python singledispatch with several arguments

Question

for example:

@singledispatch                                                                                      
def f(                                                                                               
        a: str,                                                                                      
        b: list | dict                                                                               
)->None:                                                                                             
    ...                                                                                              
                                                                                                     
@f.register                                                                                          
def flist(                                                                                           
        a: str,                                                                                      
        b: list                                                                                      
)->None:                                                                                             
    print("flist:",type(b))                                                                          
                                                                                                     
@f.register                                                                                          
def fdict(                                                                                           
        a: str,                                                                                      
        b: dict                                                                                      
)->None:                                                                                             
    print("fdict:",type(b))                                                                          
                                                                                                     
                                                                                                     
a = "---"                                                                                           
                                                                                                     
b = [1,2]                                                                                            
f(a,b)                                                                                               
                                                                                                     
b = {1:2}                                                                                            
f(a,b)   

My (apparently incorrect) understanding, is that this should print (I hope for obvious reasons):

flist: <class 'list'>
fdict: <class 'dict'>

but in fact, this prints:

fdict: <class 'list'>
fdict: <class 'dict'>

why does the first call to f redirect to 'fdict', despite 'b' being a list ?