From 7bf681dc8e853aecad7a6cb1c8542971d0ff8290 Mon Sep 17 00:00:00 2001
From: David Stansby <dstansby@gmail.com>
Date: Fri, 3 Jan 2020 21:11:38 +0000
Subject: [PATCH] Merge pull request #137 from dstansby/bg-overwrite

Prevent bg being modified in place
---
 doc/source/changes.rst | 6 ++++++
 pfsspy/__init__.py     | 4 +++-
 pfsspy/tracing.py      | 2 +-
 3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/doc/source/changes.rst b/doc/source/changes.rst
index 9341cdc8..4bfef54a 100644
--- a/doc/source/changes.rst
+++ b/doc/source/changes.rst
@@ -1,6 +1,12 @@
 Changelog
 =========
 
+0.4.2
+-----
+
+- Fix a bug where :class`pfsspy.tracing.FotranTracer` would overwrite the
+  magnetic field values in an :class:`~pfsspy.Output` each time it was used.
+
 0.4.1
 -----
 
diff --git a/pfsspy/__init__.py b/pfsspy/__init__.py
index 136c8474..be33a224 100644
--- a/pfsspy/__init__.py
+++ b/pfsspy/__init__.py
@@ -362,7 +362,9 @@ def bg(self):
             bpg[i, :, :] /= (Sbp[:-1, :-1] + Sbp[1:, :-1] +
                              Sbp[1:, 1:] + Sbp[:-1, 1:])
         bsg *= -1
-        return np.stack((bpg, bsg, brg), axis=-1)
+        out = np.stack((bpg, bsg, brg), axis=-1)
+        out.flags.writeable = False
+        return out
 
     def _common_b(self):
         """
diff --git a/pfsspy/tracing.py b/pfsspy/tracing.py
index 5f67b4e6..3786cfe7 100644
--- a/pfsspy/tracing.py
+++ b/pfsspy/tracing.py
@@ -78,7 +78,7 @@ def trace(self, seeds, output):
         self.validate_seeds_shape(seeds)
 
         # The indexing order on the last index is (phi, s, r)
-        vectors = output.bg
+        vectors = output.bg.copy()
 
         # Correct s direction for coordinate system distortion
         sqrtsg = output.grid._sqrtsg_correction