Larch Plugin Tutorial

The tricky details

In the Larch document, Groups are described here. In that section, you are told that, to make your own Groups, you use the group() function. While that is certainly correct practice at the Larch command line, you have no doubt noticed that the params Group was created using the Group() function, with a capital "G".

Similarly, the Larch document tells us that the param() function is used to make fitting parameters. Yet, the Parameter() function was used throughout this example.

And what is that _larch thing littering the entire MBACK plugin?

It turns out that there are some substantive differences between how Larch is used at the command line and how Larch is used when writing a plugin. For me, these differences represented a large and confusing hurdle when I first began working with Larch. Helping you understand these tricky but essential details is the main reason for writing this tutorial.

Function names, symbol names, and the Larch interpreter

Suppose that you want to include a function in your plugin for plotting the result of the MBACK calculation. At the larch command line, you might type something like this to make a quick-n-dirty plot showing the tabulated cross-section along with the matched data.

larch> newplot(, data.f2)
larch> plot(, data.fpp)

You might then think that a function like this would be a cool idea:

 ### !!! Note: this is not correct!!!!
def plot_mback(group):
    """Quick-n-dirty plot of MBACK calculation"""
    newplot(, group.f2)
    plot(, group.fpp)

and finish the plugin off by registering your new function:

def registerLarchPlugin(): # must have a function with this name!
    return ('_xafs', { 'mback': mback,
                       'plot_mback': plot_mback })

Go ahead! Edit the plugins/xafs/ file on your computer and give it shot at the Larch command line.

You will be disappointed.

In fact, here is what happens:

larch> a=read_ascii('Talc.xmu')
larch> mback(a.e, a.xmu, group=a, z=14, edge='K', order=2, whiteline=50, fit_erfc=True)
larch> plot_mback(a)
  File "/usr/local/share/larch/plugins/xafs/", line 162, in plot_mback
    newplot(, group.f2)file <stdin>, line 0
NameError: global name 'newplot' is not defined

Say what? Of course newplot is defined! We use it all the time at the Larch command line!

To understand the first reason why that function won't work, let's examine the plugin that implements plotting: plugins/wx/ Like all plugins, it ends with a call to registerLarchPlugin, which says, in part,

def registerLarchPlugin():
    return (MODNAME, {'plot':_plot,

followed by over a dozen more symbol definitions. MODNAME is defined earlier in the file as

MODNAME = '_plotter'

So, that call to registerLarchPlugin puts several symbols into the _plotter group. The function _plot is given the symbol name plot and so on. Unlike the example given in the MBACK plugin, the function name and the symbol name are not the same!

The symbol name is how the user accesses the function from the Larch command line. However, when authoring a plugin -- when programming with Larch -- you do not have access to the symbol names in the same way. To use a function, you must import and use the function name.

So how is this done in practice? First, you need to import the function name into your plugin. Near the top of the file containing the plugin, you would need something like this (you should continue editing the plugin file):

from plotter import _newplot, _plot

This makes those plotting functions available to your plugin. After including that at the top of the file, you might then change the plot_mback function to use the underscores:

 ### !!! Note: this is still not quite correct!!!!
def plot_mback(group):
    """Quick-n-dirty plot of MBACK calculation"""
    _newplot(, group.f2)
    _plot(, group.fpp)

Now, when you try to make the plot, this happens:

larch> a=read_ascii('Talc.xmu')
larch> mback(a.e, a.xmu, group=a, z=14, edge='K', order=2, whiteline=50, fit_erfc=True)
larch> plot_mback(a)
  File "/usr/local/share/larch/plugins/xafs/", line 162, in plot_mback
    _newplot(group.e, group.f2)
  File "/usr/local/lib/python2.7/dist-packages/larch/", line 35, in wrapper
    raise LarchPluginException(errmsg % fcn.__name__)file <stdin>, line 0
LarchPluginException: plugin function '_newplot' needs a valid '_larch' argument

That's a bit cryptic.

It turns out that the Larch interpreter is, itself, an instance of a python object. It is possible to have more that one in existence at a time. Your plotting function needs to somehow know that it is supposed to use the same instance of the interpreter that is being used at the command line you are interacting with. Here is how that is done:

def plot_mback(group, _larch=None):
    """Quick-n-dirty plot of MBACK calculation"""
    _newplot(group.e, group.f2, _larch=_larch)
    _plot(group.e, group.fpp, _larch=_larch)

Yay! Success!

Successful plot of MBACK results

The argument list takes an argument called _larch, which defaults to None. That argument is then passed to the calls to _newplot and _plot. If the _larch argument is an actual interpreter, it will be used. If the _larch argument is None, Larch will use the default interpreter. When you are working from the command line, the interpreter at the command line is the default interpreter.

Of course, not labeling axes is so unprofessional. Let's make our function a bit spiffier:

def plot_mback(group, _larch=None):
    """Quick-n-dirty plot of MBACK calculation"""
    _newplot(group.e, group.f2, _larch=_larch, xlabel='Energy (eV)', ylabel='match absorption',
             label='$f_2(E)$', show_legend=True, legend_loc='ur')
    _plot(group.e, group.fpp, _larch=_larch, label="$f''(E)$")


Attractive plot of MBACK results

Symbol names and the Larch interpreter in the MBACK function

Now that we understand the difference between symbol names and function names, we can return to the use of Group() and Parameter() in the MBACK plugin. The story is actually slightly different because those are functions that are intrinsic to Larch rather than defined in a plugin. But the concept is very similar.

Group is the name of the class that implements a Group. Parameter is the name of the class the implements a Parameter. At the Larch command line, those are accessed using symbols pointing to the group() and parameter() functions. These functions return references to Group and Parameter objects, respectively.

In the MBACK plugin, Group and Parameter are imported from larch in the first line of the boilerplate, then used throughout. Whenever Group or Parameter are used, the _larch=_larch argument is given so that Larch can correctly resolve the interpreter.

You will notice that the _larch=_larch argument is passed to almost every function from Larch that was imported among the boilerplate at the top of the plugin file ... set_xafsGroup, xray_edge, xray_line, f1_chantler, and so on. All of them.

The _larch=_larch argument has to be used for almost every function call. An exception is the objective function discussed in the previous section. This is because that function does not interact directly with the interpreter. Forgetting the _larch=_larch argument is by far my most common error when writing code for Larch.

tl,dr: Don't forget the _larch=_larch argument.

Also note that the mback function is defined with _larch=None in its signature. That is necessary so it can pass the correct interpreter to functions it calls. Almost every function you define in a plugin should include that in its signature.

Note that none of the Larch functions used in the MBACK plugin share the confusing feature with newplot and plot of having the symbol name different from the function name. In many cases, Larch plugins use the same name for the function and the symbol. Indeed, we did so for the mback function.

To examine how a function that you want to use is exported by its Larch plugin, do this from the Larch command line:

larch> show _plotter.plot
<function plot, file=/usr/local/share/larch/plugins/wx/>

This tells us where the plot function comes from. Open the indicated file, in this case /usr/local/share/larch/plugins/wx/, and examine the call to registerLarchPlugin at the end of the file. That will tell you the name of the function. You can then examine the actual function in that file if you need to better understand its semantics.