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pythonxmllxmlelementtreedeep-copy

Can't deepcopy and append etree element at same time

I'm hoping someone can explain this behavior, it gave me quite a headache trying to figure out what was going wrong with my code.

Say we set up some simple etree elements like so

from copy import deepcopy
from lxml import etree
elem1=etree.Element('e1')
elem2=etree.Element('e2')

If I do this,

elem_copy=deepcopy(elem1).append(elem2)

elem_copy comes out as NoneType

However, if I just break out the steps like this

elem_copy=deepcopy(elem1)
elem_copy.append(elem2)

I get the expected behavior with a new element in elem_copy and elem2 as a child element.

Can anyone explain why this is?

Solution

  • In the first case

    elem_copy=deepcopy(elem1).append(elem2)

    the result of append i.e. None is getting assigned back to elem_copy

    In the second case

    elem_copy=deepcopy(elem1)
elem_copy.append(elem2)

    the result of append is not getting assigned back to elem_copy. As a result of this, elem_copy has the element returned by deepcopy with the second element appended to it.

    Hope that helps.