dictionary – Union of dict objects in Python

dictionary – Union of dict objects in Python

This question provides an idiom. You use one of the dicts as keyword arguments to the dict() constructor:

dict(y, **x)

Duplicates are resolved in favor of the value in x; for example

dict({a : y[a]}, **{a, x[a]}) == {a : x[a]}

You can also use update method of dict like

a = {a : 0, b : 1}
b = {c : 2}

print a

dictionary – Union of dict objects in Python

For a static dictionary, combining snapshots of other dicts:

As of Python 3.9, the binary or operator | has been defined to concatenate dictionaries. (A new, concrete dictionary is eagerly created):

>>> a = {a:1}
>>> b = {b:2}
>>> a|b
{a: 1, b: 2}

Conversely, the |= augmented assignment has been implemented to mean the same as calling the update method:

>>> a = {a:1}
>>> a |= {b: 2}
>>> a
{a: 1, b: 2}

For details, check PEP-584

Prior to Python 3.9, the simpler way to create a new dictionary is to create a new dictionary using the star expansion to add teh contents of each subctionary in place:

c = {**a, **b}

For dynamic dictionary combination, working as view to combined, live dicts:

If you need both dicts to remain independent, and updatable, you can create a single object that queries both dictionaries in its __getitem__ method (and implement get, __contains__ and other mapping method as you need them).

A minimalist example could be like this:

class UDict(object):
   def __init__(self, d1, d2):
       self.d1, self.d2 = d1, d2
   def __getitem__(self, item):
       if item in self.d1:
           return self.d1[item]
       return self.d2[item]

And it works:

>>> a = UDict({1:1}, {2:2})
>>> a[2]
>>> a[1]
>>> a[3]
Traceback (most recent call last):
  File <stdin>, line 1, in <module>
  File <stdin>, line 7, in __getitem__
KeyError: 3

NB: If one wants to lazily maintain a Union view of two
or more dictionaries, check collections.ChainMap in the standard library – as it has all dictionary methods and cover corner cases not
contemplated in the example above.

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