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[WIP/discuss] More strict check that equation is conditionally linear #1143

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[WIP/discuss] More strict check that equation is conditionally linear #1143

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9 changes: 6 additions & 3 deletions brian2/stateupdaters/exponential_euler.py
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Original file line number Diff line number Diff line change
Expand Up @@ -56,13 +56,16 @@ def get_conditionally_linear_system(eqs, variables=None):
s_expr = sp.collect(s_expr,
var, evaluate=False)

if len(s_expr) > 2 or var not in s_expr:
if (len(s_expr) > 2 or
var not in s_expr or
s_expr.get(sp.S.One, sp.S.Zero).has(var)
):
raise ValueError(('The expression "%s", defining the variable %s, '
'could not be separated into linear components') %
(expr, name))
coefficients[name] = (s_expr[var], s_expr.get(1, 0))
coefficients[name] = (s_expr[var], s_expr.get(sp.S.One, sp.S.Zero))
else:
coefficients[name] = (0, s_expr)
coefficients[name] = (sp.S.Zero, s_expr)

return coefficients

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11 changes: 11 additions & 0 deletions brian2/tests/test_stateupdaters.py
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Expand Up @@ -398,6 +398,17 @@ def test_priority():

check_integration(eqs, variables, can_integrate)

# Equation that both linearly and non-linearly depends on the variable,
# and is therefore not "conditionally linear". The exponential Euler updater
# should therefore decline to integrate the equations.
param = 1
eqs = Equations('''dv/dt = (-v + exp(-v) + 1.0)/tau : 1''')
updater(eqs, variables) # should not raise an error
can_integrate = {linear: False, euler: True, exponential_euler: False,
rk2: True, rk4: True, heun: True, milstein: True}

check_integration(eqs, variables, can_integrate)

# Equations resulting in complex linear solution for older versions of sympy
eqs = Equations('''dv/dt = (ge+gi-(v+49*mV))/(20*ms) : volt
dge/dt = -ge/(5*ms) : volt
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