Move position and deflection initialization before step direction check.

Otherwise, if position is not specified, step would always end up
being negative, and position would always end up as zero.  Now
position will end up being the extreme in the initial step direction,
as advertised.

diff --git a/pypiezo/afm.py b/pypiezo/afm.py
index 76ee928efd74f0eaf4923d379e3983d9d5154d82..c3fc14b44a88ebe31b3e164bdef2ae0ee0934d9a 100644 (file)
--- a/pypiezo/afm.py
+++ b/pypiezo/afm.py
@@ -189,13 +189,6 @@ class AFMPiezo (_base.Piezo):
         else:
             aquire_data = False
 
-        if step == 0:
-            raise ValueError('must have non-zero step size')
-        elif step < 0 and position > self.last_output[axis_name]:
-            step = -step
-        elif step > 0 and position < self.last_output[axis_name]:
-            step = -step
-
         if position is None:
             # default to the extreme value in the step direction
             if step > 0:
@@ -208,6 +201,13 @@ class AFMPiezo (_base.Piezo):
             channel = self._deflection_channel(self)
             deflection = channel.get_maxdata()
 
+        if step == 0:
+            raise ValueError('must have non-zero step size')
+        elif step < 0 and position > self.last_output[axis_name]:
+            step = -step
+        elif step > 0 and position < self.last_output[axis_name]:
+            step = -step
+
         log_string = (
             'move to position %d or deflection %g on axis %s in steps of %d'
             % (position, deflection, axis_name, step))