Recursive attr.evolve

In the same manner as attr.asdict(..., recursive=True) exists, I have a use case for a similar option to attr.evolve(). I want all attr instances evolved, but with no need to deepcopy any non-attr attributes. I ended up implementing the code below. Could this be a consideration for addition into python-attrs?

def evolve_recursive(inst, **changes):
    """ Recursive attr.evolve() method, where any attr-based attributes
        will be evolved too.
    """
    cls = inst.__class__
    attrs = attr.fields(cls)
    for a in attrs:
        if not a.init:
            continue
        attr_name = a.name  # To deal with private attributes.
        init_name = attr_name if attr_name[0] != "_" else attr_name[1:]
        if init_name not in changes:
            value = getattr(inst, attr_name)

            # New code
            if attr.has(value.__class__):
                value = evolve_recursive(value)

            changes[init_name] = value
    return cls(**changes)

Sorry for the late answer. Would you mind showing/elaborating some use cases for this?

I’ve had a similar issue in typed settings when I want to updated nested settings and fixed it by serializing instances to dicts, calling a _set_path function and later converting the dicts back to attrs instances (which only works when using (auto) converters, though).

We have a volunteer! ✨

Wat? 👀

attr.evolve() can be adjusted relatively easy to work recursively:

diff --git a/src/attr/_funcs.py b/src/attr/_funcs.py
index 56e3fcf..1cac98e 100644
--- a/src/attr/_funcs.py
+++ b/src/attr/_funcs.py
@@ -333,8 +333,13 @@ def evolve(inst, **changes):
             continue
         attr_name = a.name  # To deal with private attributes.
         init_name = attr_name if attr_name[0] != "_" else attr_name[1:]
+        value = getattr(inst, attr_name)
         if init_name not in changes:
-            changes[init_name] = getattr(inst, attr_name)
+            # Add original value to changes
+            changes[init_name] = value
+        elif has(value):
+            # Evolve nested attrs classes
+            changes[init_name] = evolve(value, **changes[init_name])

     return cls(**changes)

I’m not sure whether this is a backwards incompatible change or rather a bug fix that prevents nested classes from being replaced by a dict. 🤷‍♂️

@hynek

@hynek What are your requirements regarding the public API?

Do you prefer a new function or would you rather update the behavior of the existing one as in the diff above?

I think if you can do it backward compatible, there's no need for a new function?

I guess, it’ll be mostly backward compatible. *mostly, because handling of nested dicts will be different if they map to attributes with attrs classes. ;-)

But the above changes did not break any of the existing tests.

I’ll do a PR once I figure out how to proberly use hypothesis and nested classes for the tests.

Recursively evolve nested attrs classes

Recursively evolve nested attrs classes (#759)

Unfortunately we had to revert the changes due to #804. I think we'll have to write a new function to make stuff unequivocal.

Sorry everyone! :(

Allow evolve() to recursively evolve attrs classes

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Recursive attr.evolve #634

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Recursive attr.evolve #634

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hynek commented on Oct 19, 2020

sscherfke commented on Oct 19, 2020

hynek commented on Oct 30, 2020

sscherfke commented on Oct 30, 2020

sscherfke commented on Dec 13, 2020

sscherfke commented on Dec 19, 2020

hynek commented on Dec 21, 2020

sscherfke commented on Dec 21, 2020

hynek commented on May 6, 2021

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