ValueError: too many values to unpack in fmin_l_bfgs_b

Question

I have the following code that is running to run xfoil to get me optimum values of weights in an aerofoil.

def four_dim_opt(x0, weight_limits, cl, file_path, xfoil_path):
opt_out = fmin_l_bfgs_b(run_xfoil_wcl, x0, args = (cl, file_path, xfoil_path), bounds =    weight_limits, epsilon = 0.01, approx_grad = True)
return opt_out

where run_xfoil_wcl is a functionreturning coefficient of drag values (float).

The error I keep getting is:

Traceback (most recent call last):
File "<pyshell#13>", line 1, in <module>
four_dim_opt(x0, weight_limits, 1.2, file_path, xfoil_path)
File "H:/appliedlab4 - Copy(2).py", line 157, in four_dim_opt
opt_out = fmin_l_bfgs_b(run_xfoil_wcl, x0, args = (cl, file_path, xfoil_path), bounds =     weight_limits, epsilon = 0.01, approx_grad = True)
File "C:\Python27\lib\site-packages\scipy\optimize\lbfgsb.py", line 166, in fmin_l_bfgs_b
l,u = bounds[i]
ValueError: too many values to unpack

Answer 1

The weight_limits argument to scipy.optimize.fmin_l_bfgs_b is causing the error and it should be either None or a list containing two elements:

weight_limits - (min, max) pairs for each element in x, defining the bounds on that parameter. Use None for one of min or max when there is no bound in that direction.

Answer 2

What are you passing to four_dim_opt as your weight_limits ? The value for weight_limits gets passed to bounds, which is then accessed in the line in your error message:

l,u = bounds[i]

This means weight_limits is expected to be a list or tuple, and each item in the list should ALSO be a list or tuple (holding exactly two values).