XML ElementTree Python: Find all the relations of a node

You can select elements based on an attribute via [@attrib='value'] according to the official documentation. When testing this i exchanged your tag <XML Data> and </XML Data> with <Data> and </Data>. Example code:

from xml.etree import ElementTree as ET

data = ET.parse(r"/path/to/your/input.xml")
root = data.getroot()
for product in root.findall("./Record/Service/Product[@id='A']"):
    print(product.attrib["id"])
    print(product.text)

Edit

After reading your question again i noticed that you first want to check whether a product with id A exists within a Service, and only then store the IDs (uniquely & sorted), so i adapted the code:

from xml.etree import ElementTree as ET

data = ET.parse(r"/path/to/your/input.xml")
root = data.getroot()
product_ids = set()
for service in root.findall("./Record/Service"):
    list_contains_a = False

    # iterate once to identify if list contains product with ID = 'A'
    for product in service.findall("./Product"):
        if product.attrib["id"] == "A":
            list_contains_a = True

    # if list contains product with ID = 'A', iterate second time and fetch IDs
    if list_contains_a:
        for product in service.findall("./Product"):
            if product.attrib["id"] == "A":
                continue

            # add to set to prevent duplicates
            product_ids.add(product.attrib["id"])

ret_list = ["ServiceID"] + list(sorted(product_ids))
print(ret_list)