使用lxml.objectify测试元素的存在

Question

What is the standard way for testing if an element exists or not with lxml.objectify ?

Sample XML :

<?xml version="1.0" encoding="utf-8"?>
<Test>
  <MyElement1>sdfsdfdsfd</MyElement1>
</Test>

Code

from lxml import etree, objectify
with open('config.xml') as f:
    xml = f.read()
root = objectify.fromstring(xml)

print root.MyElement1
print root.MyElement17  # AttributeError: no such child: MyElement17

---

Then, what is the simplest solution to write something on a specific path ?

root.MyElement1.Blah = 'New'  # this works because MyElement1 already exists
root.MyElement17.Blah = 'New'  # this doesn't work because MyElement17 doesn't exist
root.MyElement1.Foo.Bar = 'Hello' # this doesn't as well... How to do this shortly ?

Answer 1

find method will return None if the element does not exist.

>>> xml = '''<?xml version="1.0" encoding="utf-8"?>
... <Test>
...   <MyElement1>sdfsdfdsfd</MyElement1>
... </Test>'''
>>>
>>> from lxml import objectify
>>> root = objectify.fromstring(xml)
>>> root.find('.//MyElement1')
'sdfsdfdsfd'
>>> root.find('.//MyElement17')
>>> root.find('.//MyElement17') is None
True

UPDATE according to the question edit:

>>> from lxml import objectify
>>>
>>> def add_string(parent, attr, s):
...     if len(attr) == 1:
...         setattr(parent, attr[0], s)
...     else:
...         child = getattr(parent, attr[0], None)
...         if child is None:
...             child = objectify.SubElement(parent, attr[0])
...         add_string(child, attr[1:], s)
...
>>> root = objectify.fromstring(xml)
>>> add_string(root, ['MyElement1', 'Blah'], 'New')
>>> add_string(root, ['MyElement17', 'Blah'], 'New')
>>> add_string(root, ['MyElement1', 'Foo', 'Bar'], 'Hello')
>>>
>>> root.MyElement1.Blah
'New'
>>> root.MyElement17.Blah
'New'
>>> root.MyElement1.Foo.Bar
'Hello'

Answer 2

You can use getattr :

if getattr(root, 'MyElement17', None):
     # do something