Skip to content

Merging values

The preserves.merge module exports various utilities for merging Values.

merge(v0, *vs, merge_embedded=None)

Repeatedly merges v0 with each element in vs using merge2, returning the final result. The merge_embedded parameter is passed on to merge2.

Source code in preserves/merge.py 
 8
 9
10
11
12
13
14
def merge(v0, *vs, merge_embedded=None):
    """Repeatedly merges `v0` with each element in `vs` using [merge2][preserves.merge.merge2],
    returning the final result. The `merge_embedded` parameter is passed on to merge2."""
    v = v0
    for vN in vs:
        v = merge2(v, vN, merge_embedded=merge_embedded)
    return v

merge2(a, b, merge_embedded=None)

Merges a and b, returning the result. Raises ValueError if, during the merge, a pair of incompatible values is discovered.

If a and b are Embedded objects, their embeddedValues are merged using merge_embedded, and the result is again wrapped in an Embedded object.

>>> merge2(123, 234)
Traceback (most recent call last):
  ...
ValueError: Cannot merge items
>>> merge2(123, 123)
123
>>> merge2('hi', 0)
Traceback (most recent call last):
  ...
ValueError: Cannot merge items
>>> merge2([1, 2], [1, 2])
[1, 2]
>>> merge2([1, 2], [1, 3])
Traceback (most recent call last):
  ...
ValueError: Cannot merge items
>>> merge2({'a': 1, 'b': 2}, {'a': 1, 'c': 3})
{'a': 1, 'b': 2, 'c': 3}
>>> merge2({'a': 1, 'b': 2}, {'a': 10, 'c': 3})
Traceback (most recent call last):
  ...
ValueError: Cannot merge items
>>> merge2(Record('a', [1, {'x': 2}]), Record('a', [1, {'y': 3}]))
a(1, {'x': 2, 'y': 3})
Source code in preserves/merge.py 
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
def merge2(a, b, merge_embedded=None):
    """Merges `a` and `b`, returning the result. Raises `ValueError` if, during the merge, a
    pair of incompatible values is discovered.

    If `a` and `b` are [Embedded][preserves.values.Embedded] objects, their `embeddedValue`s
    are merged using `merge_embedded`, and the result is again wrapped in an
    [Embedded][preserves.values.Embedded] object.

    ```python
    >>> merge2(123, 234)
    Traceback (most recent call last):
      ...
    ValueError: Cannot merge items
    >>> merge2(123, 123)
    123
    >>> merge2('hi', 0)
    Traceback (most recent call last):
      ...
    ValueError: Cannot merge items
    >>> merge2([1, 2], [1, 2])
    [1, 2]
    >>> merge2([1, 2], [1, 3])
    Traceback (most recent call last):
      ...
    ValueError: Cannot merge items
    >>> merge2({'a': 1, 'b': 2}, {'a': 1, 'c': 3})
    {'a': 1, 'b': 2, 'c': 3}
    >>> merge2({'a': 1, 'b': 2}, {'a': 10, 'c': 3})
    Traceback (most recent call last):
      ...
    ValueError: Cannot merge items
    >>> merge2(Record('a', [1, {'x': 2}]), Record('a', [1, {'y': 3}]))
    a(1, {'x': 2, 'y': 3})

    ```

    """
    if a == b:
        return a
    if isinstance(a, (list, tuple)) and isinstance(b, (list, tuple)):
        return merge_seq(a, b)
    if isinstance(a, (set, frozenset)) and isinstance(b, (set, frozenset)):
        _die()
    if isinstance(a, dict) and isinstance(b, dict):
        r = {}
        for (ak, av) in a.items():
            bv = b.get(ak, None)
            r[ak] = av if bv is None else merge2(av, bv, merge_embedded=merge_embedded)
        for (bk, bv) in b.items():
            if bk not in r:
                r[bk] = bv
        return r
    if isinstance(a, Record) and isinstance(b, Record):
        return Record(merge2(a.key, b.key, merge_embedded=merge_embedded),
                      merge_seq(a.fields, b.fields, merge_embedded=merge_embedded))
    if isinstance(a, Embedded) and isinstance(b, Embedded):
        m = (merge_embedded or merge_embedded_id)(a.embeddedValue, b.embeddedValue)
        if m is None: _die()
        return Embedded(m)
    _die()
Last update: March 16, 2023
Created: March 16, 2023