Recursively merging dictionaries with boltons.iterutils.remap. Useful for @hynek's configs. https://twitter.com/hynek/status/696720593002041345

remerge.py

"""
This is an extension of the technique first detailed here:

http://sedimental.org/remap.html#add_common_keys

In short, it calls remap on each container, back to front, using the accumulating
previous values as the default for the current iteration.
"""


from boltons.iterutils import remap, get_path, default_enter, default_visit

defaults = {'host': '127.0.0.1',
            'port': 8000,
            'endpoints': {'persistence': {'host': '127.0.0.1',
                                          'port': 8888},
                          'cache': {'host': '127.0.0.1',
                                    'port': 8889}},
            'owners': {'secondary': ['alice']},
            'zones': [{'a': 1}],
            'notes': ['this is the default']}


overlay = {'host': '127.0.0.1',
           'port': 8080,
           'endpoints': {'persistence': {'host': '10.2.2.2',
                                         'port': 5433}},
           'overlay_version': '5.0',
           'owners': {'primary': ['bob'], 'secondary': ['charles']},
           'zones': [{'a': 2}],
           'notes': ['this is the overlay']}


cache_host_override = {'endpoints': {'cache': {'host': '127.0.0.2'}}}


def remerge(target_list, sourced=False):
    """Takes a list of containers (e.g., dicts) and merges them using
    boltons.iterutils.remap. Containers later in the list take
    precedence (last-wins).

    By default, returns a new, merged top-level container. With the
    *sourced* option, `remerge` expects a list of (*name*, container*)
    pairs, and will return a source map: a dictionary mapping between
    path and the name of the container it came from.

    """

    if not sourced:
        target_list = [(id(t), t) for t in target_list]

    ret = None
    source_map = {}

    def remerge_enter(path, key, value):
        new_parent, new_items = default_enter(path, key, value)
        if ret and not path and key is None:
            new_parent = ret
        try:
            cur_val = get_path(ret, path + (key,))
        except KeyError:
            pass
        else:
            # TODO: type check?
            new_parent = cur_val

        if isinstance(value, list):
            # lists are purely additive. See https://github.com/mahmoud/boltons/issues/81
            new_parent.extend(value)
            new_items = []

        return new_parent, new_items

    for t_name, target in target_list:
        if sourced:
            def remerge_visit(path, key, value):
                source_map[path + (key,)] = t_name
                return True
        else:
            remerge_visit = default_visit

        ret = remap(target, enter=remerge_enter, visit=remerge_visit)

    if not sourced:
        return ret
    return ret, source_map


def main():
    from pprint import pprint

    merged, source_map = remerge([('defaults', defaults),
                                  ('overlay', overlay),
                                  ('cache_host_override', cache_host_override)],
                                 sourced=True)

    assert merged['host'] == '127.0.0.1'
    assert merged['port'] == 8080
    assert merged['endpoints']['persistence']['host'] == '10.2.2.2'
    assert merged['endpoints']['persistence']['port'] == 5433
    assert merged['endpoints']['cache']['host'] == '127.0.0.2'
    assert merged['endpoints']['cache']['port'] == 8889
    assert merged['overlay_version'] == '5.0'

    pprint(merged)
    print
    pprint(source_map)
    print(len(source_map), 'paths')


if __name__ == '__main__':
    main()

the_output.py

{'endpoints': {'cache': {'host': '127.0.0.2', 'port': 8889},
               'persistence': {'host': '10.2.2.2', 'port': 5433}},
 'host': '127.0.0.1',
 'notes': ['this is the default', 'this is the overlay'],
 'overlay_version': '5.0',
 'owners': {'primary': ['bob'], 'secondary': ['alice', 'charles']},
 'port': 8080,
 'zones': [{'a': 1}, {'a': 2}]}

{('endpoints',): 'cache_host_override',
 ('endpoints', 'cache'): 'cache_host_override',
 ('endpoints', 'cache', 'host'): 'cache_host_override',
 ('endpoints', 'cache', 'port'): 'defaults',
 ('endpoints', 'persistence'): 'overlay',
 ('endpoints', 'persistence', 'host'): 'overlay',
 ('endpoints', 'persistence', 'port'): 'overlay',
 ('host',): 'overlay',
 ('notes',): 'overlay',
 ('overlay_version',): 'overlay',
 ('owners',): 'overlay',
 ('owners', 'primary'): 'overlay',
 ('owners', 'secondary'): 'overlay',
 ('port',): 'overlay',
 ('zones',): 'overlay'}
(15, 'paths')

@gsemet, I wouldn't mind having it in boltons (though probably in iterutils, just for ease of dependence), we could continue the review process there if you want to prepare a PR.

@JoanEliot, that's true, you need the structures to roughly match. Maybe it makes sense for Nones to be overridden, but by that same token, it might make sense to preprocess one side to remove Nones? I could go either way. I'm glad you got it to work!

Thank you for this utility.

I believe that functions that change their return-type based on their inputs,
are hard to work with, even if it's only for debugging aid.
Also needed provenance information for merged lists,
so i did the following changes:

• refact: feed the source_map when calling the function to be populated only if not None;
  now the return type is always the same.
• enh: when merging lists, source_map lists all mergers (important when debugging).
• optimize: do not to create a new remerge_visit() closure on each(!) container to merge, but decide it up-front.
• optimize: don't recreate input container with dummy id() just to fit the remerge_visit() for when source_map is asked;
  actually don't even override default enter-function when nosource_map is asked, simply run a simpler loop.
• refact: renamed target_list --> *containers, so as not to have to create the list-of-containers when calling it.
• refact: combine trivial else code with try-except.
• Doc: terse sphinx docstring with a doctested example.
• Doc: explain that the input-dicts order is NOT preserved in the results when source_map is asked.
• [edit:] dropped replace_lists argument (my apologies, just wasn't necessary to me, and copy pasted from my project),

def remerge(*containers, source_map: list = None):
    """
    Merge recursively dicts or lists with :func:`boltons.iterutils.remap()`.

    :param containers:
        a list of dicts or lists to merge; later ones take precedence
        (last-wins).
        If `source_map` is given, these must be 2-tuples of ``(name: container)``.
    :param source_map:
        If given, it must be a dictionary, and `containers` arg must be 2-tuples
        like ``(name: container)``.
        The `source_map` will be populated with mappings between path and the name
        of the container it came from.

        .. Warning::
            if source_map given, the order of input dictionaries is NOT preserved
            is the results  (important if your code rely on PY3.7 stable dictionaries).

    :return:
        returns a new, merged top-level container.

    - Adapted from https://gist.github.com/mahmoud/db02d16ac89fa401b968
    - Discusson in: https://gist.github.com/pleasantone/c99671172d95c3c18ed90dc5435ddd57


    **Example**

    >>> defaults = {
    ...     'subdict': {
    ...         'as_is': 'hi',
    ...         'overridden_key1': 'value_from_defaults',
    ...         'overridden_key1': 2222,
    ...         'merged_list': ['hi', {'untouched_subdict': 'v1'}],
    ...     }
    ... }

    >>> overrides = {
    ...     'subdict': {
    ...         'overridden_key1': 'overridden value',
    ...         'overridden_key2': 5555,
    ...         'merged_list': ['there'],
    ...     }
    ... }


    >>> source_map = {}
    >>> remerge(
    ...     ("defaults", defaults),
    ...     ("overrides", overrides),
    ...     source_map=source_map)
     {'subdict': {'as_is': 'hi',
                  'overridden_key1': 'overridden value',
                  'merged_list': ['hi', {'untouched_subdict': 'v1'}, 'there'],
                  'overridden_key2': 5555}}
    >>> source_map
    {('subdict', 'as_is'): 'defaults',
     ('subdict', 'overridden_key1'): 'overrides',
     ('subdict', 'merged_list'):  ['defaults', 'overrides'],
     ('subdict',): 'overrides',
     ('subdict', 'overridden_key2'): 'overrides'}
    """

    ret = None

    def remerge_enter(path, key, value):
        new_parent, new_items = default_enter(path, key, value)
        if ret and not path and key is None:
            new_parent = ret
        try:
            # TODO: type check?
            new_parent = get_path(ret, path + (key,))
        except KeyError:
            pass

        if isinstance(value, list):
            # lists are purely additive. See https://github.com/mahmoud/boltons/issues/81
            new_parent.extend(value)
            new_items = []

        return new_parent, new_items

    if source_map is not None:

        def remerge_visit(path, key, value):
            full_path = path + (key,)
            if isinstance(value, list):
                old = source_map.get(full_path)
                if old:
                    old.append(t_name)
                else:
                    source_map[full_path] = [t_name]
            else:
                source_map[full_path] = t_name
            return True

        for t_name, cont in containers:
            ret = remap(cont, enter=remerge_enter, visit=remerge_visit)
    else:
        for cont in containers:
            ret = remap(cont, enter=remerge_enter)


        ret = remap(cont, enter=remerge_enter, visit=remerge_visit)

    return ret

Unit-tests (without replace-list :-()

# coding: utf-8

# Standard Library
from pprint import pprint

# Gitlab Project Configurator Modules
# from gpc.helpers.remerge import remerge


def test_override_string():
    defaults = {"key_to_override": "value_from_defaults"}

    first_override = {"key_to_override": "value_from_first_override"}

    source_map = {}
    merged = remerge(
        ("defaults", defaults),
        ("first_override", first_override),
        source_map=source_map,
    )

    expected_merged = {"key_to_override": "value_from_first_override"}
    assert merged == expected_merged
    assert source_map == {("key_to_override",): "first_override"}

    merged = remerge(defaults, first_override, source_map=None)
    assert merged == expected_merged


def test_override_subdict():
    defaults = {
        "subdict": {
            "other_subdict": {
                "key_to_override": "value_from_defaults",
                "integer_to_override": 2222,
            }
        }
    }

    first_override = {
        "subdict": {
            "other_subdict": {
                "key_to_override": "value_from_first_override",
                "integer_to_override": 5555,
            }
        }
    }

    expected_merge = {
        "subdict": {
            "other_subdict": {
                "integer_to_override": 5555,
                "key_to_override": "value_from_first_override",
            }
        }
    }

    source_map = {}
    merged = remerge(
        ("defaults", defaults),
        ("first_override", first_override),
        source_map=source_map,
    )
    assert merged == expected_merge
    assert source_map == {
        ("subdict",): "first_override",
        ("subdict", "other_subdict"): "first_override",
        ("subdict", "other_subdict", "integer_to_override"): "first_override",
        ("subdict", "other_subdict", "key_to_override"): "first_override",
    }

    merged = remerge(defaults, first_override, source_map=None)
    assert merged == expected_merge


def test_override_list_append():
    defaults = {"list_to_append": [{"a": 1}]}
    first_override = {"list_to_append": [{"b": 1}]}

    source_map = {}
    merged = remerge(
        ("defaults", defaults),
        ("first_override", first_override),
        source_map=source_map,
    )
    expected_merged = {"list_to_append": [{"a": 1}, {"b": 1}]}

    assert merged == expected_merged
    assert source_map == {("list_to_append",): "first_override"}

    merged = remerge(defaults, first_override, source_map=None)
    assert merged == expected_merged


def test_complex_dict():
    defaults = {
        "key_to_override": "value_from_defaults",
        "integer_to_override": 1111,
        "list_to_append": [{"a": 1}],
        "subdict": {
            "other_subdict": {
                "key_to_override": "value_from_defaults",
                "integer_to_override": 2222,
            },
            "second_subdict": {
                "key_to_override": "value_from_defaults",
                "integer_to_override": 3333,
            },
        },
    }

    first_override = {
        "key_to_override": "value_from_first_override",
        "integer_to_override": 4444,
        "list_to_append": [{"b": 2}],
        "subdict": {
            "other_subdict": {
                "key_to_override": "value_from_first_override",
                "integer_to_override": 5555,
            }
        },
        "added_in_first_override": "some_string",
    }

    second_override = {
        "subdict": {"second_subdict": {"key_to_override": "value_from_second_override"}}
    }

    source_map = {}
    merged = remerge(
        ("defaults", defaults),
        ("first_override", first_override),
        ("second_override", second_override),
        source_map=source_map,
    )
    print("")
    print("'merged' dictionary:")
    pprint(merged)
    print("")
    pprint(source_map)
    print(len(source_map), "paths")

    assert merged["key_to_override"] == "value_from_first_override"
    assert merged["integer_to_override"] == 4444
    assert (
        merged["subdict"]["other_subdict"]["key_to_override"]
        == "value_from_first_override"
    )
    assert merged["subdict"]["other_subdict"]["integer_to_override"] == 5555
    assert (
        merged["subdict"]["second_subdict"]["key_to_override"]
        == "value_from_second_override"
    )
    assert merged["subdict"]["second_subdict"]["integer_to_override"] == 3333
    assert merged["added_in_first_override"] == "some_string"
    assert merged["list_to_append"] == [{"a": 1}, {"b": 2}]

@ankostis Cool! Glad this old gem continues to provide utility. Thanks for sharing :)

Realized the code was a bit inefficient, so i did these two changes on the code above:

• optimize: do not to create a new remerge_visit() closure on each(!) container to merge, but decide it up-front.
• optimize: don't recreate input container with dummy id() just to fit the remerge_visit() for when source_map is asked;

I kept the edited code it in the same comment, above, for future reference.

[edit:] bu i'm still bugged by the handling of None when extending lists :-(