A deep merging tool for Python core data structures
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Latest commit 68f64b0 Mar 20, 2018



A tools to handle merging of nested data structures in python.


deepmerge is available on pypi:

pip install deepmerge


Generic Strategy

from deepmerge import always_merger

base = {"foo": ["bar"]}
next = {"foo": ["baz"]}

expected_result = {'foo': ['bar', 'baz']}
result = always_merger.merge(base, next)

assert expected_result == result

Custom Strategy

from deepmerge import Merger

my_merger = Merger(
    # pass in a list of tuple, with the
    # strategies you are looking to apply
    # to each type.
        (list, ["append"]),
        (dict, ["merge"])
    # next, choose the fallback strategies,
    # applied to all other types:
    # finally, choose the strategies in
    # the case where the types conflict:
base = {"foo": ["bar"]}
next = {"bar": "baz"}
my_merger.merge(base, next)
assert base == {"foo": ["bar"], "bar": "baz"}

Strategies are passed as a list, and the merge runs through each strategy sequentially, and raises an exception if none are able to resolve the merge.

You can also pass in your own merge functions, instead of a string. Your function should take the arguments of (merger, path, base_value, value_to_merge_in).

A default merge does not exist, due to the numerous choices that have to be made for every merger. However, some very generic mergers are supplied:

  • always_merger: will never raise a merge exception, and will merge when possible.
  • conservative_merger: will never raise a merge exception and will use the original value when a conflict occurs.
  • merge_or_raise: will merge when possible, raise an exception when there is a conflict.

The best resource for now is the unit tests.


$ pip install pytest
$ pytest deepmerge/tests/