  As we approach the James Webb Space Telescope (JWST) era, several studies
have emerged that aim to: 1) characterize how the instruments will perform and
2) determine what atmospheric spectral features could theoretically be detected
using transmission and emission spectroscopy. To some degree, all these studies
have relied on modeling of JWST's theoretical instrument noise. With under two
years left until launch, it is imperative that the exoplanet community begins
to digest and integrate these studies into their observing plans, as well as
think about how to leverage the Hubble Space Telescope (HST) to optimize JWST
observations. In order to encourage this and to allow all members of the
community access to JWST & HST noise simulations, we present here an
open-source Python package and online interface for creating observation
simulations of all observatory-supported time-series spectroscopy modes. This
noise simulator, called PandExo, relies on some aspects of Space Telescope
Science Institute's Exposure Time Calculator, Pandeia. We describe PandExo and
the formalism for computing noise sources for JWST. Then, we benchmark
PandExo's performance against each instrument team's independently written
noise simulator for JWST, and previous observations for HST. We find that
\texttt{PandExo} is within 10% agreement for HST/WFC3 and for all JWST
instruments.
