Python is a powerful, easy-to-use language which now has a wide range of numerical and machine-learning open source libraries. At Civis Analytics, we've built a cloud-based platform for data science which empowers analysts to extract insights from their data with less effort. The platform itself runs on Amazon Web Services, and the machine learning workflows at the core of the platform are coded in Python. Open-source Python libraries such as pandas, numpy, statsmodels, and scikit-learn let our data scientists focus on high-level workflows and greatly accelerate our development process. In this talk, I'll give an overview of Civis's new data science platform, focusing on the machine-learning aspects. I'll talk about how we use Python open-source libraries to help with data analysis, and some of the challenges we've overcome along the way.