It is a benchmark to find what fast way when merging two dictionaries, with multiple methods.

output.txt

$ pytest --benchmark-compare
/Users/moreal/.pyenv/versions/3.7.4/envs/my-ground/lib/python3.7/site-packages/pytest_benchmark/logger.py:44: PytestBenchmarkWarning: Can't compare. No benchmark files in '/private/tmp/test/pytest/.benchmarks'. Can't load the previous benchmark.
  warner(PytestBenchmarkWarning(text))
==================================================================================== test session starts ====================================================================================
platform darwin -- Python 3.7.4, pytest-5.4.1, py-1.8.1, pluggy-0.13.1
benchmark: 3.2.3 (defaults: timer=time.perf_counter disable_gc=False min_rounds=5 min_time=0.000005 max_time=1.0 calibration_precision=10 warmup=False warmup_iterations=100000)
rootdir: /private/tmp/test/pytest
plugins: benchmark-3.2.3
collected 3 items                                                                                                                                                                           

tests/test_dict.py ...                                                                                                                                                                [100%]


-------------------------------------------------------------------------------------- benchmark: 3 tests --------------------------------------------------------------------------------------
Name (time in ms)                    Min                Max               Mean            StdDev             Median               IQR            Outliers      OPS            Rounds  Iterations
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
test_benchmark_update            30.3878 (1.0)      31.2581 (1.0)      30.6925 (1.0)      0.2579 (1.0)      30.6283 (1.0)      0.3767 (1.0)          10;0  32.5813 (1.0)          31           1
test_benchmark_unpack_repack     49.9390 (1.64)     51.6635 (1.65)     50.4087 (1.64)     0.4511 (1.75)     50.3805 (1.64)     0.5255 (1.40)          2;1  19.8379 (0.61)         13           1
test_construct_double_update     78.9237 (2.60)     81.4208 (2.60)     79.9099 (2.60)     0.7377 (2.86)     79.8507 (2.61)     0.7743 (2.06)          4;1  12.5141 (0.38)         12           1
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Legend:
  Outliers: 1 Standard Deviation from Mean; 1.5 IQR (InterQuartile Range) from 1st Quartile and 3rd Quartile.
  OPS: Operations Per Second, computed as 1 / Mean
===================================================================================== 3 passed in 4.06s =====================================================================================

test_benchmark_dict.py

dict_samples = [({'x': i}, {'y': i+1}) for i in range(100000)]

def unpack_repack(xd, yd):
    return {
        **xd,
        **yd,
    }

def repeat_unpack_repack():
    return [unpack_repack(*ss) for ss in dict_samples.copy()]

def update(xd, yd):
    xd.update(yd)
    return xd

def repeat_update():
    return [update(*ss) for ss in dict_samples.copy()]

def construct_double_update(xd, yd):
    d = {}
    d.update(xd)
    d.update(yd)
    return d

def repeat_construct_double_update():
    return [construct_double_update(*ss) for ss in dict_samples.copy()]

def test_benchmark_unpack_repack(benchmark):
    benchmark(repeat_unpack_repack)

def test_benchmark_update(benchmark):
    benchmark(repeat_update)

def test_construct_double_update(benchmark):
    benchmark(repeat_construct_double_update)

In conclusion, unpack_repack should be the fastest, because it has a instruction, 'BUILD_MAP_UNPACK'.