Lazy Dict in Python

lazy_dict.py

# Lazy dict allows storing function and argument pairs when initializing the dictionary, 
# it calculates the value only when fetching it.
# In this examole, if the key starts with '#', it would accept a (function, args) tuple as value and 
# returns the calculated result when fetching the values.

from collections.abc import Mapping 


class LazyDict(Mapping):
    def __init__(self, *args, **kw):
        self._raw_dict = dict(*args, **kw)

    def __getitem__(self, key):
        if key.startswith('#'):
            func, arg = self._raw_dict.__getitem__(key)
            return func(arg)
        return self._raw_dict.__getitem__(key)

    def __iter__(self):
        return iter(self._raw_dict)

    def __len__(self):
        return len(self._raw_dict)


# Initialize a lazy dict
d = LazyDict(
    {
        '#1': (lambda x: x + 1, 0), 
        '#2': (lambda x: x + 2, 0), 
        '#3': (lambda x: x + 3, 0)
    }
)

# Let's try to fetch first two values
count = 0
for index, value in d.items():
    print(d[index])

    count += 1
    if count >= 1:
        break

# Output:
# 1
# 2

# Function for "#3" doesn't run