__getattr__ weirdness

[Greg Brunet]

In adding the ability to refer to field values using dbfFile.field
notation, I learned how to use __getattr__ and __setattr__ . After some
trial and error, I got it working. But as part of my trials, I added
some print statements to debug stuff. The ones I added to __setattr__
work as expected, but the one in __getattr__ seems to get called just
under 1000 times for every __getattr__ call!

Something is obviously not right here - but I'm at a loss to understand
what's going on. I've pared down my code to still show it happening &
included it below. If you run this program & pipe the output to a file,
you'll get just under 14000 debug lines. Any ideas? Thanks!

--
Greg


# ----------------------------------------------------------------------
----------
class test:
"""Introspectio n test"""

#----------------------------------------
def __init__(self,f ilename):
print '** __init__'
self._filename= filename
self._del=' '
self._dirty=Fal se
self._open=Fals e
self._rec=[]
self._recno = 1

#----------------------------------------
def __getattr__(sel f, key):
""" Return DBF record values by field name """
print "_ga: " + key
try:
return self._rec[self._fldNames. index(key.upper ())]
except:
raise AttributeError( "Unknown Field: %s" % ( key ))

#----------------------------------------
def __setattr__(sel f, key, val):
""" Update DBF record values by field name """
print "_sa: %s: %s" % (key, val)
try:
self._rec[self._fldNames. index(key.upper ())] = val
print " (DBF field assignment)"
except:
self.__dict__[key] = val # use the regular variable!
#raise AttributeError( "Unknown Field: %s" % ( key ))

#----------------------------------------
#----------------------------------------
if __name__ == "__main__":

f = test('test.dbf' )
f._del='*'
f._recno=123
f._recno=1
f._recno=2
f._recno=3
f._recno=4
f._recno=5
f._recno=6

[Greg Brunet]

Re: __getattr__ weirdness

I've figured out one thing: that be re-arranging the __init__ code
assignments (and adding one that was placed later in code I snipped
out), I am able to get the extra _ga calls to stop a lot earlier. This
code:

#----------------------------------------
def __init__(self,f ilename):
print '** __init__'
self._rec=[]
self._fldNames=[]
self._filename= filename
self._del=' '
self._dirty=Fal se
self._open=Fals e
self._recno = 1

.... by assigning _rec & _fldNames first, gets the repeated _ga calls to
stop after the first assignment following those two assignments. I'm
still at a loss as to why _ga is being called all these times to begin
with though! I'm only assigning values, not requesting them! Thanks,

--
Greg

[Stephan Diehl]

Re: __getattr__ weirdness

Greg Brunet wrote:
[color=blue]
> In adding the ability to refer to field values using dbfFile.field
> notation, I learned how to use __getattr__ and __setattr__ . After some
> trial and error, I got it working. But as part of my trials, I added
> some print statements to debug stuff. The ones I added to __setattr__
> work as expected, but the one in __getattr__ seems to get called just
> under 1000 times for every __getattr__ call!
>
> Something is obviously not right here - but I'm at a loss to understand
> what's going on. I've pared down my code to still show it happening &
> included it below. If you run this program & pipe the output to a file,
> you'll get just under 14000 debug lines. Any ideas? Thanks!
>[/color]

Within the __getattr__ method, you can't just do normal attribute access as
this will trigger another call to __getattr__, so you where stuck in an
infinite loop.
try to access the required attribute directly from the __dict__

def __getattr__(sel f, key):
""" Return DBF record values by field name """
print "_ga: " + key
try:
# return self._rec[self._fldNames. index(key.upper ())]
^^^^
return self.__dict__['_rec'][self._fldNames. index(key.upper ())]
except:
raise AttributeError( "Unknown Field: %s" % ( key ))

Hope that helps

Stephan

[Greg Brunet]

Re: __getattr__ weirdness

"Stephan Diehl" <stephan.diehl@ gmx.net> wrote in message
news:bi5ofn$uo3 $04$1@news.t-online.com...[color=blue]
> Within the __getattr__ method, you can't just do normal attribute[/color]
access as[color=blue]
> this will trigger another call to __getattr__, so you where stuck in[/color]
an[color=blue]
> infinite loop.
> try to access the required attribute directly from the __dict__
>
> def __getattr__(sel f, key):
> """ Return DBF record values by field name """
> print "_ga: " + key
> try:
> # return self._rec[self._fldNames. index(key.upper ())]
> ^^^^
> return[/color]
self.__dict__['_rec'][self._fldNames. index(key.upper ())][color=blue]
> except:
> raise AttributeError( "Unknown Field: %s" % ( key ))
>
> Hope that helps
>
> Stephan[/color]

Hey Stephan:

That did the trick! Actually, I had to do the same for self._fldNames
as well. Also, I changed them in the _sa code section instead of the
_ga code, since now that I look at it again with a little better
understanding, the original _sa code was referring to the _rec &
_fldNames variables/attributes directly, so as long as they weren't
defined, it was triggering the _ga calls. So now the _ga code doesn't
get called unless I specifically ask for it - like I would want. The
code now looks like:

#----------------------------------------
def __getattr__(sel f, key):
""" Return DBF record values by field name """
print "_ga: " + key
try:
return self._rec[self._fldNames. index(key.upper ())]
except:
raise AttributeError( "Unknown Field: %s" % ( key ))

#----------------------------------------
def __setattr__(sel f, key, val):
""" Update DBF record values by field name """
print "_sa: %s: %s" % (key, val)
try:

self.__dict__['_rec'][self.__dict__['_fldNames'].index(key.uppe r())] =
val
print " (DBF field assignment)"
except:
self.__dict__[key] = val # use the regular variable!
#raise AttributeError( "Unknown Field: %s" % ( key ))

Thanks for everyone's help!

--
Greg