Role: Backend Developer & Tooling Engineer
Project: Independent CLI accounting tool for freelancers (Spain)

Built Conta, a Python-based command-line tool that helps self-employed professionals keep track of invoices, deductible expenses, social security payments, and quarterly tax obligations in Spain. The application combines a typed domain model, SQLite-backed persistence, and task-focused commands to provide a fast workflow for everyday bookkeeping.

View the project: Source code on GitHub

Core Responsibilities & Contributions

Product Architecture & System Design

• Designed a modular accounting engine using Python 3.10, sqlmodel, pydantic, and SQLite for local, file-based storage
• Modeled core bookkeeping entities (FacturaEmitida, GastoDeducible, PagoAutonomo, PagoFraccionado130) with explicit enums for activity types (programacion, musica)
• Structured the application as a reusable package with a dedicated conta entry point and environment-based configuration

CLI UX & Workflow Automation

• Implemented a multi-command CLI using Typer, with structured help text and argument parsing tailored to Spanish tax workflows
• Added commands to register issued invoices (emite), deductible expenses (gasto), autonomous social security payments, and fractioned IRPF payments
• Provided rich terminal output using the Rich library, including tabular listings for invoices and expenses
• Included validation and helpful error messages for dates, numeric fields, and quarter formats to reduce user mistakes

Tax & Reporting Logic for Spanish Freelancers

• Encapsulated quarterly VAT (IVA) calculations in a dedicated service, computing devengado (owed) and deducible (deductible) VAT per quarter
• Implemented IRPF accumulated snapshot logic matching the structure of Modelo 130 (base, withholdings, previous payments) via the irpf_snapshot_acumulado service
• Accounted for social security contributions, prior fractioned payments, and activity filters to give a realistic view of quarterly income tax obligations

Data Model & Persistence

• Used SQLModel to define typed tables with indexes on dates and invoice numbers for efficient filtering and reporting
• Ensured idempotent operations for critical entities (e.g. preventing duplicate invoice numbers on creation)
• Managed a lightweight SQLite database file, initialized via a single conta init command

Exports, Backups & Extensibility

• Built CSV export of official VAT books (libros de IVA) using pandas, generating separate files for issued and received invoices per quarter
• Added a backup-db command to create timestamped copies of the SQLite database for safe archiving
• Implemented PDF import hooks for invoices, laying the groundwork for automating data entry from scanned documents
• Structured the project with FastAPI and Uvicorn dependencies to allow a future evolution from pure CLI to a small web dashboard without changing the core domain layer

Read the full case study →