code: def test_stack(self):
      ts_uni = linear_timeseries(length=10)
      ts_multi = ts_uni.stack(ts_uni)
  
      static_covs_uni1 = pd.DataFrame(
          [[0, 1]], columns=["st1", "st2"]
      ).astype(int)
      static_covs_uni2 = pd.DataFrame(
          [[3, 4]], columns=["st3", "st4"]
      ).astype(int)
      static_covs_uni3 = pd.DataFrame(
          [[2, 3, 4]], columns=["st1", "st2", "st3"]
      ).astype(int)
  
      static_covs_multi = pd.DataFrame(
          [[0, 0], [1, 1]], columns=["st1", "st2"]
      ).astype(int)
  
      ts_uni = ts_uni.with_static_covariates(static_covs_uni1)
      ts_multi = ts_multi.with_static_covariates(static_covs_multi)
  
      # valid static covariates for concatenation/stack
      ts_stacked1 = ts_uni.stack(ts_uni)
      assert ts_stacked1.static_covariates.index.equals(
          ts_stacked1.components
      )
      np.testing.assert_almost_equal(
          ts_stacked1.static_covariates_values(copy=False),
          pd.concat([ts_uni.static_covariates] * 2, axis=0).values,
      )
  
      # valid static covariates for concatenation/stack: first only has static covs
      # -> this gives multivar ts with univar static covs
      ts_stacked2 = ts_uni.stack(ts_uni.with_static_covariates(None))
      np.testing.assert_almost_equal(
          ts_stacked2.static_covariates_values(copy=False),
          ts_uni.static_covariates_values(copy=False),
      )
  
      # mismatch between column names
      with pytest.raises(ValueError):
          _ = ts_uni.stack(
              ts_uni.with_static_covariates(static_covs_uni2)
          )
  
      # mismatch between number of covariates
      with pytest.raises(ValueError):
          _ = ts_uni.stack(
              ts_uni.with_static_covariates(static_covs_uni3)
          )
  
      # valid univar ts with univar static covariates + multivar ts with multivar static covariates
      ts_stacked3 = ts_uni.stack(ts_multi)
      np.testing.assert_almost_equal(
          ts_stacked3.static_covariates_values(copy=False),
          pd.concat(
              [ts_uni.static_covariates, ts_multi.static_covariates],
              axis=0,
          ).values,
      )
  
      # invalid univar ts with univar static covariates + multivar ts with univar static covariates
      with pytest.raises(ValueError):
          _ = ts_uni.stack(
              ts_multi.with_static_covariates(static_covs_uni1)
          )
  
imports: import copy
  import os
  import numpy
  import pandas
  import pytest
  from darts import TimeSeries, concatenate
  from darts.dataprocessing.transformers import BoxCox, Scaler
  from darts.timeseries import DEFAULT_GLOBAL_STATIC_COV_NAME, STATIC_COV_TAG
  from darts.utils.timeseries_generation import generate_index, linear_timeseries
label_desc: Use image node_id 6 for calling the TestTimeSeriesStaticCovariate obj's underlying member method code with example usage: obj.test_stack() without return types