issues

Reduce number of imports inside functions. I think it helps making the code easier to read as well, as now you know that plt is always available.

• [x] Tests added
• [x] Passes pre-commit run --all-files
• [ ] User visible changes (including notable bug fixes) are documented in whats-new.rst

Seems to not be a major difference in initial imports from (my small sample of) repeated tests:

This branch: ```python %timeit -n1 -r1 import xarray

3.81 s ± 0 ns per loop (mean ± std. dev. of 1 run, 1 loop each) 3.83 s ± 0 ns per loop (mean ± std. dev. of 1 run, 1 loop each) 3.87 s ± 0 ns per loop (mean ± std. dev. of 1 run, 1 loop each) 3.7 s ± 0 ns per loop (mean ± std. dev. of 1 run, 1 loop each) 3.77 s ± 0 ns per loop (mean ± std. dev. of 1 run, 1 loop each) 3.91 s ± 0 ns per loop (mean ± std. dev. of 1 run, 1 loop each) 3.8 s ± 0 ns per loop (mean ± std. dev. of 1 run, 1 loop each)

np.mean([3.81, 3.83, 3.87, 3.7, 3.77, 3.91, 3.8]) Out[3]: 3.812857142857143 ```

Main: ```python %timeit -n1 -r1 import xarray

3.93 s ± 0 ns per loop (mean ± std. dev. of 1 run, 1 loop each) 3.69 s ± 0 ns per loop (mean ± std. dev. of 1 run, 1 loop each) 3.64 s ± 0 ns per loop (mean ± std. dev. of 1 run, 1 loop each) 3.76 s ± 0 ns per loop (mean ± std. dev. of 1 run, 1 loop each) 3.79 s ± 0 ns per loop (mean ± std. dev. of 1 run, 1 loop each) 3.81 s ± 0 ns per loop (mean ± std. dev. of 1 run, 1 loop each) 3.68 s ± 0 ns per loop (mean ± std. dev. of 1 run, 1 loop each)

np.mean([3.93, 3.69, 3.64, 3.76, 3.79, 3.81, 3.68]) Out[4]: 3.7571428571428567 ```

There's some error in the minimum environments (for example ubuntu-latest py37-bare-minimum) introduced in #5682 but was hidden in all other errors because of #5654: =========================== short test summary info ============================ ERROR xarray/tests/test_plot.py::test_latex_name_isnt_split - NameError: name... = 7908 passed, 4163 skipped, 202 xfailed, 42 xpassed, 89 warnings, 1 error in 279.31s (0:04:39) =

But when looking further in the details it mentions a completely different function crashing because matplotlib isn't installed, which is true but the function doesn't test any mpl-specific things. ``` ==================================== ERRORS ==================================== ___ ERROR at teardown of test_latex_name_isnt_split ____ [gw0] linux -- Python 3.7.10 /usr/share/miniconda/envs/xarray-tests/bin/python

@pytest.fixture(scope="function", autouse=True)
def test_all_figures_closed():
    """meta-test to ensure all figures are closed at the end of a test

    Notes:  Scope is kept to module (only invoke this function once per test
    module) else tests cannot be run in parallel (locally). Disadvantage: only
    catches one open figure per run. May still give a false positive if tests
    are run in parallel.
    """
    yield None

  open_figs = len(plt.get_fignums())

E NameError: name 'plt' is not defined

/home/runner/work/xarray/xarray/xarray/tests/test_plot.py:72: NameError ``` Might be something that pytest runs on every function. Not sure if there's a clean way of turning it off, but requiring matplotlib to be installed works of course.

• [x] Passes pre-commit run --all-files

Attempt to fix errors found in #5365. I've changed it to integers because when does it ever make sense to input 1.5 or a complex number? It will always be forced to integers anyway: https://github.com/dask/dask/blob/8aea537d925b794a94f828d35211a5da05ad9dce/dask/array/core.py#L2815

Integers don't work with Numbers: xarray/core/computation.py:1576: error: Dict entry 0 has incompatible type "Hashable": "int"; expected "Hashable": "Union[None, Number, str, Tuple[Number, ...]]" [dict-item] xarray/core/computation.py:1576: error: Dict entry 0 has incompatible type "Hashable": "int"; expected "Hashable": "Union[None, Number, Tuple[Number, ...]]" [dict-item] xarray/core/computation.py:1536: error: Dict entry 0 has incompatible type "Hashable": "int"; expected "Hashable": "Union[None, Number, str, Tuple[Number, ...]]" [dict-item]

Floats don't work with Numbers: xarray/core/computation.py:1536: error: Dict entry 0 has incompatible type "Hashable": "int"; expected "Hashable": "Union[None, Number, str, Tuple[Number, ...]]" [dict-item] xarray/core/computation.py:1578: error: Dict entry 0 has incompatible type "Hashable": "float"; expected "Hashable": "Union[None, Number, str, Tuple[Number, ...]]" [dict-item] xarray/core/computation.py:1578: error: Dict entry 0 has incompatible type "Hashable": "float"; expected "Hashable": "Union[None, Number, Tuple[Number, ...]]" [dict-item] This workaround seems related to: https://github.com/python/mypy/issues/3186

• [ ] Closes #xxxx
• [ ] Tests added
• [ ] Passes pre-commit run --all-files
• [ ] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [ ] New functions/methods are listed in api.rst