PySCF is a general-purpose electronic structure platform designed from theground up to emphasize code simplicity, both to aid new method development, aswell as for flexibility in computational workflow. The package provides a widerange of tools to support simulations of finite size systems, extended systemswith periodic boundary conditions, low dimensional periodic systems, and customHamiltonians, using mean-field and post-mean-field methods with standardGaussian basis functions. To ensure easy of extensibility, PySCF uses thePython language to implement almost all its features, while computationallycritical paths are implemented with heavily optimized C routines. Using thiscombined Python/C implementation, the package is as efficient as the bestexisting C or Fortran based quantum chemistry programs. In this paper wedocument the capabilities and design philosophy of the current version of thePySCF package.