Specifying Argument Type For List of Dictionary For a Python Function

user_1227860 0
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I have a function whose argument is a list of dictionaries. I am trying to specify the type of elements for the dictionary in the function argument. The dictionary has the following form:

{'my_object':my_object, 'data':int}

my_object is a custom datatype. Is there any way to specify the type of dictionary, something like the one in below:

def my_function(x : list[dict[my_object, int]]):

Python complains about how I defined the type and gives me the following error:

TypeError: 'type' object is not subscriptable
arguments dictionary python types
2 Answers

You can try this:

from typing import List, Dict


def get_list_of_dicts(name: str, surname: str) -> List[Dict[str, str]]:
    return [{'name': name, 'surname': surname}]


print(get_list_of_dicts('Frank', 'Zappa'))

I hope it is useful for you.

user_106918280

If you are asking can you specify a list of dictionaries containing a custom object as keys, then the answer is below. If you wish to specify more than one type of object as a key within the same dictionary though, this won't work.

You can do the former in Python 3.9 with type aliases. Reference https://docs.python.org/3/library/typing.html gives example with list of dicts.

Example below shows a list of dicts containing custom objects as keys.

from collections.abc import Sequence

class MyClass: 
    def __init__(self, arg1, arg2):
        self.attr1 = arg1 + arg2

class AnotherClass: 
    def __init__(self, arg1, arg2):
        self.attr1 = arg1 + arg2
    
seq1 = dict[MyClass, int]
seq2 = list[seq1]

def myfunc(arg1: Sequence[seq2]) -> seq2:
    for dict in arg1:
        for key, value in dict.items():
                print(key.attr1, value)

#Correct type provided.
myobj1 = MyClass('A', '1')
myobj2 = MyClass('B','2')
listdict= [{myobj1: 10, myobj2: 20}, {myobj2: 100, myobj1: 200}]
listdict
myfunc(listdict)

#Incorrect type provided.  
myobj1 = AnotherClass('A', '1')
myobj2 = AnotherClass('B','2')
listdict= [{myobj1: 10, myobj2: 20}, { myobj2: 100, myobj1: 200}]
myfunc(listdict)
        

A1 10
B2 20
B2 100
A1 200

Note: Linters may not recognize valid inputs though. E.g. mypy complains that the list of dict(object, int) is not a sequence thereof.

user_39940920
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