How to Implement a Simple Parser in Python in 2025

In today's data-driven world, parsing is an essential skill for anyone working with programming and data manipulation.

Python, being a versatile language, provides several ways to implement parsers with ease. In this article, you'll learn how to create a simple parser in Python, focusing on flexibility and efficiency in 2025.

Introduction to Parsing

Parsing involves analyzing a sequence of inputs to confirm its adherence to a particular grammar. This process is crucial in scenarios like interpreting data formats, compiling source code, or processing configuration files. With advancements in Python libraries and community contributions in 2025, building parsers is more efficient than ever.

Step-by-Step Guide to Implement a Simple Parser in Python

Step 1: Define the Grammar

Before you start coding, clearly define the grammar or syntax rules that your input must follow. For example, if you're parsing a simple arithmetic expression, your grammar might look something like this:

• Expression: Number ( '+' | '-' ) Number
• Number: Digit+

Step 2: Tokenize the Input

Tokenization is breaking an input string into meaningful components called tokens. We'll use Python's in-built libraries for this task.

import re

def tokenize(expression):
    token_specification = [
        ('NUMBER',   r'\d+'),   # Integer
        ('PLUS',     r'\+'),    # Plus symbol
        ('MINUS',    r'-'),     # Minus symbol
        ('SKIP',     r'[ \t]+'),# Skip spaces and tabs
    ]
    token_regex = '|'.join(f'(?P<{pair[0]}>{pair[1]})' for pair in token_specification)
    get_token = re.compile(token_regex).match
    line = expression.strip()
    tokens = []
    pos = 0
    match = get_token(line)
    while match is not None:
        kind = match.lastgroup
        if kind != 'SKIP':
            value = match.group(kind)
            tokens.append((kind, value))
        pos = match.end()
        match = get_token(line, pos)
    return tokens

expression = "3 + 5 - 2"
tokens = tokenize(expression)
print(tokens)

Step 3: Parse Tokens into an Abstract Syntax Tree (AST)

An Abstract Syntax Tree is a tree representation of the abstract syntactic structure of source code. The nodes of the tree represent constructions in the source code.

class ASTNode:
    def __init__(self, type, value=None):
        self.type = type
        self.value = value
        self.children = []

def parse(tokens):
    position = 0

    def parse_expression():
        nonlocal position
        left = parse_number()
        while position < len(tokens):
            tok_type, tok_value = tokens[position]
            if tok_type in ('PLUS', 'MINUS'):
                position += 1
                right = parse_number()
                node = ASTNode(tok_type, tok_value)
                node.children.append(left)
                node.children.append(right)
                left = node
            else:
                break
        return left

    def parse_number():
        nonlocal position
        tok_type, tok_value = tokens[position]
        if tok_type == 'NUMBER':
            position += 1
            return ASTNode('NUMBER', tok_value)
        raise ValueError("Number expected")

    return parse_expression()

ast = parse(tokens)

Step 4: Evaluate the AST

Finally, we need a mechanism to evaluate our AST to produce a result from the parsed expressions.

def evaluate_ast(node):
    if node.type == 'NUMBER':
        return int(node.value)
    elif node.type == 'PLUS':
        return evaluate_ast(node.children[0]) + evaluate_ast(node.children[1])
    elif node.type == 'MINUS':
        return evaluate_ast(node.children[0]) - evaluate_ast(node.children[1])

result = evaluate_ast(ast)
print(f"Result of '{expression}' is {result}")

Advanced Parsing Techniques

The simple parser we created is suitable for straightforward expressions. However, for complex grammars, consider using parser libraries like ANTLR or PLY for more nuanced control.

Related Topics

Explore more on parsing across various environments:

• HTML Parsing with PowerShell Core
• Prolog Parsing: Converting Lists to Numbers
• Parsing SQL Results in PowerShell
• JSON Parsing with Regex
• Parsing Month-Year Strings Using Presto

Conclusion

By following these steps, you can implement a basic parser in Python and adapt it to more complex parsing tasks. Stay updated with the latest libraries and tools to enhance your parsing capabilities and efficiency in 2025 and beyond.