<SYSTEM_TASK:>
Given the following text description, write Python code to implement the functionality described below step by step
<END_TASK>
<USER_TASK:>
Description:
 Example xml file
Step1:  Traversing the parsed tree
To visit all the children in order, user iter() to create a generator that iterates over the ElementTree instance.
Step2:  To print only the groups of names and feed URL for the podcasts, leaving out all of the data in the header section by iterating over only the outline nodes and print the text and xmlURL attributes by looking up the values in the attrib dictionary
Step3:  Finding Nodes in a Documents
Walking the entire tree like this, searching for relevant nodes, can be error prone. The previous example had to look at each outline node to determine if it was a group (nodes with only a text attribute) or podcast (with both text and xmlUrl). To produce a simple list of the podcast feed URLs, without names or groups, the logic could be simplified using findall() to look for nodes with more descriptive search characteristics.
As a first pass at converting the first version, an XPath argument can be used to look for all outline nodes.
Step4:  It is possible to take advantage of the fact that the outline nodes are only nested two levels deep. Changing the search path to .//outline/outline means the loop will process only the second level of outline nodes.
Step5:  Parsed Node Attributes
The items returned by findall() and iter() are Element objects, each representing a node in the XML parse tree. Each Element has attributes for accessing data pulled out of the XML. This can be illustrated with a somewhat more contrived example input file, data.xml.
```
<?xml version="1.0" encoding="UTF-8"?>
<top>
  <child>Regular text.</child>
  <child_with_tail>Regular text.</child_with_tail>"Tail" text.
  <with_attributes name="value" foo="bar" />
  <entity_expansion attribute="This &#38; That">
    That &#38; This
  </entity_expansion>
</top>
```
Step6:  The text content of the nodes is available, along with the tail text, which comes after the end of a close tag.
Step7:  XML entity references embedded in the document are converted to the appropriate characters before values are returned.
Step8:  Watching Events While Parsing
The other API for processing XML documents is event-based. The parser generates start events for opening tags and end events for closing tags. Data can be extracted from the document during the parsing phase by iterating over the event stream, which is convenient if it is not necessary to manipulate the entire document afterwards and there is no need to hold the entire parsed document in memory.
Events can be one of
Step9:  The event-style of processing is more natural for some operations, such as converting XML input to some other format. This technique can be used to convert list of podcasts from the earlier examples from an XML file to a CSV file, so they can be loaded into a spreadsheet or database application.
Step10:  Parsing Strings
To work with smaller bits of XML text, especially string literals that might be embedded in the source of a program, use XML() and the string containing the XML to be parsed as the only argument.
Step11:  For structured XML that uses the id attribute to identify unique nodes of interest, XMLID() is a convenient way to access the parse results.
XMLID() returns the parsed tree as an Element object, along with a dictionary mapping the id attribute strings to the individual nodes in the tree.
<ASSISTANT_TASK:>
Python Code:
from xml.etree import ElementTree
with open('podcasts.opml', 'rt') as f:
    tree = ElementTree.parse(f)
    
print(tree)
Explanation: Example xml file:
```
<?xml version="1.0" encoding="UTF-8"?>
<opml version="1.0">
<head>
    <title>My Podcasts</title>
    <dateCreated>Sat, 06 Aug 2016 15:53:26 GMT</dateCreated>
    <dateModified>Sat, 06 Aug 2016 15:53:26 GMT</dateModified>
</head>
<body>
  <outline text="Non-tech">
    <outline
        text="99% Invisible" type="rss"
        xmlUrl="http://feeds.99percentinvisible.org/99percentinvisible"
        htmlUrl="http://99percentinvisible.org" />
  </outline>
  <outline text="Python">
    <outline
        text="Talk Python to Me" type="rss"
        xmlUrl="https://talkpython.fm/episodes/rss"
        htmlUrl="https://talkpython.fm" />
    <outline
        text="Podcast.__init__" type="rss"
        xmlUrl="http://podcastinit.podbean.com/feed/"
        htmlUrl="http://podcastinit.com" />
  </outline>
</body>
</opml>
```
To parse the file, pass an open file handle to parse()
End of explanation
from xml.etree import ElementTree
import pprint
with open('podcasts.opml', 'rt') as f:
    tree = ElementTree.parse(f)
    
for node in tree.iter():
    print(node.tag)
Explanation: Traversing the parsed tree
To visit all the children in order, user iter() to create a generator that iterates over the ElementTree instance.
End of explanation
from xml.etree import ElementTree
with open('podcasts.opml', 'rt') as f:
    tree = ElementTree.parse(f)
for node in tree.iter('outline'):
    name = node.attrib.get('text')
    url = node.attrib.get('xmlUrl')
    if name and url:
        print('  %s' % name)
        print('    %s' % url)
    else:
        print(name)
Explanation: To print only the groups of names and feed URL for the podcasts, leaving out all of the data in the header section by iterating over only the outline nodes and print the text and xmlURL attributes by looking up the values in the attrib dictionary
End of explanation
from xml.etree import ElementTree
with open('podcasts.opml', 'rt') as f:
    tree = ElementTree.parse(f)
for node in tree.findall('.//outline'):
    url = node.attrib.get('xmlUrl')
    if url:
        print(url)
Explanation: Finding Nodes in a Documents
Walking the entire tree like this, searching for relevant nodes, can be error prone. The previous example had to look at each outline node to determine if it was a group (nodes with only a text attribute) or podcast (with both text and xmlUrl). To produce a simple list of the podcast feed URLs, without names or groups, the logic could be simplified using findall() to look for nodes with more descriptive search characteristics.
As a first pass at converting the first version, an XPath argument can be used to look for all outline nodes.
End of explanation
from xml.etree import ElementTree
with open('podcasts.opml', 'rt') as f:
    tree = ElementTree.parse(f)
for node in tree.findall('.//outline/outline'):
    url = node.attrib.get('xmlUrl')
    print(url)
Explanation: It is possible to take advantage of the fact that the outline nodes are only nested two levels deep. Changing the search path to .//outline/outline means the loop will process only the second level of outline nodes.
End of explanation
from xml.etree import ElementTree
with open('data.xml', 'rt') as f:
    tree = ElementTree.parse(f)
node = tree.find('./with_attributes')
print(node.tag)
for name, value in sorted(node.attrib.items()):
    print('  %-4s = "%s"' % (name, value))
Explanation: Parsed Node Attributes
The items returned by findall() and iter() are Element objects, each representing a node in the XML parse tree. Each Element has attributes for accessing data pulled out of the XML. This can be illustrated with a somewhat more contrived example input file, data.xml.
```
<?xml version="1.0" encoding="UTF-8"?>
<top>
  <child>Regular text.</child>
  <child_with_tail>Regular text.</child_with_tail>"Tail" text.
  <with_attributes name="value" foo="bar" />
  <entity_expansion attribute="This &#38; That">
    That &#38; This
  </entity_expansion>
</top>
```
End of explanation
from xml.etree import ElementTree
with open('data.xml', 'rt') as f:
    tree = ElementTree.parse(f)
for path in ['./child', './child_with_tail']:
    node = tree.find(path)
    print(node.tag)
    print('  child node text:', node.text)
    print('  and tail text  :', node.tail)
Explanation: The text content of the nodes is available, along with the tail text, which comes after the end of a close tag.
End of explanation
from xml.etree import ElementTree
with open('data.xml', 'rt') as f:
    tree = ElementTree.parse(f)
node = tree.find('entity_expansion')
print(node.tag)
print('  in attribute:', node.attrib['attribute'])
print('  in text     :', node.text.strip())
Explanation: XML entity references embedded in the document are converted to the appropriate characters before values are returned.
End of explanation
from xml.etree.ElementTree import iterparse
depth = 0
prefix_width = 8
prefix_dots = '.' * prefix_width
line_template = ''.join([
    '{prefix:<0.{prefix_len}}',
    '{event:<8}',
    '{suffix:<{suffix_len}} ',
    '{node.tag:<12} ',
    '{node_id}',
])
EVENT_NAMES = ['start', 'end', 'start-ns', 'end-ns']
for (event, node) in iterparse('podcasts.opml', EVENT_NAMES):
    if event == 'end':
        depth -= 1
    prefix_len = depth * 2
    print(line_template.format(
        prefix=prefix_dots,
        prefix_len=prefix_len,
        suffix='',
        suffix_len=(prefix_width - prefix_len),
        node=node,
        node_id=id(node),
        event=event,
    ))
    if event == 'start':
        depth += 1
Explanation: Watching Events While Parsing
The other API for processing XML documents is event-based. The parser generates start events for opening tags and end events for closing tags. Data can be extracted from the document during the parsing phase by iterating over the event stream, which is convenient if it is not necessary to manipulate the entire document afterwards and there is no need to hold the entire parsed document in memory.
Events can be one of:
start  
    A new tag has been encountered. The closing angle bracket of the tag was processed, but not the contents.
end  
     The closing angle bracket of a closing tag has been processed. All of the children were already processed.
start-ns  
    Start a namespace declaration.
end-ns  
    End a namespace declaration.
End of explanation
import csv
from xml.etree.ElementTree import iterparse
import sys
writer = csv.writer(sys.stdout, quoting=csv.QUOTE_NONNUMERIC)
group_name = ''
parsing = iterparse('podcasts.opml', events=['start'])
for (event, node) in parsing:
    if node.tag != 'outline':
        # Ignore anything not part of the outline
        continue
    if not node.attrib.get('xmlUrl'):
        # Remember the current group
        group_name = node.attrib['text']
    else:
        # Output a podcast entry
        writer.writerow(
            (group_name, node.attrib['text'],
             node.attrib['xmlUrl'],
             node.attrib.get('htmlUrl', ''))
        )
Explanation: The event-style of processing is more natural for some operations, such as converting XML input to some other format. This technique can be used to convert list of podcasts from the earlier examples from an XML file to a CSV file, so they can be loaded into a spreadsheet or database application.
End of explanation
from xml.etree.ElementTree import XML
def show_node(node):
    print(node.tag)
    if node.text is not None and node.text.strip():
        print('  text: "%s"' % node.text)
    if node.tail is not None and node.tail.strip():
        print('  tail: "%s"' % node.tail)
    for name, value in sorted(node.attrib.items()):
        print('  %-4s = "%s"' % (name, value))
    for child in node:
        show_node(child)
parsed = XML('''
<root>
  <group>
    <child id="a">This is child "a".</child>
    <child id="b">This is child "b".</child>
  </group>
  <group>
    <child id="c">This is child "c".</child>
  </group>
</root>
''')
print('parsed =', parsed)
for elem in parsed:
    show_node(elem)
Explanation: Parsing Strings
To work with smaller bits of XML text, especially string literals that might be embedded in the source of a program, use XML() and the string containing the XML to be parsed as the only argument.
End of explanation
from xml.etree.ElementTree import XMLID
tree, id_map = XMLID('''
<root>
  <group>
    <child id="a">This is child "a".</child>
    <child id="b">This is child "b".</child>
  </group>
  <group>
    <child id="c">This is child "c".</child>
  </group>
</root>
''')
for key, value in sorted(id_map.items()):
    print('%s = %s' % (key, value))
Explanation: For structured XML that uses the id attribute to identify unique nodes of interest, XMLID() is a convenient way to access the parse results.
XMLID() returns the parsed tree as an Element object, along with a dictionary mapping the id attribute strings to the individual nodes in the tree.
End of explanation
<END_TASK>