scipy - Fitting only one parameter of a function with many parameters in python

Question

In python I have a function which has many parameters. I want to fit this function to a data set, but using only one parameter, the rest of the parameters I want to supply on on my own. Here is an example:

def func(x,a,b):
   return a*x*x + b

for b in xrange(10):
   popt,pcov = curve_fit(func,x1,x2)

In this I want that the fitting is done only for a and the parameter b takes the value of the loop variable. How can this be done?

Answer 1

You can wrap func in a lambda, as follows:

def func(x,a,b):
   return a*x*x + b

for b in xrange(10):
   popt,pcov = curve_fit(lambda x, a: func(x, a, b), x1, x2)

A lambda is an anonymous function, which in Python can only be used for simple one line functions. Basically, it's normally used to reduce the amount of code when don't need to assign a name to the function. A more detailed description is given in the official documentation: http://docs.python.org/tutorial/controlflow.html#lambda-forms

In this case, a lambda is used to fix one of the arguments of func. The newly created function accepts only two arguments: x and a, whereas b is fixed to the value taken from the local b variable. This new function is then passed into curve_fit as an argument.