check existence of object

Category: Python(Script)

#to check to see if id exists in current folder:
myId = 'id to check for'

#in Script(Python) this will check current folder,
#as well as Acquired objects

if hasattr(context, myId):
    return getattr(context, myId)

#another way is to use the default argument of getattr
return getattr(context, myId, 0)

#in a DTML Method you can
<dtml-if "_.hasattr(this(), myId )"> <dtml-var myId> exists
<dtml-else> doesnt exist
</dtml-if>

#or
<dtml-if "_.getattr(this(), myId, 0)"> <dtml-var myId> exists
<dtml-else> nope
</dtml-if>

#or simply
<dtml-if "myId in objectIds()"> in objectIds()
<dtml-else> not in objectIds()
</dtml-if>


#in a DTML Document
<dtml-if "_.getattr(this().aq_parent, myId, 0)"> <dtml-var myId> exists
<dtml-else> doesnt exist
</dtml-if>

#using objectIds
<dtml-if "myId in this().aq_parent.objectIds()"> in objectIds()
<dtml-else> not in objectIds()
</dtml-if>

#in PageTemplates, NOTE: you are returning the *actual* instance
<span tal:replace="python:test(hasattr(here, myId), getattr(here, myId), 'object not found')" />

# EXPLICIT: 
# getattr/hasattr checks acquistion, if you want to explictly
# check a folder if it has a object you .aq_explicit

# Script(Python)
f = context.myFolder
if getattr(f.aq_explicit, myId, 0): return myId + ' exists!'

#DTML
<dtml-if "_.hasattr(this().aq_explicit, myId)"> <dtml-var myId> exists
<dtml-else> doesnt exist
</dtml-if>

#if you want to check the parent folder you can, go up the acqusition
#chain using aq_parent

f = context.myFolder
if getattr(f.aq_parent.aq_explicit, myId, 0): return 'exists'
return 'nope'

#this will check explicitly myFolder's parent directory for myId


#we can also use ZDOM
f = context.myFolder
if getattr(f.getParentNode().aq_explicit, myId, 0): return 'exists'
return 'nope'

#if you want to programatically acquire lets say, a Property from the Acquistion
#chain you can use 
propertyValue=object.aq_acquire('propertyId')

Confusion by efge - 2004-10-14 This is a bit confused, _.hasattr(this(), myId) has *nothing to do* with myId in objectIds(), as the latter does not use acquisition, and only looks at "contained" objects, not other methods or attributes.

Re: Confusion by runyaga - 2004-10-14 works just fine for me. this() is the current context and includes acquistion in its lookup. I dont see the problem.   Re: Confusion by efge - 2004-10-14 Well for instance suppose you want to test the presence of "foo" in the current folder. Also suppose it's not there **but** there is another "foo" higher in the acquisition chain. Then "foo" in objectIds() will return the correct result (false), whereas _.hasattr(this(), "foo") will return true, which is not what we want.   Re: Re: Confusion by runyaga - 2004-11-13 your absolutely correct! _.hasattr(this().aq_expilict, 'foo') the aq_explicit (as shown above) would check the existence of 'foo' as a object in the current folder (if run in a DTML Method) or if the property 'foo' if this() was a DTML Document. ~runyaga   Re: Confusion by efge - 2004-11-13 Actually no, aq_explicit doesn't work like that. The recipe is incorrect. You have to use ob.aq_inner.aq_explicit. This came up again recently, see http://mail.zope.org/pipermail/zope/2004-October/154155.html or more detailed http://mail.zope.org/pipermail/zope/2000-December/122978.html

This works for DTML and is easier to read...
<dtml-if "myDocument.hasProperty('myProperty')">yep</dtml-if>