Try Liza Daly's fast_iter. After processing an element, elem
, it calls elem.clear()
to remove descendants and also removes preceding siblings.
def fast_iter(context, func, *args, **kwargs):
"""
http://lxml.de/parsing.html#modifying-the-tree
Based on Liza Daly's fast_iter
http://www.ibm.com/developerworks/xml/library/x-hiperfparse/
See also http://effbot.org/zone/element-iterparse.htm
"""
for event, elem in context:
func(elem, *args, **kwargs)
# It's safe to call clear() here because no descendants will be
# accessed
elem.clear()
# Also eliminate now-empty references from the root node to elem
for ancestor in elem.xpath('ancestor-or-self::*'):
while ancestor.getprevious() is not None:
del ancestor.getparent()[0]
del context
def process_element(elem):
print elem.xpath( 'description/text( )' )
context = etree.iterparse( MYFILE, tag='item' )
fast_iter(context,process_element)
Daly's article is an excellent read, especially if you are processing large XML files.
Edit: The fast_iter
posted above is a modified version of Daly's fast_iter
. After processing an element, it is more aggressive at removing other elements that are no longer needed.
The script below shows the difference in behavior. Note in particular that orig_fast_iter
does not delete the A1
element, while the mod_fast_iter
does delete it, thus saving more memory.
import lxml.etree as ET
import textwrap
import io
def setup_ABC():
content = textwrap.dedent('''\
<root>
<A1>
<B1></B1>
<C>1<D1></D1></C>
<E1></E1>
</A1>
<A2>
<B2></B2>
<C>2<D></D></C>
<E2></E2>
</A2>
</root>
''')
return content
def study_fast_iter():
def orig_fast_iter(context, func, *args, **kwargs):
for event, elem in context:
print('Processing {e}'.format(e=ET.tostring(elem)))
func(elem, *args, **kwargs)
print('Clearing {e}'.format(e=ET.tostring(elem)))
elem.clear()
while elem.getprevious() is not None:
print('Deleting {p}'.format(
p=(elem.getparent()[0]).tag))
del elem.getparent()[0]
del context
def mod_fast_iter(context, func, *args, **kwargs):
"""
http://www.ibm.com/developerworks/xml/library/x-hiperfparse/
Author: Liza Daly
See also http://effbot.org/zone/element-iterparse.htm
"""
for event, elem in context:
print('Processing {e}'.format(e=ET.tostring(elem)))
func(elem, *args, **kwargs)
# It's safe to call clear() here because no descendants will be
# accessed
print('Clearing {e}'.format(e=ET.tostring(elem)))
elem.clear()
# Also eliminate now-empty references from the root node to elem
for ancestor in elem.xpath('ancestor-or-self::*'):
print('Checking ancestor: {a}'.format(a=ancestor.tag))
while ancestor.getprevious() is not None:
print(
'Deleting {p}'.format(p=(ancestor.getparent()[0]).tag))
del ancestor.getparent()[0]
del context
content = setup_ABC()
context = ET.iterparse(io.BytesIO(content), events=('end', ), tag='C')
orig_fast_iter(context, lambda elem: None)
# Processing <C>1<D1/></C>
# Clearing <C>1<D1/></C>
# Deleting B1
# Processing <C>2<D/></C>
# Clearing <C>2<D/></C>
# Deleting B2
print('-' * 80)
"""
The improved fast_iter deletes A1. The original fast_iter does not.
"""
content = setup_ABC()
context = ET.iterparse(io.BytesIO(content), events=('end', ), tag='C')
mod_fast_iter(context, lambda elem: None)
# Processing <C>1<D1/></C>
# Clearing <C>1<D1/></C>
# Checking ancestor: root
# Checking ancestor: A1
# Checking ancestor: C
# Deleting B1
# Processing <C>2<D/></C>
# Clearing <C>2<D/></C>
# Checking ancestor: root
# Checking ancestor: A2
# Deleting A1
# Checking ancestor: C
# Deleting B2
study_fast_iter()