Plotting a 2D iso-probability curve

I have a library function that returns a pair of two numbers like this

def library_function(x0, x1):
    return x0**2 + x1**2, 10*x0*x1 + 1

(Actually, it is a function that returns predicted probabilities of two classes for some object, but it doesn't matter, so I created an artificial one.) I want to draw contours where the difference between two numbers returned by library_function is zero. To do so, I have to create a two-dimensional list (or array) with the differences.

I have the following possibilities:

X0 = np.linspace(-10, 10, 200)
X1 = np.linspace(-10, 10, 200)

results = [[(lambda u: u[0]-u[1])(library_function(x0, x1)) 
            for x0 in X0] for x1 in X1]
plt.contour(X0, X1, results, [0])

This approach is concise but I have to introduce lambda which seem to be considered not so Pythonic.

Another way:

values = [[library_function(x0, x1) for x0 in X0] for x1 in X1]
results = [[a-b for a, b in row] for row in values]

I have to introduce temporary two-dimensional list and this approach is longer.

Are there any ideas how to do it better (more concise and readable)?

Since you are using NumPy, you should deal with grids the NumPy way instead of performing list comprehensions.

def probability_diff(x0, x1):
    probs = library_function(x0, x1)
    return probs[0] - probs[1]

results = np.vectorize(probability_diff)(X0[:, np.newaxis], X1)

I wouldn't try to get too clever with the lambdas.

Also, since the grid happens to be a square, you could use X0 again instead of defining X1.

StackExchange Code Review Q#148039, answer score: 3