不能从timedelta继承

Question

i was annoyed by the fact that timedelta has no __format__ method. so i wanted to inherit from timedelta (monkey-patching is no option for built-in/extension type 'datetime.timedelta' )

so i tired inheritance ( my_dict is just here for reference; this one works!).

from datetime import timedelta

class my_dict(dict):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

class my_timedelta(timedelta):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

print(my_dict.__mro__)
# (<class '__main__.my_dict'>, <class 'dict'>, <class 'object'>)

print(my_timedelta.__mro__)
# (<class '__main__.my_timedelta'>,
#  <class 'datetime.timedelta'>, <class 'object'>)

d0 = my_dict(); d1 = my_dict(a=3)
print(d0, d1)
# -> {} {'a': 3}

# no arguments: this works!
t0 = my_timedelta()
print(t0)      # -> 0:00:00

# one argument: TypeError
# t1 = my_timedelta(5)  # -> TypeError: object.__init__() takes no parameters
# print(t1)

# a key-word argument: TypeError
# t2 = my_timedelta(seconds=5)  # -> TypeError: object.__init__() 
#                                               takes no parameters
# print(t2)

the method resolution order of both looks similar; but it seems that __init__ in the timedelta case is skipping timedelta.__init__ and jumping directoy to object.__init__ .

am i doing something very wrong? how can i inherit from timedelta ? what goes wrong here?

Answer 1

According to How to initialize an inherited object with either the 'usual' (keyword) arguments or an instance of the parent , you have to override new() method. Something like:

class my_timedelta(timedelta):
    def __new__(cls, *args, **kwargs):
        # whatever here
        return super().__new__(cls, *args, **kwargs)