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\Large{\textbf{yt Cheat Sheet}} \\
\end{center}
\subsection{General Info}
For everything yt please see \url{http://yt-project.org}.
Documentation \url{http://yt-project.org/doc/index.html}.
Need help? Start here \url{http://yt-project.org/doc/help/} and then
try the IRC chat room \url{http://yt-project.org/irc.html},
or the mailing list \url{https://mail.python.org/archives/list/yt-users@python.org/}. \\
\subsection{Installing yt} The easiest way to install yt is to use the
installation script found on the yt homepage or the docs linked above. If you
already have python set up with \texttt{numpy}, \texttt{scipy},
\texttt{matplotlib}, \texttt{h5py}, and \texttt{cython}, you can also use
\texttt{pip install yt}
\subsection{Command Line yt}
yt, and its convenience functions, are launched from a command line prompt.
Many commands have flags to control behavior.
Commands can be followed by
{\bf {-}{-}help} (e.g. {\bf yt render {-}{-}help}) for detailed help for that command
including a list of the available flags.
\texttt{yt load} \textit{dataset} \textemdash\ Load a single dataset. \\
\texttt{yt help} \textemdash\ Print yt help information. \\
\texttt{yt stats} \textit{dataset} \textemdash\ Print stats of a dataset. \\
\texttt{yt update} \textemdash\ Update yt to most recent version.\\
\texttt{yt update --all} \textemdash\ Update yt and dependencies to most recent version. \\
\texttt{yt version} \textemdash\ yt installation information. \\
\texttt{yt upload\_image} \textit{image.png} \textemdash\ Upload PNG image to imgur.com. \\
\texttt{yt upload\_notebook} \textit{notebook.nb} \textemdash\ Upload IPython notebook to \url{https://girder.hub.yt}.\\
\texttt{yt plot} \textit{dataset} \textemdash\ Create a set of images.\\
\texttt{yt render} \textit{dataset} \textemdash\ Create a simple
volume rendering. \\
\texttt{yt mapserver} \textit{dataset} \textemdash\ View a plot/projection in a Gmaps-like
interface. \\
\texttt{yt pastebin} \textit{text.out} \textemdash\ Post text to the pastebin at
paste.yt-project.org. \\
\texttt{yt pastebin\_grab} \textit{identifier} \textemdash\ Print content of pastebin to
STDOUT. \\
\texttt{yt bugreport} \textemdash\ Report a yt bug. \\
\texttt{yt hop} \textit{dataset} \textemdash\ Run hop on a dataset. \\
\subsection{yt Imports}
In order to use yt, Python must load the relevant yt modules into memory.
The import commands are entered in the Python/IPython shell or
used as part of a script.
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\texttt{import yt} \textemdash\
Load yt. \\
\texttt{from yt.config import ytcfg} \textemdash\
Used to set yt configuration options.
If used, must be called before importing any other module.\\
\texttt{from yt.analysis\_modules.\emph{halo\_finding}.api import \textasteriskcentered} \textemdash\
Load halo finding modules. Other modules
are loaded in a similar way by swapping the
\emph{emphasized} text.
See the \textbf{Analysis Modules} section for a listing and short descriptions of each.
\subsection{YTArray}
Simulation data in yt is returned as a YTArray. YTArray is a numpy array that
has unit data attached to it and can automatically handle unit conversions and
detect unit errors. Just like a numpy array, YTArray provides a wealth of
built-in functions to calculate properties of the data in the array. Here is a
very brief list of some useful ones.
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\texttt{v = a.in\_cgs()} \textemdash\ Return the array in CGS units \\
\texttt{v = a.in\_units('Msun/pc**3')} \textemdash\ Return the array in solar masses per cubic parsec \\
\texttt{v = a.max(), a.min()} \textemdash\ Return maximum, minimum of \texttt{a}. \\
\texttt{index = a.argmax(), a.argmin()} \textemdash\ Return index of max,
min value of \texttt{a}.\\
\texttt{v = a[}\textit{index}\texttt{]} \textemdash\ Select a single value from \texttt{a} at location \textit{index}.\\
\texttt{b = a[}\textit{i:j}\texttt{]} \textemdash\ Select the slice of values from
\texttt{a} between
locations \textit{i} to \textit{j-1} saved to a new Numpy array \texttt{b} with length \textit{j-i}. \\
\texttt{sel = (a > const)} \textemdash\ Create a new boolean Numpy array
\texttt{sel}, of the same shape as \texttt{a},
that marks which values of \texttt{a > const}. Other operators (e.g. \textless, !=, \%) work as well.\\
\texttt{b = a[sel]} \textemdash\ Create a new Numpy array \texttt{b} made up of
elements from \texttt{a} that correspond to elements of \texttt{sel}
that are \textit{True}. In the above example \texttt{b} would be all elements of \texttt{a} that are greater than \texttt{const}.\\
\texttt{a.write\_hdf5(\textit{filename.h5})} \textemdash\ Save \texttt{a} to the hdf5 file \textit{filename.h5}.\\
\subsection{IPython Tips}
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These tips work if IPython has been loaded, typically either by invoking
\texttt{yt load} on the command line.
\texttt{Tab complete} \textemdash\ IPython will attempt to auto-complete a
variable or function name when the \texttt{Tab} key is pressed, e.g. \textit{HaloFi}\textendash\texttt{Tab} would auto-complete
to \textit{HaloFinder}. This also works with imports, e.g. \textit{from numpy.random.}\textendash\texttt{Tab}
would give you a list of random functions (note the trailing period before hitting \texttt{Tab}).\\
\texttt{?, ??} \textemdash\ Appending one or two question marks at the end of any object gives you
detailed information about it, e.g. \textit{variable\_name}?.\\
Below a few IPython ``magics'' are listed, which are IPython-specific shortcut commands.\\
\texttt{\%paste} \textemdash\ Paste content from the system clipboard into the IPython shell.\\
\texttt{\%hist} \textemdash\ Print recent command history.\\
\texttt{\%quickref} \textemdash\ Print IPython quick reference.\\
\texttt{\%pdb} \textemdash\ Automatically enter the Python debugger at an exception.\\
\texttt{\%debug} \textemdash\ Drop into a debugger at the location of the last unhandled exception. \\
\texttt{\%time, \%timeit} \textemdash\ Find running time of expressions for benchmarking.\\
\texttt{\%lsmagic} \textemdash\ List all available IPython magics. Hint: \texttt{?} works with magics.\\
Please see \url{http://ipython.org/documentation.html} for the full
IPython documentation.
\subsection{Load and Access Data}
The first step in using yt is to reference a simulation snapshot.
After that, simulation data is generally accessed in yt using \textit{Data Containers} which are Python objects
that define a region of simulation space from which data should be selected.
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\texttt{ds = yt.load(}\textit{dataset}\texttt{)} \textemdash\ Reference a single snapshot.\\
\texttt{dd = ds.all\_data()} \textemdash\ Select the entire volume.\\
\texttt{a = dd[}\textit{field\_name}\texttt{]} \textemdash\ Copies the contents of \textit{field} into the
YTArray \texttt{a}. Similarly for other data containers.\\
\texttt{ds.field\_list} \textemdash\ A list of available fields in the snapshot. \\
\texttt{ds.derived\_field\_list} \textemdash\ A list of available derived fields
in the snapshot. \\
\texttt{val, loc = ds.find\_max("Density")} \textemdash\ Find the \texttt{val}ue of
the maximum of the field \texttt{Density} and its \texttt{loc}ation. \\
\texttt{sp = ds.sphere(}\textit{cen}\texttt{,}\textit{radius}\texttt{)} \textemdash\ Create a spherical data
container. \textit{cen} may be a coordinate, or ``max'' which
centers on the max density point. \textit{radius} may be a float in
code units or a tuple of (\textit{length, unit}).\\
\texttt{re = ds.region(\textit{cen}, \textit{left edge}, \textit{right edge})} \textemdash\ Create a
rectilinear data container. \textit{cen} is required but not used.
\textit{left} and \textit{right edge} are coordinate values that define the region.
\texttt{di = ds.disk(\textit{cen}, \textit{normal}, \textit{radius}, \textit{height})} \textemdash\
Create a cylindrical data container centered at \textit{cen} along the
direction set by \textit{normal},with total length
2$\times$\textit{height} and with radius \textit{radius}. \\
\texttt{ds.save\_object(sp, \textit{``sp\_for\_later''})} \textemdash\ Save an object (\texttt{sp}) for later use.\\
\texttt{sp = ds.load\_object(\textit{``sp\_for\_later''})} \textemdash\ Recover a saved object.\\
\subsection{Defining New Fields}
\texttt{yt} expects on-disk fields, fields generated on-demand and in-memory.
Field can either be created before a dataset is loaded using \texttt{add\_field}:
\texttt{def \_metal\_mass(\textit{field},\textit{data})}\\
\texttt{\hspace{4 mm} return data["metallicity"]*data["cell\_mass"]}\\
\texttt{add\_field("metal\_mass", units='g', function=\_metal\_mass)}\\
Or added to an existing dataset using \texttt{ds.add\_field}:
\texttt{ds.add\_field("metal\_mass", units='g', function=\_metal\_mass)}\\
\subsection{Slices and Projections}
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\texttt{slc = yt.SlicePlot(ds, \textit{axis or normal vector}, \textit{fields}, \textit{center=}, \textit{width=}, \textit{weight\_field=}, \textit{additional parameters})} \textemdash\ Make a slice plot
perpendicular to \textit{axis} (specified via 'x', 'y', or 'z') or a normal vector for an off-axis slice of \textit{fields} weighted by \textit{weight\_field} at (code-units) \textit{center} with
\textit{width} in code units or a (value, unit) tuple. Hint: try \textit{yt.SlicePlot?} in IPython to see additional parameters.\\
\texttt{slc.save(\textit{file\_prefix})} \textemdash\ Save the slice to a png with name prefix \textit{file\_prefix}.
\texttt{.save()} works similarly for the commands below.\\
\texttt{prj = yt.ProjectionPlot(ds, \textit{axis or normal vector}, \textit{fields}, \textit{additional params})} \textemdash\ Same as \texttt{yt.SlicePlot} but for projections.\\
\subsection{Plot Annotations}
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Plot callbacks are functions itemized in a registry that is attached to every plot object. They can be accessed and then called like \texttt{ prj.annotate\_velocity(factor=16, normalize=False)}. Most callbacks also accept a \textit{plot\_args} dict that is fed to matplotlib annotator. \\
\texttt{velocity(\textit{factor=},\textit{scale=},\textit{scale\_units=}, \textit{normalize=})} \textemdash\ Uses field "x-velocity" to draw quivers\\
\texttt{magnetic\_field(\textit{factor=},\textit{scale=},\textit{scale\_units=}, \textit{normalize=})} \textemdash\ Uses field "Bx" to draw quivers\\
\texttt{quiver(\textit{field\_x},\textit{field\_y},\textit{factor=},\textit{scale=},\textit{scale\_units=}, \textit{normalize=})} \\
\texttt{contour(\textit{field=},\textit{levels=},\textit{factor=},\textit{clim=},\textit{take\_log=}, \textit{additional parameters})} \textemdash Plots a number of contours \textit{ncont} to interpolate \textit{field} optionally using \textit{take\_log}, upper and lower \textit{c}ontour\textit{lim}its and \textit{factor} number of points in the interpolation.\\
\texttt{grids(\textit{alpha=}, \textit{draw\_ids=}, \textit{periodic=}, \textit{min\_level=}, \textit{max\_level=})} \textemdash Add grid boundaries. \\
\texttt{streamlines(\textit{field\_x},\textit{field\_y},\textit{factor=},\textit{density=})}\\
\texttt{clumps(\textit{clumplist})} \textemdash\ Generate \textit{clumplist} using the clump finder and plot. \\
\texttt{arrow(\textit{pos}, \textit{code\_size})} Add an arrow at a \textit{pos}ition. \\
\texttt{point(\textit{pos}, \textit{text})} \textemdash\ Add text at a \textit{pos}ition. \\
\texttt{marker(\textit{pos}, \textit{marker=})} \textemdash\ Add a matplotlib-defined marker at a \textit{pos}ition. \\
\texttt{sphere(\textit{center}, \textit{radius}, \textit{text=})} \textemdash\ Draw a circle and append \textit{text}.\\
\texttt{hop\_circles(\textit{hop\_output}, \textit{max\_number=}, \textit{annotate=}, \textit{min\_size=}, \textit{max\_size=}, \textit{font\_size=}, \textit{print\_halo\_size=}, \textit{fixed\_radius=}, \textit{min\_mass=}, \textit{print\_halo\_mass=}, \textit{width=})} \textemdash\ Draw a halo, printing it's ID, mass, clipping halos depending on number of particles (\textit{size}) and optionally fixing the drawn circle radius to be constant for all halos.\\
\texttt{hop\_particles(\textit{hop\_output},\textit{max\_number=},\textit{p\_size=},\\
\textit{min\_size},\textit{alpha=})} \textemdash\ Draw particle positions for member halos with a certain number of pixels per particle.\\
\texttt{particles(\textit{width},\textit{p\_size=},\textit{col=}, \textit{marker=}, \textit{stride=}, \textit{ptype=}, \textit{stars\_only=}, \textit{dm\_only=}, \textit{minimum\_mass=}, \textit{alpha=})} \textemdash\ Draw particles of \textit{p\_size} pixels in a slab of \textit{width} with \textit{col}or using a matplotlib \textit{marker} plotting only every \textit{stride} number of particles.\\
\texttt{title(\textit{text})}\\
\subsection{The $\sim$/.yt/ Directory}
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yt will automatically check for configuration files in a special directory (\texttt{\$HOME/.yt/}) in the user's home directory.
The \texttt{config} file \textemdash\ Settings that control runtime behavior. \\
The \texttt{my\_plugins.py} file \textemdash\ Add functions, derived fields, constants, or other commonly-used Python code to yt.
\subsection{Analysis Modules}
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The import name for each module is listed at the end of each description (see \textbf{yt Imports}).
\texttt{Absorption Spectrum} \textemdash\ (\texttt{absorption\_spectrum}). \\
\texttt{Clump Finder} \textemdash\ Find clumps defined by density thresholds (\texttt{level\_sets}). \\
\texttt{Halo Finding} \textemdash\ Locate halos of dark matter particles (\texttt{halo\_finding}). \\
\texttt{Light Cone Generator} \textemdash\ Stitch datasets together to perform analysis over cosmological volumes. \\
\texttt{Light Ray Generator} \textemdash\ Analyze the path of light rays.\\
\texttt{Rockstar Halo Finding} \textemdash\ Locate halos of dark matter using the Rockstar halo finder (\texttt{halo\_finding.rockstar}). \\
\texttt{Star Particle Analysis} \textemdash\ Analyze star formation history and assemble spectra (\texttt{star\_analysis}). \\
\texttt{Sunrise Exporter} \textemdash\ Export data to the sunrise visualization format (\texttt{sunrise\_export}). \\
\subsection{Parallel Analysis}
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Nearly all of yt is parallelized using
MPI\@. The \textit{mpi4py} package must be installed for parallelism in yt. To
install \textit{pip install mpi4py} on the command line usually works.
Execute python in parallel similar to this:\\
\textit{mpirun -n 12 python script.py}\\
The file \texttt{script.py} must call the \texttt{yt.enable\_parallelism()} to
turn on yt's parallelism. If this doesn't happen, all cores will execute the
same serial yt script. This command may differ for each system on which you use
yt; please consult the system documentation for details on how to run parallel
applications.
\texttt{parallel\_objects()} \textemdash\ A way to parallelize analysis over objects
(such as halos or clumps).\\
\subsection{Git}
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Please see \url{https://git-scm.com/} for the latest Git documentation.
\texttt{git clone https://github.com/yt-project/yt} \textemdash\ Clone the yt
repository. \\
\texttt{git status} \textemdash\ Show status of working tree.\\
\texttt{git diff} \textemdash\ Show changed files in the working tree. \\
\texttt{git log} \textemdash\ Show a log of changes in reverse chronological
order.\\
\texttt{git revert <commit>} \textemdash\ Revert the changes in an existing
commit and create a new commit with reverted changes. \\
\texttt{git add <pathspec>} \textemdash\ Stage changes in the working tree to
the index. \\
\texttt{git commit} \textemdash\ Commit staged changes to the repository. \\
\texttt{git merge <branch>} Merge the revisions from the specified branch on
top of the current branch.\\
\texttt{git push <remote>} \textemdash\ Push changes to remote repository. \\
\texttt{git push <remote> <branch>} \textemdash\ Push changes in specified
branch to remote repository. \\
\texttt{git pull <remote> <branch>} \textemdash\ Pull changes from the
specified branch of the remote repository. This is equivalent to \texttt{git
fetch <remote>} and then \texttt{git merge <remote>/<branch>}.\\
\subsection{FAQ}
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\texttt{slc.set\_log('field', False)} \textemdash\ When plotting \texttt{field}, use linear scaling instead of log scaling.
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