Check if object path exists in tree of objects

Question

I have a tree of objects and I need to check that particular object contains specific branch of objects. For example:

def specificNodeHasTitle(specificNode):
    # something like this
    return specificNode.parent.parent.parent.header.title != None

Is there an elegant way to do this without throwing exception if needed attribute is missing?

Answer 1

Use try..except :

def specificNodeHasTitle(specificNode):
    try:
        return specificNode.parent.parent.parent.header.title is not None
    except AttributeError:
        # handle exception, for example
        return False

There is nothing wrong with raising exceptions, by the way. It is a normal part of Python programming. Using try..except is the way to handle them.

Answer 2

This works as long as you don't need indexes of arrays in your path to the item.

def getIn(d, arraypath, default=None):
    if not d:
        return d 
    if not arraypath:
        return d
    else:
        return getIn(d.get(arraypath[0]), arraypath[1:], default) \
            if d.get(arraypath[0]) else default

getIn(specificNode,["parent", "parent", "parent", "header", "title"]) is not None

Answer 3

For your specific case, the solution provided by unutbu is the best and the most pythonic, but I can't help trying to show the great capabilities of python and its getattr method:

#!/usr/bin/env python
# https://stackoverflow.com/questions/22864932/python-check-if-object-path-exists-in-tree-of-objects

class A(object):
  pass

class Header(object):
  def __init__(self):
    self.title = "Hello"

specificNode=A()
specificNode.parent = A()
specificNode.parent.parent = A()
specificNode.parent.parent.parent = A()
specificNode.parent.parent.parent.header = Header()

hierarchy1="parent.parent.parent.header.title"
hierarchy2="parent.parent.parent.parent.header.title"

tmp = specificNode
for attr in hierarchy1.split('.'):
  try:
    tmp = getattr(tmp, attr)
  except AttributeError:
    print "Ouch... nopes"
    break
else:
  print "Yeeeps. %s" % tmp


tmp = specificNode
for attr in hierarchy2.split('.'):
  try:
    tmp = getattr(tmp, attr)
  except AttributeError:
    print "Ouch... nopes"
    break
else:
  print "Yeeeps. %s" % tmp

That outputs:

Yeeeps. Hello
Ouch... nopes

Python's great :)