Remove the restriction that Clock.dt can only be set once -- state up…

…daters etc. all work fine with changing dt. Closes #84

brian2/core/clocks.py

@@ -8,7 +8,7 @@
 
 from brian2.utils.logger import get_logger
 from brian2.core.names import Nameable
-from brian2.units.fundamentalunits import check_units
+from brian2.units.fundamentalunits import check_units, Quantity
 from brian2.units.allunits import second, msecond
 
 __all__ = ['Clock', 'defaultclock']
@@ -40,12 +40,12 @@ class Clock(Nameable):
     '''
 
     @check_units(dt=second)
-    def __init__(self, dt=None, name='clock*'):
-        self._dt_spec = dt
+    def __init__(self, dt=0.1*msecond, name='clock*'):
+        self._dt = float(dt)
         self.i = 0  #: The time step of the simulation as an integer.
         self.i_end = 0  #: The time step the simulation will end as an integer
         Nameable.__init__(self, name=name)
-        logger.debug("Created clock {self.name} with dt={self._dt_spec}".format(self=self))
+        logger.debug("Created clock {self.name} with dt={self._dt}".format(self=self))
 
     def reinit(self):
         '''
@@ -77,30 +77,19 @@ def _set_t_end(self, end):
         self.i_end = int(float(end) / self.dt_)
 
     def _get_dt_(self):
-        if hasattr(self, '_dt'):
-            return self._dt
-        else:
-            dtspec = self._dt_spec
-            if dtspec is None:
-                dtspec = 0.1*msecond
-            self._dt = float(dtspec)
-            return self._dt
+        return self._dt
 
     def _set_dt_(self, dt_):
-        if hasattr(self, '_dt'):
-            raise RuntimeError("Cannot change dt, it has already been set to "+str(self.dt))
         self._dt = dt_
         logger.debug("Set dt for clock {self.name} to {self.dt}".format(self=self))
 
     @check_units(dt=second)
     def _set_dt(self, dt):
         self.dt_ = float(dt)
 
-    dt = property(fget=lambda self: self.dt_*second,
+    dt = property(fget=lambda self: Quantity(self.dt_, dim=second.dim),
                   fset=_set_dt,
-                  doc='''The time step of the simulation in seconds
-                         Returns a `Quantity`, and can only
-                         be set once. Defaults to ``0.1*ms``.''',
+                  doc='''The time step of the simulation in seconds.''',
                   )
 
     dt_ = property(fget=_get_dt_, fset=_set_dt_,

brian2/tests/test_clocks.py

@@ -17,7 +17,6 @@ def test_clocks():
     assert_equal(clock.i_end, 100)
     clock.i_end = 200
     assert_equal(clock.t_end, 200*ms)
-    assert_raises(RuntimeError, lambda: setattr(clock, 'dt', 2*ms))
     clock.t_ = float(8*ms)
     assert_equal(clock.i, 8)
     clock.t = 0*ms
@@ -31,15 +30,14 @@ def test_clocks():
     assert_equal(clock.t, 0*ms)
 
     clock = Clock()
+    assert_equal(clock.dt, 0.1*ms)
     clock.dt = 1*ms
     assert_equal(clock.dt, 1*ms)
-    assert_raises(RuntimeError, lambda: setattr(clock, 'dt', 2*ms))
 
 @with_setup(teardown=restore_initial_state)
 def test_defaultclock():        
     defaultclock.dt = 1*ms
     assert_equal(defaultclock.dt, 1*ms)
-    assert_raises(RuntimeError, lambda: setattr(defaultclock, 'dt', 2*ms))
 
 if __name__=='__main__':
     test_clocks()

docs_sphinx/developer/new_magic_and_clocks.rst

@@ -4,7 +4,7 @@ New magic and clock behaviour
 Clocks
 ------
 
-The rule for clocks was that
+The rule for clocks in Brian 1 was that
 you would either specify a clock explicitly, or it would guess it based on the
 following rule: if there is no clock defined in the execution frame of the
 object being defined, use the default clock; if there is a single clock
@@ -21,16 +21,11 @@ introduce.
 
 Incidentally, you could also change the dt of a
 clock after it had been defined, which would invalidate any state updaters that
-were based on a fixed dt. Consequently, in the new design you cannot change the
-dt of a clock once it has been specified. This has one huge problem, it would
-mean that you couldn't change the dt of the defaultclock, which would be too
-annoying in practice. So, you can now leave the dt of a clock unspecified when
-you create the clock, and it can be left unspecified until the .dt attribute
-is accessed. If no value is specified, it uses ``dt=0.1*ms``, and the .dt
-attribute can be set precisely once (if it was left unspecified when created).
-This means you can now set ``defaultclock.dt`` precisely once. Again, this is
-slightly less flexible than the old system, but avoids many potentially
-confusing bugs.
+were based on a fixed dt. This is no longer a problem in Brian 2, since state
+updaters are re-built at every run so they work fine with a changed dt. It is
+important to note that the dt of the respective clock (i.e. in many cases,
+``defaultclock.dt``) at the time of the `run` call, not the dt during
+the `NeuronGroup` creation, for example, is relevant for the simulation.
 
 Magic
 -----