Commit d2145780 authored by Joseph Weston's avatar Joseph Weston

explain that kwant no longer accepts default values in value functions

parent 0a7bc483
......@@ -48,6 +48,35 @@ the scattering region and leads, one could do the following::
kwant.smatrix(syst, params=dict(V_dot=0, V_lead=1))
Value functions no longer accept default values for parameters
Using value functions with default values for parameters can be
problematic, especially when re-using value functions between simulations.
When parameters have default values it is easy to forget that such a
parameter exists at all, because it is not necessary to provide them explicitly
to functions that use the Kwant system. This means that other value functions
might be introduced that also depend on the same parameter,
but in an inconsistent way (e.g. a parameter 'phi' that is a superconducting
phase in one value function, but a peierls phase in another). This leads
to bugs that are confusing and hard to track down.
Concretely, the above means that the following no longer works::
syst = kwant.Builder()
# Parameter 't' has a default value of 1
def onsite(site, V, t=1):
return V = 2 * t
def hopping(site_a, site_b, t=1):
return -t
syst[...] = onsite
syst[...] = hopping
# Raises ValueError
syst = syst.finalized()
Interpolated density plots
A new function `~kwant.plotter.density` has been added that can be used to
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