Roger Davis wrote:
Completely off topic but I think the try clause could be rewritten that way:
...
Don't use bare except clause, you're masking syntax errors for instance,
which will be flagged as 'unexpected error in generation ...".
In a more general manner, if something unexpected happens it's better to
just let the exception raise uncought. If you want to handle some
errors, meaning you're kindof expecting them then add a explicit clause
(like you did with KeyboardInterrupt).
JM
PS : "except Exception :" will catch most of the exceptions (all
inheriting from that class). It's better than using a bare "except :"
clause. (Exception won't catch SyntaxError)
Thanks for the suggestion JM, it is off-topic and, although I will
first just say that the exception mechanism is *not* one of the
reasons I use Python (and stop there with regard to the whole
exception mechanism and various usage strategies in general), I do
have a few specific questions about a couple of your statements if you
don't mind following up.
First, inserting a syntax error into my existing code does not hide a
SyntaxError exception as you have stated:
% cat pid.py
#!/usr/bin/python
import os
import sys
import subprocess
def main():
psargs= ["/bin/ps", "-e"]
try:
ps= subprocess.Popen(psargs, stdout=subprocess.PIPE,
close_fds=True)
psout= ps.communicate()[0]
pslines= psout.splitlines()
if pslines not good Python talky-talk
for line in pslines:
print "%s" % line
except KeyboardInterrupt:
print "Keyboard interrupt received -- terminating."
sys.stdout.flush()
sys.exit(-1)
except:
print "%s: unexpected error in generation of system process
list" %
prognm
sys.stdout.flush()
sys.exit(-1)
main()
% ./pid.py
File "./pid.py", line 14
if pslines not good Python talky-talk
^
SyntaxError: invalid syntax
It appears that the interpreter is catching the syntax error before
the code is even executed.
That's right, 'cause my example is approximative. You would need to
import that file.
A better example would be to fire a NameError exception.
In the above code of fix the syntax error, but replace "for line in
pslines" by "for line in psLines". note the typo, very common mistake.
Then see how the error is hidden by your handler.
Second, python.org's exception hierarchy documentation (Section 6.1 at
http://docs.python.org/library/exceptions.html) shows all exception
types except SystemExit, KeyboardInterrupt and GeneratorExit as being
descendants of Exception. This includes SyntaxError, a child of
StandardError which is itself a child of Exception. So, if I say
'except Exception:' then isn't that clause going to process any child
exception type of Exception (including SyntaxError) that I haven't
already covered in a separate except clause?
You're absolutely right. Using Exception is bad, using bare except is
even worse. Exception is slightly better than except because you can at
least get the exception instance that has been raised, print it in your
handler, so you don't loose information (well, with a bare except you
can still inspect some sys objects to get info about the last exception
but it's a bit hackish).
(Except of course that my
interpreter doesn't seem to treat a syntax error as any kind of
exception at all!)
Finally, and this does not apply to your comments in particular, in
researching around about exception handling I often see the usage
except Exception, e:
suggested, but can't for the life of me figure out what the heck the
', e' part does. Can anybody explain what this means and why it might
be desirable (or not)?
Thanks!
Exception is the class, e is an instance of that class that has been
actually fired, it contains informations, most of the time a message.
This syntax has changed in python 3 I think, that would mean that you're
right to be confused, this python 2 syntax is ... strange...
import sys
# what if someone writes sys.hihi ?
try:
raise AttributeError("sys object has no attribute 'hihi'")
except Exception, e:
print e.args
print e.__class__
print e.message
print e.__doc__
output:
("sys object has no attribute 'hihi'",)
<type 'exceptions.AttributeError'>
sys object has no attribute 'hihi'
Attribute not found.
Apart from all these details, just keep in mind this golden rule:
- Don't use bare except clause.
And you'll be fine.
JM
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