XMLParser

Overview

XMLParser extends Parser to extract structured data from XML-tagged completions. It’s useful for prompts that request specific output formats like <reasoning>...</reasoning><answer>...</answer>.

Constructor

XMLParser(
    fields: list[str | tuple[str, ...]],
    answer_field: str = "answer",
    extract_fn: Callable[[str], str] = lambda x: x,
)

fields

list[str | tuple[str, ...]]

List of field definitions. Each can be:

A string: Fixed XML tag name (e.g., "reasoning")
A tuple: Multiple allowed tag names with the first as canonical (e.g., ("code", "answer"))

answer_field

str

default:"answer"

Which field contains the final answer for parse_answer().

extract_fn

Callable[[str], str]

default:"lambda x: x"

Additional transformation applied to extracted field values.

Do NOT include "think" in fields for models like Qwen3 or DeepSeek-R1 that auto-parse thinking tags. This will cause parsing failures.

Methods

parse

def parse(self, text: str, strip: bool = True, last: bool = False) -> Any

Parse XML tags from text and return a namespace object with attributes for each field.

text

str

The text containing XML tags.

strip

bool

default:"True"

Whether to strip whitespace from field values.

last

bool

default:"False"

If True, extract the last occurrence of each tag. If False, extract the first.

Returns: SimpleNamespace with attributes for each allowed field name. Missing fields are set to None.

parse_answer

def parse_answer(self, completion: Messages) -> str | None

Extract the answer field from the last assistant message containing it.

completion

Messages

String or list of messages.

Returns: Content of the answer field, or None if not found.

format

def format(self, **kwargs) -> str

Format keyword arguments into an XML string using canonical field names.

**kwargs

Field values to format. Keys must match canonical or alternative field names.

Returns: XML-formatted string. Raises: ValueError if a required field is missing.

get_format_str

def get_format_str(self) -> str

Get a description of the expected XML format. Returns: String showing the XML structure with field names.

get_fields

def get_fields(self) -> list[str]

Get the list of canonical field names. Returns: List of field names in order.

get_format_reward_func

def get_format_reward_func(self) -> Callable

Create a reward function that checks format compliance. Returns: Function that scores completions based on:

Presence of expected fields (40% weight)
Proper spacing (20%)
Starting with first field (20%)
Ending with last field (20%)

Attributes

answer_field

str

The field name used for extracting answers.

All attributes from Parser are also available.

Example Usage

Basic XML Parsing

import verifiers as vf

# Define parser expecting reasoning and answer
parser = vf.XMLParser(
    fields=["reasoning", "answer"],
    answer_field="answer"
)

text = """<reasoning>
Let's solve step by step.
2 + 2 = 4
</reasoning>
<answer>
4
</answer>"""

result = parser.parse(text)
print(result.reasoning)  # "Let's solve step by step.\n2 + 2 = 4"
print(result.answer)     # "4"

Alternative Field Names

# Allow either "code" or "answer" for the final field
parser = vf.XMLParser(
    fields=["reasoning", ("code", "answer")],
    answer_field="answer"  # Canonical name for extraction
)

# This works with <code>
text1 = "<reasoning>...</reasoning><code>print('hi')</code>"
result1 = parser.parse(text1)
print(result1.code)    # "print('hi')"
print(result1.answer)  # "print('hi')" (same value, different attribute)

# This works with <answer>
text2 = "<reasoning>...</reasoning><answer>42</answer>"
result2 = parser.parse(text2)
print(result2.answer)  # "42"
print(result2.code)    # "42" (same value, different attribute)

Extracting Answers from Messages

parser = vf.XMLParser(fields=["reasoning", "answer"])

completion = [
    {"role": "user", "content": "What is 5*5?"},
    {"role": "assistant", "content": "<reasoning>5*5</reasoning><answer>25</answer>"}
]

answer = parser.parse_answer(completion)
print(answer)  # "25"

Formatting XML Output

parser = vf.XMLParser(fields=["reasoning", "answer"])

xml = parser.format(
    reasoning="First, we calculate...",
    answer="42"
)
print(xml)
# <reasoning>
# First, we calculate...
# </reasoning>
# <answer>
# 42
# </answer>

Format Validation Reward

parser = vf.XMLParser(fields=["reasoning", "answer"])

# Get format checking function
format_checker = parser.get_format_reward_func()

# Well-formatted completion
good = [{"role": "assistant", "content": "<reasoning>...</reasoning><answer>5</answer>"}]
print(format_checker(good))  # 1.0

# Missing reasoning tag
bad = [{"role": "assistant", "content": "<answer>5</answer>"}]
print(format_checker(bad))  # < 1.0 (partial credit)

# No tags at all
worse = [{"role": "assistant", "content": "just text"}]
print(format_checker(worse))  # 0.0

Using with Rubric

parser = vf.XMLParser(fields=["reasoning", "answer"])

# Create rubric with format checking
rubric = vf.Rubric(parser=parser)

# Add format reward
rubric.add_reward_func(parser.get_format_reward_func(), weight=0.2)

# Add correctness reward
def correctness(completion, answer, parser, **kwargs):
    parsed = parser.parse_answer(completion)
    return 1.0 if parsed == answer else 0.0

rubric.add_reward_func(correctness, weight=1.0)

# Now: reward = 1.0 * correctness + 0.2 * format_compliance

Last Occurrence Extraction

parser = vf.XMLParser(fields=["answer"])

# Multiple answer tags - get the last one
text = "<answer>wrong</answer> <answer>correct</answer>"

first = parser.parse(text, last=False)
print(first.answer)  # "wrong"

last = parser.parse(text, last=True)
print(last.answer)   # "correct"

Complex Multi-Field Example

parser = vf.XMLParser(
    fields=[
        "problem_understanding",
        "approach",
        ("code", "solution"),  # Either code or solution
        "explanation",
        "answer"
    ],
    answer_field="answer"
)

text = """<problem_understanding>
We need to find the sum.
</problem_understanding>
<approach>
Use a loop.
</approach>
<code>
result = sum(range(10))
</code>
<explanation>
This calculates 0+1+2+...+9
</explanation>
<answer>
45
</answer>"""

result = parser.parse(text)
print(result.problem_understanding)  # "We need to find the sum."
print(result.code)                   # "result = sum(range(10))"
print(result.answer)                 # "45"

# Get format description
print(parser.get_format_str())
# <problem_understanding>
# ...
# </problem_understanding>
# <approach>
# ...
# </approach>
# <[ code | solution ]>
# ...
# </[ code | solution ]>
# ...

Error Handling

parser = vf.XMLParser(fields=["reasoning", "answer"])

# Malformed XML - unclosed tags
text = "<reasoning>text</reasoning><answer>42"
result = parser.parse(text)
print(result.reasoning)  # "text"
print(result.answer)     # None (unclosed tag)

# Missing required field when formatting
try:
    parser.format(reasoning="only this")
except ValueError as e:
    print(e)  # "Missing value for field 'answer' (allowed: ['answer'])"

No Whitespace Stripping

parser = vf.XMLParser(fields=["code"])

text = "<code>\n    def foo():\n        pass\n</code>"

# With stripping (default)
stripped = parser.parse(text, strip=True)
print(repr(stripped.code))  # "def foo():\n        pass"

# Without stripping
unstripped = parser.parse(text, strip=False)
print(repr(unstripped.code))  # "\n    def foo():\n        pass\n"

Multi-turn Parsing

parser = vf.XMLParser(fields=["reasoning", "answer"])

completion = [
    {"role": "user", "content": "What is 2+2?"},
    {"role": "assistant", "content": "<reasoning>Let me think</reasoning>"},
    {"role": "user", "content": "Continue."},
    {"role": "assistant", "content": "<answer>4</answer>"}
]

# parse_answer searches backwards through assistant messages
answer = parser.parse_answer(completion)
print(answer)  # "4" (from last assistant message with answer field)

Format Reward Function Details

The format reward function scores completions based on:

Field presence (40%): Proportion of expected field sets present
Proper spacing (20%): Tags have content between them (not just whitespace)
Starts correctly (20%): Begins with first field’s opening tag
Ends correctly (20%): Ends with last field’s closing tag

Partial credit is given for partial compliance.

Environment Classes

Rubrics & Parsers

Clients

Integration Classes

Experimental

Data Types

Utilities

Overview

Constructor

Methods

parse

parse_answer

format

get_format_str

get_fields

get_format_reward_func

Attributes

Example Usage

Basic XML Parsing

Alternative Field Names

Extracting Answers from Messages

Formatting XML Output

Format Validation Reward

Using with Rubric

Last Occurrence Extraction

Complex Multi-Field Example

Error Handling

No Whitespace Stripping

Multi-turn Parsing

Format Reward Function Details

See Also

Build docs developers (and LLMs) love

Environment Classes

Rubrics & Parsers

Clients

Integration Classes

Experimental

Data Types

Utilities

​Overview

​Constructor

​Methods

​parse

​parse_answer

​format

​get_format_str

​get_fields

​get_format_reward_func

​Attributes

​Example Usage

​Basic XML Parsing

​Alternative Field Names

​Extracting Answers from Messages

​Formatting XML Output

​Format Validation Reward

​Using with Rubric

​Last Occurrence Extraction

​Complex Multi-Field Example

​Error Handling

​No Whitespace Stripping

​Multi-turn Parsing

​Format Reward Function Details

​See Also

Build docs developers (and LLMs) love

Overview

Constructor

Methods

parse

parse_answer

format

get_format_str

get_fields

get_format_reward_func

Attributes

Example Usage

Basic XML Parsing

Alternative Field Names

Extracting Answers from Messages

Formatting XML Output

Format Validation Reward

Using with Rubric

Last Occurrence Extraction

Complex Multi-Field Example

Error Handling

No Whitespace Stripping

Multi-turn Parsing

Format Reward Function Details

See Also