Concepts / Working with Python Lists and For Loops

Working with Python Lists and For Loops

The findall method retrieves a list of XML nodes using a path that must include all parent-level elements (except the root) separated by forward slashes.

  • Programming

A List of XML Nodes

When XML data is nested, finding one kind of element often begins by retrieving several matching nodes at once. Python's findall method returns those matching nodes as a list. A for loop can then visit each node in order, allowing you to extract the child-element text and attributes belonging to that node.

Tracing Each Loop Iteration

The for loop takes one item from the list at a time. On the first iteration, the loop variable item refers to the first user node. After that iteration finishes, item refers to the second user node. When every node in the list has been processed, the loop stops.

lsttwo user nodesuserChuck, 001, x=2userBrent, 009, x=7endlist exhaustedfirst itemnext itemno items remain
How does the loop move through the list, and when does it stop?
Code walkthrough
for item in lst:
    name = item.find('name').text
    user_id = item.find('id').text
    attribute_x = item.get('x')
    print(name, user_id, attribute_x)
  1. 1
    The first iteration assigns the first user node to item.
  2. 2
    The first node provides the child text Chuck, the child text 001, and the attribute value 2.
  3. 3
    The second iteration assigns the second user node to item.
  4. 4
    The second node provides the child text Brent, the child text 009, and the attribute value 7.
  5. 5
    The list is then exhausted, so the loop ends.
  • The loop assigns each user node to item in document order.
  • find('name') locates the name child of the current user node.
  • The .text property extracts Chuck or Brent from that child.
  • find('id').text extracts 001 or 009.
  • get('x') retrieves the x attribute from the current user node.
Output
Chuck 001 2
Brent 009 7

Reading Children and Attributes

A current XML node can contain both child elements and attributes, but they are accessed differently. For child-element content, use find with the child name and then read that element's .text property. For an attribute attached directly to the current node, use get with the attribute name.

userx=2nameChuckid001x2find then .textfind then .textget
Which parts of the current user node are read with find and which part is read with get?
XML dataAccess patternExtracted value
<name>Chuck</name>item.find('name').textChuck
<id>001</id>item.find('id').text001
<user x="2">item.get('x')2

Child-element text and node attributes use different access methods.

Diagnosing Empty Results

A path mistake in findall can be difficult to notice because the method can return an empty list without producing an error. If the list is empty, the for loop has no items to process, so its body never runs and no output appears.

userparent omittedusers/userparent includedusers/memberelement name differsusers/usertarget name matchesusers//userextra slashusers/usersingle slashinclude usersuse target nameuse required separators
What changes when the path omits the parent, names the wrong element, or does not follow the required slash-separated structure?
  • Common mistake

    Omitting the parent element from the path

    What it looks like
    stuff.findall('user')
    Why it fails

    The user elements are inside users, so user is not a direct child of the current root node. The result is an empty list and the loop does not execute.

    Do this instead
    Use stuff.findall('users/user').
  • Common mistake

    Using the parent name without the target element

    What it looks like
    stuff.findall('users')
    Why it fails

    This path identifies the users element rather than the user elements that the loop is intended to process.

    Do this instead

    Include the target level with users/user.

  • Common mistake

    Using an element name that does not match the XML structure

    What it looks like
    stuff.findall('users/member')
    Why it fails

    The path must describe the names of the parent and target elements in the XML tree.

    Do this instead

    Check the XML names and use users/user.

  • Common mistake

    Adding an extra slash to the path

    What it looks like
    stuff.findall('users//user')
    Why it fails

    The described path format uses the parent-level element names separated by forward slashes, as in users/user.

    Do this instead

    Write the path with the required single separator between the listed levels.

Before
lst = stuff.findall('user')
for item in lst:
    print(item.find('name').text)

lst is empty, so the loop body produces no output.

After
lst = stuff.findall('users/user')
for item in lst:
    print(item.find('name').text)

lst contains the user nodes, so the loop processes Chuck and Brent.

Practice the Extraction

Worked example

Trace the Two Users

Given a list created with stuff.findall('users/user'), determine the values extracted during each loop iteration.

  1. 1
    First node

    The first user node contains name text Chuck, id text 001, and attribute x with value 2.

  2. 2
    Second node

    The second user node contains name text Brent, id text 009, and attribute x with value 7.

  3. 3
    Loop completion

    After the second node, every item in the list has been visited, so the loop ends.

Result

The loop extracts Chuck, 001, 2 and then Brent, 009, 7.

EASY

A root node contains a users element, and users contains user elements. Write the findall path that retrieves the user nodes, then state which method retrieves the name text and which method retrieves the x attribute.

Hints
  • Include each parent-level element between the current node and user.
  • Child-element content uses find followed by .text.
  • An attribute attached to the current node uses get.

Key Takeaways

  1. 1
    findall returns a list of matching XML nodes.
  2. 2
    The path must include the parent-level element names, such as users/user, while excluding the root name.
  3. 3
    A for loop visits each returned node once and in document order.
  4. 4
    Use find followed by .text for child-element content.
  5. 5
    Use get for attributes attached directly to the current node.
  6. 6
    An incorrect path can produce an empty list silently, causing the loop body not to run.

At a glance

  • findall retrieves a list of XML nodes by following a slash-separated path through the tree.
  • A for loop processes those nodes one at a time in document order.
  • Child-element text is extracted with find and .text, while attributes are retrieved with get.
  • A missing parent in the path can silently create an empty list and prevent the loop from running.

Related Topics

Related concepts

More concepts starting with W