PySCF is a Python-based electronic structure platform designed to emphasize code simplicity, facilitating new method development and enabling flexible computational workflows. The package supports a wide range of tools for simulations of finite-size systems, extended systems with periodic boundary conditions, low-dimensional periodic systems, and custom Hamiltonians using mean-field and post-mean-field methods with standard Gaussian basis functions. PySCF uses Python for most features and heavily optimized C routines for computationally critical paths, ensuring efficiency comparable to existing C or Fortran-based quantum chemistry programs. The package includes self-consistent field methods, post-SCF methods, multireference methods, molecular properties, relativistic effects, orbital localization, extended systems with periodic boundary conditions, general AO integral evaluation, density fitting, and custom Hamiltonians. PySCF also provides interfaces to external programs and numerical tools, and is designed to be easily extensible and interactive, making it a versatile and powerful tool for quantum chemistry and electronic structure simulations.PySCF is a Python-based electronic structure platform designed to emphasize code simplicity, facilitating new method development and enabling flexible computational workflows. The package supports a wide range of tools for simulations of finite-size systems, extended systems with periodic boundary conditions, low-dimensional periodic systems, and custom Hamiltonians using mean-field and post-mean-field methods with standard Gaussian basis functions. PySCF uses Python for most features and heavily optimized C routines for computationally critical paths, ensuring efficiency comparable to existing C or Fortran-based quantum chemistry programs. The package includes self-consistent field methods, post-SCF methods, multireference methods, molecular properties, relativistic effects, orbital localization, extended systems with periodic boundary conditions, general AO integral evaluation, density fitting, and custom Hamiltonians. PySCF also provides interfaces to external programs and numerical tools, and is designed to be easily extensible and interactive, making it a versatile and powerful tool for quantum chemistry and electronic structure simulations.