如何在Python中区分参数类型（元组/元组列表）

Question

I have getDict() function that should accept tuple or list of tuples:

def getDict(items):
    item1,item2=items
    print item1,item2

getDict(({1:'a'},{2:'b'}))
getDict([({1:'a'},{2:'b'})])

But it gives me an error:

ValueError: need more than 1 value to unpack

I know that it can be fixed with following snipped:

if type(items) == list:
    item1,item2=items[0]
else:
    item1,item2=items

But I'm seeking for a solution without if.

Answer 1

There are several possible ways to do it. You can check a type of your input: is it a tuple or a list. Or you can always use it a as list.

def get_dict_cast(tuple_or_list):
    """ This function casts it's args to list. """
    now_list = list(tuple_or_list)
    print(now_list, now_list.__class__) 

def get_dict_type_check(input):
    """ 
    This function checks what to do with your input depending on it's type.
    """
    if isinstance(input, tuple):
         print('it is a tuple: {}'.format(input))
    elif isinstance(input, list):
         print('it is a list: {}'.format(input))

just_tuple = ({ 1: 'a' }, { 2: 'b' })
list_of_tuples = [
    just_tuple, 
    ({ 3: 'a' }, { 2: 'b' })
]

get_dict_cast(just_tuple)
get_dict_cast(list_of_tuples)

get_dict_type_check(just_tuple)
get_dict_type_check(list_of_tuples)

The output will be:

# get_dict_cast(just_tuple):
[{1: 'a'}, {2: 'b'}], <type 'list'>

# get_dict_cast(list_of_tuples):
[({1: 'a'}, {2: 'b'}), ({3: 'a'}, {2: 'b'})], <type 'list'>

# get_dict_type_check functions:
it is a tuple: ({1: 'a'}, {2: 'b'})
it is a list: [({1: 'a'}, {2: 'b'}), ({3: 'a'}, {2: 'b'})]

Depending on what your are doing both methods might be usefull or useless.

Answer 2

The pattern you seeking is multimethod . I have found several implementations in PyPI, but used simple module written by Guido van Rossum: Five-minute Multimethods in Python

They can be easily used like this:

@multimethod(tuple)
def getDict(items):
    item1,item2=items
    print item1,item2

@multimethod(list)
def getDict(items):
    return getDict(items[0])

getDict(({1:'a'},{2:'b'}))
getDict([({1:'a'},{2:'b'})])