pyMolDyn is an interactive viewer of atomic systems defined in a unitcell and is particularly useful for crystalline and amorphous materials. Itidentifies and visualizes cavities (vacancies, voids) in simulation cellscorresponding to all seven 3D Bravais lattices, makes no assumptions aboutcavity shapes, allows for atoms of different size, and locates the cavitycenters (the centers of the largest spheres not including an atom center). Wedefine three types of cavity and develop a method based on the split and mergealgorithm to calculate all three. The visualization of the cavities uses themarching cubes algorithm. The program allows one to calculate and export pairdistribution functions (between atoms and/or cavities), as well as bonding anddihedral angles, cavity volumes and surface areas, and measures of cavityshapes, including asphericity, acylindricity, and relative shape anisotropy.The open source Python program is based on GR framework and GR3 routines andcan be used to generate high resolution graphics and videos.