A Python library [http://hitran.org/hapi HAPI] (HITRAN Application Programming Interface) has been developed which serves as a tool for absorption and transmission calculations as well as comparisons of spectroscopic data sets. HAPI extends the functionality of the main site, in particular, for the calculation of spectra using several types of line shape calculations, including the flexible HT (Hartmann-Tran) profile. This HT line shape can also be reduced to a number of conventional line profiles such as Gaussian (Doppler), Lorentzian, Voigt, Rautian, Speed-Dependent Voigt and Speed-Dependent Rautian. In addition to accounting for pressure, temperature and optical path length, the user can include a number of instrumental functions to simulate experimental spectra. HAPI is able to account for broadening of lines due to mixtures of gases as well as make use of all broadening parameters supplied by HITRAN. This includes the traditional broadeners (air, self) as well as additional parameters for CO, HO, H and He broadening. The following spectral functions can be calculated in the current version #1 of HAPI: 
* absorption coefficient
* absorption spectrum
* transmittance spectrum
* radiance spectrum
HAPIEST (an acronym for HITRAN Application Programming Interface and Efficient Spectroscopic Tools) is a graphical user interface allowing users to access some of the functionality provided by HAPI without any knowledge of Python programming, including downloading data from HITRAN, and plotting of spectra and cross-sections. The source code for HAPIEST is available on GitHub [https://github.com/hitranonline/hapiest (HAPIEST)], along with binary distributions for Mac and PC.