Of course! The term ParseResult in Python most commonly refers to the object returned by the parsimonious library's grammar parser. It's a powerful and elegant way to parse text based on a grammar you define.

Let's break down what a ParseResult is, how to create one, and how to use it effectively.

What is a ParseResult?

A ParseResult is an object that represents the outcome of a parsing operation. It contains two main pieces of information:

1. The Parsed Data: The actual Python object(s) that were successfully extracted from the input text according to your grammar rules. This is often the most important part.
2. The Parse Tree: A complete representation of the input text, showing how every part of it (both the parts you cared about and the parts you ignored) matched the grammar rules.

Think of it like this: the grammar is a set of instructions. The parser reads the text and builds a ParseResult object which is the "report" of how well the text followed those instructions. The report includes the final summary (the parsed data) and the full, detailed log (the parse tree).

How to Get a ParseResult

You first need to install the parsimonious library:

pip install parsimonious

Then, you create a grammar, use it to parse some text, and it returns a ParseResult object.

Step 1: Define a Grammar

The grammar is defined using a domain-specific language (DSL) that looks like Extended Backus-Naur Form (EBNF). Each rule has a name and an expression.

from parsimonious.grammar import Grammar
# A simple grammar to parse a greeting like "Hello, World!"
grammar = Grammar("""
    greeting = salutation comma name exclamation
    salutation = "Hello"
    comma = ","
    name = ~"[A-Z][a-z]+"
    exclamation = "!"
""")

• greeting, salutation, etc., are rule names.
• "Hello", , are literal strings that must appear in the text.
• ~"[A-Z][a-z]+" is a regular expression. The prefix means "match this regex and capture the text".
• A B means sequence: A must be followed by B.
• A / B means alternation: match either A or B.

Step 2: Parse Text to Get the ParseResult

Now, let's use this grammar to parse a string.

# The text we want to parse
text = "Hello, World!"
# Parse the text using the grammar
# This returns a ParseResult object
result = grammar.parse(text)
print(f"The type of the result is: {type(result)}")
# The type of the result is: <class 'parsimonious.nodes.Node'>

Note: In parsimonious, the ParseResult is actually an instance of the Node class. The grammar.parse() method returns the root Node of the parse tree, which effectively is the ParseResult.

Anatomy of a ParseResult (The Node Object)

Let's inspect the result object we just created. It's a tree of nested Node objects. The root node represents the top-level rule (greeting).

# 1. Accessing the Parsed Data (The most common use case)
# The 'expr' attribute of the root node contains the Python object
# that was returned by the top-level rule's visitor function.
# Since we didn't define a visitor, it defaults to the text itself.
print(f"Parsed data (result.expr): {result.expr}")
# Parsed data (result.expr): Hello, World!
# 2. Accessing the Parse Tree
# The 'text' attribute is the full input string that this node matched.
print(f"Full text matched by root node: '{result.text}'")
# Full text matched by root node: 'Hello, World!'
# The 'children' attribute is a list of child nodes
# representing the parts of the grammar sequence.
print(f"Number of children for 'greeting': {len(result.children)}")
# Number of children for 'greeting': 4
# You can navigate the tree
salutation_node = result.children[0]
comma_node = result.children[1]
name_node = result.children[2]
exclamation_node = result.children[3]
print(f"\n--- Navigating the tree ---")
print(f"Salutation node text: '{salutation_node.text}'")
print(f"Comma node text: '{comma_node.text}'")
print(f"Name node text: '{name_node.text}'") # This is our captured regex!
print(f"Exclamation node text: '{exclamation_node.text}'")
# The 'expr' of a child node holds its parsed data
print(f"Name node's parsed data: {name_node.expr}")
# Name node's parsed data: World

The Power of Visitors: Extracting Meaningful Data

Just getting the raw text is okay, but the real power of parsimonious comes from visitors. A visitor is a class you define that walks the parse tree and converts the parsed text into meaningful Python data structures (like dictionaries, lists, or custom objects).

You do this by implementing methods named visit_<rule_name>().

Let's rewrite our example with a visitor to get a structured dictionary.

from parsimonious.grammar import Grammar
from parsimonious.nodes import NodeVisitor
# The grammar is the same
grammar = Grammar("""
    greeting = salutation comma name exclamation
    salutation = "Hello"
    comma = ","
    name = ~"[A-Z][a-z]+"
    exclamation = "!"
""")
# Define a visitor class
class GreetingVisitor(NodeVisitor):
    """
    This visitor will walk the parse tree and build a dictionary.
    """
    def visit_greeting(self, node, visited_children):
        # The top-level rule. `visited_children` is a list of the
        # results from visiting all its children.
        # The order is: [salutation_result, comma_result, name_result, exclamation_result]
        salutation, _, name, _ = visited_children
        return {
            "type": "greeting",
            "salutation": salutation,
            "name": name
        }
    def visit_salutation(self, node, visited_children):
        # This rule matched the literal "Hello"
        return node.text
    def visit_name(self, node, visited_children):
        # This rule matched our regex
        return node.text
    # The generic visit method is called for every node by default.
    # We can override it to control behavior.
    # By default, it returns the first visited child.
    # We don't need to do anything special here for our simple grammar.
# Create an instance of the visitor
visitor = GreetingVisitor()
# Parse the text
text = "Hello, World!"
result = grammar.parse(text)
# Use the visitor to transform the parse tree
structured_data = visitor.visit(result)
print("--- Using a Visitor ---")
print(f"Raw ParseResult object: {result}")
print(f"Structured data from visitor: {structured_data}")

Output:

--- Using a Visitor ---
Raw ParseResult object: <Node called "greeting">
Structured data from visitor: {'type': 'greeting', 'salutation': 'Hello', 'name': 'World'}

This is the core workflow:

1. Define a Grammar to describe your text structure.
2. Parse the Text to get a ParseResult (a Node tree).
3. Use a Visitor to walk the tree and extract the meaningful data you need.

Common Attributes and Methods of a ParseResult (Node)

Error Handling

If the input text doesn't match the grammar, grammar.parse() will raise a ParseError exception. This is very useful for providing feedback.

try:
    # This will fail because "hi" is not "Hello"
    grammar.parse("hi, World!")
except grammar.ParserError as e:
    print(f"\n--- Error Handling ---")
    print(f"Failed to parse: {e.text}")
    print(f"Error message: {e.msg}")
    print(f"Error occurred at line {e.line}, column {e.column}")

Output:

--- Error Handling ---
Failed to parse: hi, World!
Error message: Expected 'Hello' (line 1, column 1)
Error occurred at line 1, column 1