Finding file after project move

relative-project.py

import os

def path_inside(path, dir):
    path = os.path.normpath(path)
    dir = os.path.normpath(dir)
    return os.path.commonprefix([path, dir]) == dir

def path_samepath(a, b):
    return os.path.normpath(a) == os.path.normpath(b)

def load_file(actual_project_file,
              expected_project_file,
              expected_base_path,
              expected_file_path):
    
    if os.path.exists(expected_file_path):
        return expected_file_path
    else:
        expected_project_file_dir = os.path.dirname(expected_project_file)
        
        if (path_inside(expected_project_file_dir, expected_base_path) and
            path_inside(expected_file_path, expected_base_path)):
            
            project_to_base_path = os.path.relpath(expected_base_path,
                                                   expected_project_file_dir)
            actual_project_file_dir = os.path.dirname(actual_project_file)
            actual_base_path = os.path.join(actual_project_file_dir,
                                            project_to_base_path)
            
            # make sure the project still is inside the tree like it was before
            base_path_to_project = os.path.relpath(expected_project_file_dir,
                                                   expected_base_path)
            actual_project_file_dir_guess = os.path.join(actual_base_path,
                                                         base_path_to_project)
            if (path_samepath(actual_project_file_dir,
                              actual_project_file_dir_guess)):
                
                file_relpath = os.path.relpath(expected_file_path,
                                               expected_base_path)
                actual_file_path = os.path.join(actual_base_path, file_relpath)
                actual_file_path = os.path.normpath(actual_file_path)
                
                return actual_file_path

    return None


# these ones are correct
assert(load_file('/a/b/foo.geany',
                 '/home/user/p/foo/foo.geany',
                 '/home/user/p/foo',
                 '/home/user/p/foo/src/main.c') ==
       '/a/b/src/main.c')
assert(load_file('/a/b/project/foo.geany',
                 '/home/user/p/foo/project/foo.geany',
                 '/home/user/p/foo',
                 '/home/user/p/foo/src/main.c') ==
       '/a/b/src/main.c')
assert(load_file('/home/user/p/newname/newname.geany',
                 '/home/user/p/foo/foo.geany',
                 '/home/user/p/foo',
                 '/home/user/p/foo/src/main.c') ==
       '/home/user/p/newname/src/main.c')
assert(load_file('/home/user/p/newname/project/newname.geany',
                 '/home/user/p/foo/project/foo.geany',
                 '/home/user/p/foo',
                 '/home/user/p/foo/src/main.c') ==
       '/home/user/p/newname/src/main.c')
# this one is incorrect, because the project file moved from base path
assert(load_file('/a/b/foo.geany',
                 '/home/user/p/foo/project/foo.geany',
                 '/home/user/p/foo',
                 '/home/user/p/foo/src/main.c') ==
       None)