From d2145780e7769055da12b9fd6707383a87195e05 Mon Sep 17 00:00:00 2001
From: Joseph Weston <joseph@weston.cloud>
Date: Thu, 20 Sep 2018 10:28:13 +0200
Subject: [PATCH] explain that kwant no longer accepts default values in value
functions
---
doc/source/pre/whatsnew/1.4.rst | 29 +++++++++++++++++++++++++++++
1 file changed, 29 insertions(+)
diff --git a/doc/source/pre/whatsnew/1.4.rst b/doc/source/pre/whatsnew/1.4.rst
index 193b4056..375a2024 100644
--- a/doc/source/pre/whatsnew/1.4.rst
+++ b/doc/source/pre/whatsnew/1.4.rst
@@ -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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