py", line 207, in
upgrade_cheapest
raise SomethingBeingBuiltError()
village.SomethingBeingBuiltError: 'Unable to upgrade, check if something
is being built.'

If I change line 253 to only "except:", everything works as expected.
How come? What am I missing? Any help would be appreciated.

I can't run your code, so I'm stuck with trying to guess how it works
just by reading it. As far as I can tell, it should work. If it doesn't
work, I can only imagine that you're not running the code you think you
are running. Perhaps you're running an older version?


Other than that, I notice that your module throws away useful debugging
information, and replaces it with bland, useless pap of no nutritional
value:

try:
import account, fetch, resources, const
except Exception:
raise Exception('One or more travapi modules not available.')

Instead of a useful ImportError exception that tells the caller two
important pieces of information (namely that it is an import error, and
what the error actually was) you replace it with a useless, generic
error. It might as well say "An error occurred" for all the use it is.

Errors should be *more* specific, not less. By replacing useful, specific
exceptions with the generic Exception class, you're throwing information
away and making your own job harder. Why would you rather see:

Exception: One or more travapi modules not available.

instead of this?

ImportError: No module named resources

As a general rule, anytime you find yourself writing:

except SomeSpecificException:
raise SomeOtherException

you should stop and think *really hard* about why you are bothering.

 

[nick]

Dana Fri, 18 Jun 2010 10:36:21 -0700 (PDT),

http://www.travian.com/spielregeln.php -- rule 3???

Yeah, I know. If it's any consolation to you, I'm not doing it for the
fun of wining the game (hence not bothering to hide de code) but for
the fun of coding the API.

Anyway, catching your own exceptions?

 

[Peter Otten]

I have a problem with catching my own exception. Here is the code:
http://fly.srk.fer.hr/~nick/travapi/blame.php?repname=Travian+API&path=/village.py&

Line 252 calls a method, which on line 207 raises a
SomethingBeingBuiltError exception. On line 253 I catch that
exception, but when I run that code, I get:

Traceback (most recent call last):
File "village.py", line 252, in <module>
v.upgrade_cheapest()
File "/media/data/home/code/travapi/village.py", line 207, in
upgrade_cheapest
raise SomethingBeingBuiltError()
village.SomethingBeingBuiltError: 'Unable to upgrade, check if something
is being built.'

If I change line 253 to only "except:", everything works as
expected. How come? What am I missing? Any help would be appreciated.

P.S.: I'm aware I'm exposing my login/pass in the code. Don't care
really, those are throwaway accounts.

You are importing your main script elswhere. Your code then
effectively becomes

try:
# in another module
raise village.SomethingBuiltError
except __main__.SomethingBeingBuiltError:
print "caught"

i. e. you get two versions of every class that are built from the same code
but not (recognized as) identical.

Solution: move your startup code into a separate file and have it import the
village module.

Peter

 

[Nikola Skoric]

Dana Fri, 18 Jun 2010 20:01:45 +0200,

Solution: move your startup code into a separate file and have it import the
village module.

Excellent, thanks! Everything works now, but I still don't quite get what the problem is...

You are importing your main script elswhere. Your code then
effectively becomes

try:
# in another module
raise village.SomethingBuiltError
except __main__.SomethingBeingBuiltError:
print "caught"

i. e. you get two versions of every class that are built from the same code
but not (recognized as) identical.

What I don't get is: what do you mean I'm importing my main script
elsewhere by runing "python village.py"? SomethingBuiltError is
defined in the same script that I'm runing, I didn't import it, did I?
If you could please clear it up for me... or point me to relevant
literature, that's also cool, I couldn't find this thing explained
anywhere.

Anyway, thanks for the solution!

 

[Nikola Skoric]

Dana 18 Jun 2010 17:45:31 GMT,

Other than that, I notice that your module throws away useful debugging
information, and replaces it with bland, useless pap of no nutritional
value:

try:
import account, fetch, resources, const
except Exception:
raise Exception('One or more travapi modules not available.')

Oh, yes, thanks for pointing that out, that was a dirty hack for
solving some error I was getting with pydoc. I'll remove that, thanks
for pointing that out.

 

[Peter Otten]

Dana Fri, 18 Jun 2010 20:01:45 +0200,


Excellent, thanks! Everything works now, but I still don't quite get what
the problem is...


What I don't get is: what do you mean I'm importing my main script
elsewhere by runing "python village.py"? SomethingBuiltError is
defined in the same script that I'm runing, I didn't import it, did I?
If you could please clear it up for me... or point me to relevant
literature, that's also cool, I couldn't find this thing explained
anywhere.

In Python, if you write

class A:
pass

that is executable code just like an if-statement or a for-loop rather than
a declaration, and this code creates a new class every time it is run.
That's why

items = []
for i in range(5):

.... class A: pass
.... items.append(A)
....

issubclass(A, A) True
issubclass(items[0], items[1])

False

creates 5 distinct classes. The same would go for modules if it weren't for
the sys.modules cache:

If we clear the cache manually, the code in the module will be executed
again:

import sys
del sys.modules["tmp"]
import tmp

importing tmp

Now in your case village.py is first run as the main script and then
imported from within account.py -- but the main script is put into the cache
under the key "__main__" regardless of its actual name. Using our simple
example:

$ python -i tmp.py
importing __main__importing tmp

So you end up with a __main__ module and a village module that contain
functions and classes built from the same code but with distinct identities.

In the account module you indirectly raise village.ExceptionWithLongName and
in __main__ you try to catch __main__.ExceptionWithLongName.

Peter

 

[Nikola Skoric]

Dana Sun, 20 Jun 2010 10:53:08 +0200,

Now in your case village.py is first run as the main script and then [...]
In the account module you indirectly raise village.ExceptionWithLongName and
in __main__ you try to catch __main__.ExceptionWithLongName.

Phew. I get it now. You're my hero :-D I had to read it twice, but I
got it. Thanks a bunch!