add smart_deepcopy (originaly from #1679)

[Bobronium]

Change Summary

add smart_deepcopy():

smart_deepcopy(obj: ~Obj) -> ~Obj
    Return type as is for immutable built-in types
    Use obj.copy() for built-in empty collections
    Use copy.deepcopy() only on non-empty collections and unknown objects

It's primarily needed for faster copying of default values, since they're often are immutable types or empty collections

Here's benchmark showing the difference in speed between different types of values:

from copy import deepcopy
from timeit import timeit

from pydantic.utils import smart_deepcopy, BUILTIN_COLLECTIONS

cases = {
    "IMMUTABLE":
        (1, 1.0, '1', b'1', int, None, smart_deepcopy, len, smart_deepcopy.__code__, lambda: _, ...),
    "EMPTY_COLLECTIONS":
        map(lambda collection: collection(), BUILTIN_COLLECTIONS),
    "NON_EMPTY_COLLECTIONS":
        map(lambda c: c.fromkeys([1]) if issubclass(c, dict) else c([1]), BUILTIN_COLLECTIONS),
}

for name, values in cases.items():
    deepcopy_results = []
    smart_deepcopy_results = []
    for value in values:
        deepcopy_results.append(timeit(lambda: deepcopy(value), number=100000))
        smart_deepcopy_results.append(timeit(lambda: smart_deepcopy(value), number=100000))

    deepcopy_result = sum(deepcopy_results)
    smart_deepcopy_result = sum(smart_deepcopy_results)
    faster_by = deepcopy_result / smart_deepcopy_result
    print(f"{name}: {deepcopy_result=:.3}s, {smart_deepcopy_result=:.3}s, {faster_by=:.3} times")

IMMUTABLE: deepcopy_result=0.658s, smart_deepcopy_result=0.235s, faster_by=2.8 times
EMPTY_COLLECTIONS: deepcopy_result=1.73s, smart_deepcopy_result=0.261s, faster_by=6.62 times
NON_EMPTY_COLLECTIONS: deepcopy_result=2.33s, smart_deepcopy_result=2.5s, faster_by=0.933 times

Related issue number

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #1920 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1920   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         3909      3916    +7     
  Branches       788       788           
=========================================
+ Hits          3909      3916    +7     

Impacted Files	Coverage Δ
pydantic/fields.py	100.00% <100.00%> (ø)
pydantic/utils.py	100.00% <100.00%> (ø)

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[PrettyWood]

Nice !

[wozniakty]

Small food for thought that smart_deepcopy seems not particularly descriptive about what this function does, but I cannot come up with a better name so just offering food for thought in case someone else has an idea 😅

[Bobronium]

I also thought about «fast_deepcopy», but function is actually a bit slower in case it has to deepcopy value, so «smart» seemed a better choice.

I agree, it’s kinda vague word, so would love hear any better suggestions :)

[samuelcolvin]

I agree with both, it's not a perfect name, but I can't think of anything better.

[samuelcolvin]

thanks so much, sorry I've taken so long to review this.

[samuelcolvin]

I guess you can now update #1679