PySCF is a Python-based electronic structure package designed for efficient and flexible quantum chemistry simulations. It emphasizes code simplicity to facilitate new method development and support diverse computational workflows. The package provides tools for simulations of finite and extended systems, including periodic boundary conditions, using mean-field and post-mean-field methods with Gaussian basis functions. It is implemented primarily in Python, with computationally critical parts optimized in C, achieving performance comparable to C or Fortran-based quantum chemistry programs. PySCF supports a wide range of quantum chemistry methods, including Hartree-Fock, density functional theory, coupled cluster theory, and multireference methods. It also includes tools for molecular properties, relativistic effects, and extended systems with periodic boundary conditions. The package is designed for extensibility, with a focus on simplicity, generality, and efficiency. It provides a flexible API for users to modify and extend its functionality. PySCF supports interactive computing, integrates with external programs, and offers numerical tools for efficient simulations. The package is open-source and widely used in the quantum chemistry community for its versatility and performance.PySCF is a Python-based electronic structure package designed for efficient and flexible quantum chemistry simulations. It emphasizes code simplicity to facilitate new method development and support diverse computational workflows. The package provides tools for simulations of finite and extended systems, including periodic boundary conditions, using mean-field and post-mean-field methods with Gaussian basis functions. It is implemented primarily in Python, with computationally critical parts optimized in C, achieving performance comparable to C or Fortran-based quantum chemistry programs. PySCF supports a wide range of quantum chemistry methods, including Hartree-Fock, density functional theory, coupled cluster theory, and multireference methods. It also includes tools for molecular properties, relativistic effects, and extended systems with periodic boundary conditions. The package is designed for extensibility, with a focus on simplicity, generality, and efficiency. It provides a flexible API for users to modify and extend its functionality. PySCF supports interactive computing, integrates with external programs, and offers numerical tools for efficient simulations. The package is open-source and widely used in the quantum chemistry community for its versatility and performance.