How do you represent an explicit null in XML, and what does the schema need?

Use xsi:nil="true" on the element (with the XML Schema instance namespace), and declare the element nillable="true" in the XSD.

Why does an xs:decimal element reject ?

An empty string isn't in the lexical space of xs:decimal, so validation fails, which catches 'empty' values early instead of silently treating them as zero.

The Discount That Became Zero — XML in Production

Scenario

Example team: a pricing service receives product updates. Each product may have <discount>: a percentage to apply. A partner's new system starts sending <discount/> (empty) for "no change" and omitting the element for "no discount".

What you see in production

Thousands of products lose their existing discounts overnight. Revenue looks great for a day; then the partner's account manager calls.

The naive code

discount = float(product.findtext("discount") or 0)   # missing, empty and nil all become 0

Reproduce it

Missing, empty and nilTry it yourself

from lxml import etree

XSI = "http://www.w3.org/2001/XMLSchema-instance"
feed = etree.fromstring(f"""<products xmlns:xsi="{XSI}">
  <product sku="A"><discount>15</discount></product>
  <product sku="B"/>
  <product sku="C"><discount/></product>
  <product sku="D"><discount xsi:nil="true"/></product>
</products>""".encode())

def naive(p):
    return float(p.findtext("discount") or 0)

def careful(p):
    el = p.find("discount")
    if el is None:
        return "keep current discount"           # element absent: no information
    if el.get(f"{{{XSI}}}nil") == "true":
        return "remove discount"                 # explicitly null
    if not (el.text or "").strip():
        return "reject: empty value"             # empty is not a number
    return f"set to {float(el.text)}%"

for p in feed:
    print(p.get("sku"), "| naive:", naive(p), "| careful:", careful(p))

Output

A | naive: 15.0 | careful: set to 15.0%
B | naive: 0.0 | careful: keep current discount
C | naive: 0.0 | careful: reject: empty value
D | naive: 0.0 | careful: remove discount

Root cause

XML has three different states for an optional value: absent, present but empty, and explicitly nil (xsi:nil="true", allowed when the schema says nillable="true"). findtext() returns None for absent and "" for empty, and the or 0 turned both, plus nil, into zero.

The fix

Agree the meaning of each state in the contract (often: absent = unchanged, nil = clear, empty = invalid).
Enforce it in the schema: give numbers a numeric type (xs:decimal rejects an empty string), and use nillable="true" where null is meaningful.
Parse with explicit branches, not or default.

Trade-offs

Strict schemas reject messages partners used to get away with; roll out with a reporting period first.
Distinguishing three states adds code, but it's the only way to support partial updates correctly.

Detect it in production

Track how many values change per import; alert on mass changes to the same field.
Count each state (absent, empty, nil) per partner.

Key points

Absent, empty and nil are different in XML; don't collapse them with or default.
Put the meaning in the contract and the schema.
Alert on mass updates that change one field everywhere.