编写dict和defaultdict的代码

Question

I have the following problem:

from collections import defaultdict

def give_it_to_me(d):
    # This will crash if key 'it' does not exist
    return d['it']

def give_it2_to_me(d):
    # This will never crash, but does not trigger the default value in defaultdict
    return d.get('it2')

d1 = defaultdict(int)
d2 = { 'it' : 55 }

print give_it_to_me(d1)
print give_it_to_me(d2)

print give_it2_to_me(d1)
print give_it2_to_me(d2)

As you can see in the comments, it seems impossible to write a version of give_it_to_me which:

1. Does never crash
2. Triggers the default value for defaultdict s

Or am I wrong?

Answer 1

You might need a bit more code in your give_it_to_me function.

Use a try except statement to check for an existing key.

For example:

def give_it_to_me(d):
    # This won't crash if key 'it' does not exist in a normal dict.
    # It returns just None in that case.
    # It returns the default value of an defaultdict if the key is not found.
    try:
        return d['it']
    except KeyError:
        return None

Answer 2

Use try .. except :

try
    return d['it']
except KeyError:
    return None

Why defaultdict.get does not work as expected :

defaultdict is implemented using __missing__ method. __missing__ method is only called by dict[k] (or dict.__getitem__(k) ) when the key k is not in a dictionary.

defaultdict.__missing__ documentation also mention that:

Note that __missing__() is not called for any operations besides __getitem__() . This means that get() will, like normal dictionaries, return None as a default rather than using default_factory .