Senior Python Backend Developer - Freelance

We’re supporting a fast-growing French scale-up operating several SaaS products and a B2B marketplace, with strong technical challenges around scalability, reliability, performance, and transaction integrity in an international context.

The role is embedded within payment-related teams, working on business-critical systems that power payment flows, transaction processing, financial data reliability, and integrations with the broader platform ecosystem.

Mission – Key Responsibilities (Tech-focused)

Design, develop, and optimize backend services in Python (FastAPI, Django, or Flask) for payment and transaction-related systems.

Contribute to the evolution of a resilient microservices architecture supporting payment workflows, transaction orchestration, and financial operations.

Build and maintain high-performance, secure REST/GraphQL APIs connected to payment services, financial events, and operational flows.

Improve latency, throughput, fault tolerance, and reliability of systems handling high-volume payment-related traffic.

Work on data consistency, traceability, and transactional flows in a distributed environment, with strong attention to robustness and accuracy.

Help structure and evolve the data model on PostgreSQL and BigQuery for payment, reconciliation, and operational reporting needs.

Implement and improve CI/CD pipelines with GitHub Actions, Docker, and Kubernetes.

Industrialize infrastructure and environments following IaC, observability, and performance best practices.

Set up and maintain monitoring & alerting with Datadog and Sentry, with a strong focus on critical payment flows, incident detection, and service reliability.

Tech Stack

Languages / Backend: Python (FastAPI, Django, Flask)
Databases: PostgreSQL, BigQuery
Cloud: GCP (Compute, Pub/Sub, Cloud Run, …)
Observability: Datadog, Sentry
CI/CD & Ops: GitHub Actions, Docker, Kubernetes

Profile

7+ years of experience in backend Python on complex web architectures.

Strong experience with microservices and high-traffic / scale-up environments.

Solid experience building or maintaining business-critical systems related to payments, transactions, billing, or financial flows.

Strong understanding of scalability, performance tuning, monitoring, observability, and transaction reliability.

Good knowledge of CI/CD and cloud environments, ideally GCP.

Product-oriented mindset, able to balance robust architecture, operational constraints, and business impact.

Excellent communication skills and fluent English, comfortable in distributed teams.