lxml_3.py

# to find all unitid elements whose parent is a did tag:
tree.xpath("//did/unitid")

# using an absolute path to find exact locations:
tree.xpath("/ead/archdesc/did/physdesc/extent")

## if there are multiple "extent" tags in the parent physdesc, 
## you can find specific tags by designating an index
## unlike any other language ever, xpath indexes start at 1, not zero

# returns a list containing the first extent in the parent physdesc
tree.xpath("/ead/archdesc/did/physdesc/extent[1]")

## You can also facet by tag attributes:

# finding all extent tags with an encodinganalog attribute
tree.xpath("//extent[@encodinganalog])

# finding all container tags with a type attribute whose value is "box"
tree.xpath("//container[@type='box']")