Julien Kieffer, Developer in Paris, France
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Julien Kieffer

Verified Expert  in Engineering

Python Developer

Paris, France
Toptal Member Since
April 20, 2020

Julien has led data analytics and web development teams and projects for more than eight years after earning a master's degree in complex system engineering from Grande École Art & Métiers. With passion, intelligence, and high-efficiency, Julien ensures high-quality deliverables across multiple projects by applying effective problem-solving, communication, training, and test-driven development (TDD).



Preferred Environment

Codecov, Travis CI, GitHub, GitLab, Slack, Visual Studio Code (VS Code), MacOS

The most amazing...

...results I achieved are 30 projects, growth from 4 to 20 developers, a sales increase from €300,000 to €1.2 million, and margins from negative to double-digits.

Work Experience


2016 - PRESENT
VingtCinq – Digital Manufacture
  • Recruited, trained, and managed an efficient 12-member team from the ground up.
  • Significantly improved margins from €200,000 to €1.2 million.
  • Shipped 30 projects by responding to my customer's needs while respecting delays and budgets.
  • Actively contributed to 12 complex, interconnected web apps using current technologies and relevant methodologies across teams to deliver high-end tested code.
  • Applied and promoted my philosophy: DRY, KISS, Zen of Python, TDD.
Technologies: Amazon Web Services (AWS), Docker, Django REST Framework, Django, API Development, Python 3


This was an eCommerce website migration for international shipping solutions. The project involved TTD, multilingual eCommerce, complex business logic, an external services wrapper, data migration (accounts and previous orders imported from the previous website), fine-tuned analytics statistics, and a customized administrative dashboard.


This multisite and mobile app compound allows users to apply for legal, regulated housing by completing complex and conditional legal forms. Data is live-synced with Résidétape's system to follow applications.

Once accepted, users can pay their rent online and follow housing-related administrative procedures through the app. This makes communication between the tenant and Résidétape (the landlord) clear, easy, and trackable. Investors and partners have their own dedicated spaces where they can follow their related information such as tenant requests and project status.

Flagship features: TDD, a conditional complex user form, wrappers to sync data from the website to the customer's ERP solution, REST API consumed, complex business-related logic, and user rights.

Internal Purchasing App

I developed the new geolocated purchasing app back end for a top-ranked construction company. The app allows users to geolocate and browse products by category and use the search engine to order goods directly from the app.

Geolocation is used to show only products that can be shipped within the user's area. Once the order is confirmed through the app, the back end does all the magic: Retailers get purchase orders in their mailbox and accountants have all the information they need to make their weekly data entry super easy. The back end also handles the data sync based on the customer's available data extraction. Negotiated prices are updated to keep new purchase orders relevant without impacting previous orders.

Flagship features: DD, GIS, GeoDjango, REST API, complex business logic, external services wrappers, and complex data synchronization.

Gesica – Lawyer Network

GESICA is a French-based lawyer network. To promote their members, I developed a website allowing users to search for lawyers based on their needs and geolocation. Some conversational forms allow users to get advice on their problems and the website suggests the most relevant attorney that's nearby. While browsing an attorney's page, the user can start a live chat with them to discuss their problems and make an appointment.

On the attorneys' side, they can manage their profile data, their company page data, and requests from their front office page, all within a tailor-made administration space. They can also use live chat, exchange messages with users, and manage their chat availability (online/offline).

Flagship features: TDD, REST API, WebSocket, Django Channels, GeoDjango, GIS, and a complex geolocated search engine.
2008 - 2011

Master's Degree in Industrial Engineering

Arts et Métiers ParisTech - Paris, France

MAY 2020 - MAY 2023

AWS Certified Solutions Architect – Associate



API Development, REST APIs, Vue, Pandas, NumPy, Scikit-learn, PiLLoW


Pytest, Git, Slack, GitLab, GitHub, Travis CI, Codecov, GIS


Django, Django REST Framework, Django Channels, GeoDjango


Python 3, Python


REST, DevOps, Test-driven Development (TDD)




Amazon Web Services (AWS), MacOS, Docker, Visual Studio Code (VS Code)


Architecture, GitOps

Collaboration That Works

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