imranal - 3 months ago 51

Python Question

I have a made a solver which can interchange between

`scipy.integrate.ode`

`scipy.integrate.odeint`

`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`

`f`

`scipy.integrate.ode`

`f(y,s,C,u,v)`

`f(s,y,C,u,v)`

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

If I do this, then

`scipy.integrate.odeint`

`f`

`f(s,y,C,u,v)`

`f`

To sum the problem up :

`scipy.integrate.ode`

`f(s,y,C,u,v)`

`scipy.integrate.odeint`

`f(y,s,C,u,v)`

Scipy -- version 0.16.0

Answer

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