Reading and Writing XML Files in Python

Extensible Markup Language, commonly known as XML is a language designed specifically to be easy to interpret by both humans and computers altogether. The language defines a set of rules used to encode a document in a specific format. In this article, methods have been described to read and write XML files in python.

Note: In general, the process of reading the data from an XML file and analyzing its logical components is known as Parsing. Therefore, when we refer to reading a xml file we are referring to parsing the XML document. 

In this article, we would take a look at two libraries that could be used for the purpose of xml parsing. They are:

• BeautifulSoup used alongside the lxml parser 
• Elementtree library. 

Using BeautifulSoup alongside with lxml parser

For the purpose of reading and writing the xml file we would be using a Python library named BeautifulSoup. In order to install the library, type the following command into the terminal.  

pip install beautifulsoup4

Beautiful Soup supports the HTML parser included in Python’s standard library, but it also supports a number of third-party Python parsers. One is the lxml parser (used for parsing XML/HTML documents). lxml could be installed by running the following command in the command processor of your Operating system:  

pip install lxml

Firstly we will learn how to read from an XML file. We would also parse data stored in it. Later we would learn how to create an XML file and write data to it. 

Reading Data From an XML File

There are two steps required to parse a xml file:-  

• Finding Tags 
• Extracting from tags

Example:

XML File used: 

from bs4 import BeautifulSoup


# Reading the data inside the xml
# file to a variable under the name 
# data
with open('dict.xml', 'r') as f:
    data = f.read()

# Passing the stored data inside
# the beautifulsoup parser, storing
# the returned object 
Bs_data = BeautifulSoup(data, "xml")

# Finding all instances of tag 
# `unique`
b_unique = Bs_data.find_all('unique')

print(b_unique)

# Using find() to extract attributes 
# of the first instance of the tag
b_name = Bs_data.find('child', {'name':'Frank'})

print(b_name)

# Extracting the data stored in a
# specific attribute of the 
# `child` tag
value = b_name.get('test')

print(value)

OUTPUT:

Writing an XML File

Writing a xml file is a primitive process, reason for that being the fact that xml files aren’t encoded in a special way. Modifying sections of a xml document requires one to parse through it at first. In the below code we would modify some sections of the aforementioned xml document. 

Example: 

from bs4 import BeautifulSoup

# Reading data from the xml file
with open('dict.xml', 'r') as f:
    data = f.read()

# Passing the data of the xml
# file to the xml parser of
# beautifulsoup
bs_data = BeautifulSoup(data, 'xml')

# A loop for replacing the value
# of attribute `test` to WHAT !!
# The tag is found by the clause
# `bs_data.find_all('child', {'name':'Frank'})`
for tag in bs_data.find_all('child', {'name':'Frank'}):
    tag['test'] = "WHAT !!"


# Output the contents of the 
# modified xml file
print(bs_data.prettify())

Output:

Using Elementtree

Elementtree module provides us with a plethora of tools for manipulating XML files. The best part about it being its inclusion in the standard Python’s built-in library. Therefore, one does not have to install any external modules for the purpose. Due to the xmlformat being an inherently hierarchical data format, it is a lot easier to represent it by a tree. The module provides ElementTree provides methods to represent whole XML document as a single tree. 

In the later examples, we would take a look at discrete methods to read and write data to and from XML files.  

Reading XML Files

To read an XML file using ElementTree, firstly, we import the ElementTree class found inside xml library, under the name ET (common convension). Then passed the filename of the xml file to the ElementTree.parse() method, to enable parsing of our xml file. Then got the root (parent tag) of our xml file using getroot(). Then displayed (printed) the root tag of our xml file (non-explicit way). Then displayed the attributes of the sub-tag of our parent tag using root[0].attrib. root[0] for the first tag of parent root and attrib for getting it’s attributes. Then we displayed the text enclosed within the 1st sub-tag of the 5th sub-tag of the tag root.

Example:  

# importing element tree
# under the alias of ET
import xml.etree.ElementTree as ET

# Passing the path of the
# xml document to enable the
# parsing process
tree = ET.parse('dict.xml')

# getting the parent tag of
# the xml document
root = tree.getroot()

# printing the root (parent) tag
# of the xml document, along with
# its memory location
print(root)

# printing the attributes of the
# first tag from the parent 
print(root[0].attrib)

# printing the text contained within
# first subtag of the 5th tag from
# the parent
print(root[5][0].text)

Output:

Writing XML Files

Now, we would take a look at some methods which could be used to write data on an xml document. In this example we would create a xml file from scratch. 

To do the same, firstly, we create a root (parent) tag under the name of chess using the command ET.Element(‘chess’). All the tags would fall underneath this tag, i.e. once a root tag has been defined, other sub-elements could be created underneath it. Then we created a subtag/subelement named Opening inside the chess tag using the command ET.SubElement(). Then we created two more subtags which are underneath the tag Opening named E4 and D4. Then we added attributes to the E4 and D4 tags using set() which is a method found inside SubElement(), which is used to define attributes to a tag. Then we added text between the E4 and D4 tags using the attribute text found inside the SubElement function. In the end we converted the datatype of the contents we were creating from ‘xml.etree.ElementTree.Element’ to bytes object, using the command ET.tostring() (even though the function name is tostring() in certain implementations it converts the datatype to `bytes` rather than `str`). Finally, we flushed the data to a file named gameofsquares.xml which is a opened in `wb` mode to allow writing binary data to it. In the end, we saved the data to our file.

Example:  

import xml.etree.ElementTree as ET

# This is the parent (root) tag 
# onto which other tags would be
# created
data = ET.Element('chess')

# Adding a subtag named `Opening`
# inside our root tag
element1 = ET.SubElement(data, 'Opening')

# Adding subtags under the `Opening`
# subtag 
s_elem1 = ET.SubElement(element1, 'E4')
s_elem2 = ET.SubElement(element1, 'D4')

# Adding attributes to the tags under
# `items`
s_elem1.set('type', 'Accepted')
s_elem2.set('type', 'Declined')

# Adding text between the `E4` and `D5` 
# subtag
s_elem1.text = "King's Gambit Accepted"
s_elem2.text = "Queen's Gambit Declined"

# Converting the xml data to byte object,
# for allowing flushing data to file 
# stream
b_xml = ET.tostring(data)

# Opening a file under the name `items2.xml`,
# with operation mode `wb` (write + binary)
with open("GFG.xml", "wb") as f:
    f.write(b_xml)

Output: