473,898 Members | 2,436 Online
Bytes | Software Development & Data Engineering Community
+ Post

Home Posts Topics Members FAQ

Uncatchable AttributeError, shelve raises exceptions in __del__

Last week I encountered an AttributeError in my unit tests that I
wasn'table to catch with an "except AttributeError" statement.

The problem stemmed from a class that raised an error inside
__init__and defined a __del__ method to clean up resources. I then
discovered asimilar problem in the shelve module. This led me to two
importantdiscov eries:

1. Attributes defined in __init__ after an error is raised will not be
apart of the "self" passed to __del__. On reflection, this seems
quiteobvious, but it is easy to have this bite you because it is
"morePython ic", I think, to define class attributes inside __init__ on
anas-needed basis. In less dynamic languages, say Java, one defines
allclass attributes at the class level, which is allowed in Python,
and soall class attributes are defined when the constructor is called.

2. As documented(http://www.python.org/doc/current/re...mization.html),
If anexception is raised in __del__, a message is printed to stderr
and it isignored. This means that such an exception cannot be caught
with"except"! This can be a pain if such an error is raised in a
modulethat isn't your code.

As an aside, why isn't __del__ documentation in the index with the
other"_ (underscore)" stuff?

Here's an example that shows how the location of attribute
definitionsinsi de __init__ can lead to AttributeErrors in __del__ (and
henceuncatchabl e errors):

class AttributeErrorR aiser:
def __init__(self, be_bad=True):
self.earlyAttri bute = "early"
if be_bad:
raise SomeError()
self.lateAttrib ute = "late"
def __del__(self):
print self.earlyAttri bute
print self.lateAttrib ute

class SomeError(Excep tion):
def __init__(self, args=None):
self.args = args
try: ... aer = AttributeErrorR aiser()
... except SomeError:
... print "Caught SomeError"
Caught SomeError
Exception exceptions.Attr ibuteError: "AttributeError Raiser
instance has no attribute 'lateAttribute' " in <bound method
AttributeErrorR aiser.__del__ of <trytocatchme.A ttributeErrorRa iser
instance at 0x402e9d0c>> ignored

And here is the same programming error in the shelve module. Assume
that 'notashelf.txt' is an existing plain text file:

import shelve
import anydbm

class ShowShelfBadnes s:
def __init__(self):
self.shelf = shelve.open('no tashelf.txt')
except anydbm.error:
print "caught a shelve related error"
ssb = ShowShelfBadnes s()

caught a shelve related error
Exception exceptions.Attr ibuteError: "DbfilenameShel f instance has
no attribute 'writeback'" in ignored

The workaround that I found was to use the whichdb module to test for
a valid shelf-type file, if the file exists, before opening with
shelve, but I still don't like the fact that shelve.open raises an
"uncatchabl e" error when you give it a bad file.


Does anyone have suggestions for "best practice" w.r.t. defining
attributes in __init__ or at the class level?

Thoughts on workarounds for the behavior shown in the shelve module?
Thanks for listening.
Jul 18 '05 #1
0 2485

This thread has been closed and replies have been disabled. Please start a new discussion.

Similar topics

by: Rami A. Kishek | last post by:
Hi - this mysterious behavior with shelve is just about to kill me. I hope someone here can shed some light. First of all, I have this piece of code which uses shelve to save instances of some class I define. It works perfectly on an old machine (PII-400) running Python 2.2.1 under RedHat Linux 8.0. When I try to run it under Python for windows ME on a P-4 1.4 GHz, however, it keeps crashing on reading from the shelved file the second...
by: Rune | last post by:
Hi, I've written a very simple 'kill-server' to help me shut down processes through Telnet or HTTP. The kill-server is a function and is launched as a thread. I use the module socket.py on Python v2.3 (Windows) I can use telnet host:port and enter the secret killword or use a broser with http://host:port/secret_killword The 'kill-server' validates the secret_killword and writes a file
by: Erlend Fuglum | last post by:
I have tried and tried, but cannot figure out the source of the following error: AttributeError: 'module' object has no attribute 'menyHMTL' __doc__ = 'Attribute not found.' __getitem__ = <bound method AttributeError.__getitem__ of <exceptions.AttributeError instance at 0x8187984>> __init__ = <bound method AttributeError.__init__ of <exceptions.AttributeError instance at 0x8187984>> __module__ = 'exceptions'
by: David Turner | last post by:
Hi all I noticed something interesting while testing some RAII concepts ported from C++ in Python. I haven't managed to find any information about it on the web, hence this post. The problem is that when an exception is raised, the destruction of locals appears to be deferred to program exit. Am I missing something? Is this behaviour by design? If so, is there any reason for it? The only rationale I can think of is to speed up...
by: PiedmontBiz | last post by:
I am finishing up a simple survey for a web site. I am creating a server-side data base using the shelve module. I am a relative python newcomer. I began with anydbm, but found that shelve uses anydbm and pickle so it is a bit easier for me to do what I want. The database I am testing list 4 candidates for president. A web form ( I use javascript to validate the data before sending it on ) posts the users survey data to a server-side...
by: cs | last post by:
We have some code that walks the process tree on windows xp. It uses the process class in system diagnostic as well as some dllimport calls to kernel32.dll and ntdll.dll We also have some code that talks on terminal services virtual channels using wtsapi32.dll What we have noticed is that after calling those methods several times per minute in some cases or at least lots of times per hour all day long will end up giving a "application...
by: aomighty | last post by:
I wanted to write the following code: import shelve try: db = shelve.open(file, "r") except SomeError: print "Oh no, db not found" Only, I'm not sure what SomeError should be. I tried error, anydbm.error, shelve.open.anydb.error, etc. but can't find it. Things
by: erikcw | last post by:
Hi, I'm trying to build a SQL string sql = """INSERT INTO ag ('cid', 'ag', 'test') VALUES(%i, %s, %d)""", (cid, ag, self.data) It raises this error: AttributeError: 'tuple' object has no attribute 'encode'
by: Nikhil | last post by:
I have recently written a small module. When I import the module, I always get the error only when I do -- Traceback (most recent call last): File "<stdin>", line 1, in <module>
by: Hystou | last post by:
Most computers default to English, but sometimes we require a different language, especially when relocating. Forgot to request a specific language before your computer shipped? No problem! You can effortlessly switch the default language on Windows 10 without reinstalling. I'll walk you through it. First, let's disable language synchronization. With a Microsoft account, language settings sync across devices. To prevent any complications,...
by: jinu1996 | last post by:
In today's digital age, having a compelling online presence is paramount for businesses aiming to thrive in a competitive landscape. At the heart of this digital strategy lies an intricately woven tapestry of website design and digital marketing. It's not merely about having a website; it's about crafting an immersive digital experience that captivates audiences and drives business growth. The Art of Business Website Design Your website is...
by: Hystou | last post by:
Overview: Windows 11 and 10 have less user interface control over operating system update behaviour than previous versions of Windows. In Windows 11 and 10, there is no way to turn off the Windows Update option using the Control Panel or Settings app; it automatically checks for updates and installs any it finds, whether you like it or not. For most users, this new feature is actually very convenient. If you want to control the update process,...
by: tracyyun | last post by:
Dear forum friends, With the development of smart home technology, a variety of wireless communication protocols have appeared on the market, such as Zigbee, Z-Wave, Wi-Fi, Bluetooth, etc. Each protocol has its own unique characteristics and advantages, but as a user who is planning to build a smart home system, I am a bit confused by the choice of these technologies. I'm particularly interested in Zigbee because I've heard it does some...
by: isladogs | last post by:
The next Access Europe User Group meeting will be on Wednesday 1 May 2024 starting at 18:00 UK time (6PM UTC+1) and finishing by 19:30 (7.30PM). In this session, we are pleased to welcome a new presenter, Adolph Dupré who will be discussing some powerful techniques for using class modules. He will explain when you may want to use classes instead of User Defined Types (UDT). For example, to manage the data in unbound forms. Adolph will...
by: conductexam | last post by:
I have .net C# application in which I am extracting data from word file and save it in database particularly. To store word all data as it is I am converting the whole word file firstly in HTML and then checking html paragraph one by one. At the time of converting from word file to html my equations which are in the word document file was convert into image. Globals.ThisAddIn.Application.ActiveDocument.Select();...
by: TSSRALBI | last post by:
Hello I'm a network technician in training and I need your help. I am currently learning how to create and manage the different types of VPNs and I have a question about LAN-to-LAN VPNs. The last exercise I practiced was to create a LAN-to-LAN VPN between two Pfsense firewalls, by using IPSEC protocols. I succeeded, with both firewalls in the same network. But I'm wondering if it's possible to do the same thing, with 2 Pfsense firewalls...
by: 6302768590 | last post by:
Hai team i want code for transfer the data from one system to another through IP address by using C# our system has to for every 5mins then we have to update the data what the data is updated we have to send another system
by: bsmnconsultancy | last post by:
In today's digital era, a well-designed website is crucial for businesses looking to succeed. Whether you're a small business owner or a large corporation in Toronto, having a strong online presence can significantly impact your brand's success. BSMN Consultancy, a leader in Website Development in Toronto offers valuable insights into creating effective websites that not only look great but also perform exceptionally well. In this comprehensive...

By using Bytes.com and it's services, you agree to our Privacy Policy and Terms of Use.

To disable or enable advertisements and analytics tracking please visit the manage ads & tracking page.