Dynamically build a lambda function in python

Question

Supose that I want to generate a function to be later incorporated in a set of equations to be solved with scipy nsolve function. I want to create a function like this:

xi + xi+1 + xi+3 = 1

in which the number of variables will be dependent on the number of components. For example, if I have 2 components:

 f = lambda x: x[0] + x[1] - 1

for 3:

 f = lambda x: x[0] + x[1] + x[2] - 1

I specify the components as an array within the arguments of the function to be called:

 def my_func(components):
        for component in components:
        .....
        .....
        return f

I can't just find a way of doing this. I've to be able to make it this way as this function and other functions need to be solved together with nsolve:

 x0 = scipy.optimize.fsolve(f, [0, 0, 0, 0 ....])

Any help would be appreciated

Thanks!

---

Since I'm not sure which is the best way of doing this I will fully explain what I'm trying to do:

-I'm trying to generate this two functions to be later nsolved:

So I want to create a function teste([list of components]) that can return me this two equations (Psat(T) is a function I can call depending on the component and P is a constant(value = 760)).

Example:

  teste(['Benzene','Toluene'])

would return:

xBenzene + xToluene = 1

xBenzenePsat('Benzene') + xToluenePsat('Toluene') = 760

in the case of calling:

   teste(['Benzene','Toluene','Cumene'])

it would return:

xBenzene + xToluene + xCumene = 1

xBenzenePsat('Benzene') + xToluenePsat('Toluene') + xCumene*Psat('Cumene') = 760

All these x values are not something I can calculate and turn into a list I can sum. They are variables that are created as a function ofthe number of components I have in the system...

Hope this helps to find the best way of doing this

Answer 1

A direct translation would be:

f = lambda *x: sum(x) - 1

But not sure if that's really what you want.

Answer 2

You can dynamically build a lambda with a string then parse it with the eval function like this:

a = [1, 2, 3]

s = "lambda x: "
s += " + ".join(["x[" + str(i) + "]" for i in xrange(0, 3)]) # Specify any range
s += " - 1"

print s

f = eval(s)
print f(a)