imranal - 1 year ago 246
Python Question

# Interchanging between different scipy ode solvers

I have a made a solver which can interchange between

`scipy.integrate.ode`
and
`scipy.integrate.odeint`
. Here is the code.

``````def f(y,s,C,u,v):
y0 = y[0] # u
y1 = y[1] # u'
y2 = y[2] # v
y3 = y[3] # v'
dy = np.zeros_like(y)
dy[0] = y1
dy[2] = y3

C = C.subs({u:y0,v:y2})
dy[1] = -C[0,0][0]*dy[0]**2\
-2*C[0,0][1]*dy[0]*dy[2]\
-C[0,1][1]*dy[2]**2
dy[3] = -C[1,0][0]*dy[0]**2\
-2*C[1,0][1]*dy[0]*dy[2]\
-C[1,1][1]*dy[2]**2
return dy

def solve(C,u0,s0,s1,ds,solver=None):
from sympy.abc import u,v
if solver == None: # use lsoda from scipy.integrate.odeint
s = np.arange(s0,s1+ds,ds)
print 'Running solver ...'
return sc.odeint(f,u0,s,args=(C,u,v))
else: # use any other solver from scipy.integrate.ode
r = sc.ode(f).set_integrator(solver) # vode,zvode,lsoda,dopri5,dop853
r.set_f_params(C,u,v)
r.set_initial_value(u0)
#t = []
y = []
print 'Running solver ...'
while r.successful() and r.t <= s1:
r.integrate(r.t + ds)
y.append(r.y)#; t.append(r.t)
return np.array(y)
``````

The problem I experience is as following. If I decide to use the solver from
`scipy.integrate.odeint`
then the parameters of
`f`
have to be specified in the order as they are in the code. However, if I decide to use the solvers from
`scipy.integrate.ode`
I have to change the order of the parameters of the function
`f(y,s,C,u,v)`
to
`f(s,y,C,u,v)`
, otherwise I get the error

``````TypeError: 'float' object has no attribute '__getitem__'
``````

If I do this, then
`scipy.integrate.odeint`
generates the same error for
`f`
defined as
`f(s,y,C,u,v)`
. How can I operate with a unified
`f`
regardless of the order of the parameters ?

Edit :

To sum the problem up :

`scipy.integrate.ode`
solvers work if the function f is defined as
`f(s,y,C,u,v)`
, and
`scipy.integrate.odeint`
solver works if the function f is defined as
`f(y,s,C,u,v)`
. Why is this occurring, and how I can I fix this?

Edit :

Scipy -- version 0.16.0

Why is this occurring, and how I can I fix this?

It is occurring because of an unfortunate API design decision made years ago. `odeint` and the `ode` class require different signatures for the system to be solved.

You can fix it by adding a wrapper that changes the order of the first two arguments when you use, say, the `ode` class. For example, you could change this:

``````    r = sc.ode(f).set_integrator(solver)
``````

to

``````    r = sc.ode(lambda t, x, *args: f(x, t, *args)).set_integrator(solver)
``````

Better documentation of this discrepancy is in progress: https://github.com/scipy/scipy/pull/5151

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