Merging Lots of XML files

Question

I have lots of xml files that I need to merge. I have tried this link at merging xml files using python's ElementTree whose code is (Edited as per my need):

import os, os.path, sys
import glob
from xml.etree import ElementTree

def run(files):
    xml_files = glob.glob(files +"/*.xml")
    xml_element_tree = None
    for xml_file in xml_files:
        print xml_file
        data = ElementTree.parse(xml_file).getroot()
        # print ElementTree.tostring(data)
        for result in data.iter('TALLYMESSAGE'):
            if xml_element_tree is None:
                xml_element_tree = data 
                insertion_point = xml_element_tree.findall("./BODY/DATA/TALLYMESSAGE")[0]
            else:
                insertion_point.extend(result) 
    if xml_element_tree is not None:
        f =  open("myxmlfile.xml", "wb")
        f.write(ElementTree.tostring(xml_element_tree))
run("F:/data/data")

But the problem is that I have lots of XML file, 365 to be precise and each one is atleast 2 mb. merging them all has lead to crashing of my PC. This is the image of the xml tree of my xml file:

My new updated code is:

import os, os.path, sys
import glob
from lxml import etree
def XSLFILE(files):
    xml_files = glob.glob(files +"/*.xml")
    #print xml_files[0]
    xslstring = """<?xml version="1.0" ?> 
<xsl:transform xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0"> 
<xsl:template match="/DATA">
<DATA>
<xsl:copy>
<xsl:copy-of select="TALLYMESSAGE"/>\n"""
    #print xslstring
    for xmlfile in xml_files[1:]:
        xslstring = xslstring + '<xsl:copy-of select="document(\'' + xmlfile[-16:] + "')/BODY/DATA/TALLYMESSAGE\"/>\n"
    xslstring = xslstring + """</xsl:copy>+
</DATA>
</xsl:template> 
</xsl:transform>"""
    #print xslstring
    with open("parsingxsl.xsl", "w") as f:
        f.write(xslstring)
    with open(xml_files[0], "r") as f:
        dom = etree.XML(f.read())
    print etree.tostring(dom)
    with open('F:\data\parsingxsl.xsl', "r") as f:
        xslt_tree = etree.XML(f.read())
    print xslt_tree
    transform = etree.XSLT(xslt_tree)
    newdom = transform(dom)
    #print newdom
    tree_out = etree.tostring(newdom, encoding='UTF-8', pretty_print=True,  xml_declaration=True)
    print(tree_out)

    xmlfile = open('F:\data\OutputFile.xml','wb')
    xmlfile.write(tree_out)
    xmlfile.close()
XSLFILE("F:\data\data")

The same when run creates the following error:

Traceback (most recent call last):
  File "F:\data\xmlmergexsl.py", line 38, in <module>
    XSLFILE("F:\data\data")
  File "F:\data\xmlmergexsl.py", line 36, in XSLFILE
    xmlfile.write(tree_out)
TypeError: must be string or buffer, not None

Answer 1

Consider using XSLT and its document() function to merge XML files. Python (like many object-oriented programming languages) maintain an XSLT processor like in its lxml module. As information, XSLT is a declarative programming language to transform XML files in various formats and structures.

For your purposes, XSLT may be more efficient than using programming code to develop files as no lists or loops or other objects are held in memory during processing except what the XSLT processor would use.

XSLT (to be saved externally as .xsl file)

Consider initially running a Python write to text file looping to fill in all 365 documents to avoid copy and paste. Also notice first document is skipped since it is the starting point used in Python script below:

<?xml version="1.0" ?> 
<xsl:transform xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0"> 

 <xsl:template match="DATA">
  <DATA>
    <xsl:copy> 
       <xsl:copy-of select="TALLYMESSAGE"/>
       <xsl:copy-of select="document('Document2.xml')/BODY/DATA/TALLYMESSAGE"/>
       <xsl:copy-of select="document('Document3.xml')/BODY/DATA/TALLYMESSAGE"/>
       <xsl:copy-of select="document('Document4.xml')/BODY/DATA/TALLYMESSAGE"/>
       ...
       <xsl:copy-of select="document('Document365.xml')/BODY/DATA/TALLYMESSAGE"/>             
    </xsl:copy>
  </DATA>
 </xsl:template> 

</xsl:transform>

Python (to be included in you overall script)

import lxml.etree as ET

dom = ET.parse('C:\Path\To\XML\Document1.xml')
xslt = ET.parse('C:\Path\To\XSL\file.xsl')
transform = ET.XSLT(xslt)
newdom = transform(dom)

tree_out = ET.tostring(newdom, encoding='UTF-8', pretty_print=True,  xml_declaration=True)
print(tree_out)

xmlfile = open('C:\Path\To\XML\OutputFile.xml','wb')
xmlfile.write(tree_out)
xmlfile.close()