Given a binary tree, find the maximum path sum
If you want \$−∞\$, just write float('-inf').
The names could be improved by removing the common prefix max_path_, using to instead of from, and node instead of current. (The replacements are shorter than the originals, so reduce the size of the code without reducing readability.)
The computation of max_path_from_current (which becomes to_node after my renaming), can be simplified by pulling the addition of node.val out of the call to max:

to_node = node.val + max(0, to_left, to_right)

Since max_path is only called from maxPathSum it makes sense for it to be a local function there. Then you could avoid the unnecessary use of self.


Revised code:

def maxPathSum(self, root):
    def max_path(node):
        if node is None:
            return 0, float('-inf')
        to_left, in_left = max_path(node.left)
        to_right, in_right = max_path(node.right)
        to_node = node.val + max(0, to_left, to_right)
        in_node = max(in_left, in_right, to_left + node.val + to_right, to_node)
        return to_node, in_node
    return max_path(root)[1]