code: def test_add_aes():
      data = pd.DataFrame({"var1": [1, 2, 3, 4], "var2": 2})
      p = ggplot(data) + geom_point()
      p += aes("var1", "var2")
  
      assert p.mapping == aes("var1", "var2")
      assert p.labels.x == "var1"
      assert p.labels.y == "var2"
  
imports: from copy import deepcopy
  import numpy
  import pandas
  import pytest
  from plotnine import aes, after_scale, after_stat, annotate, coord_trans, facet_null, geom_bar, geom_histogram, geom_line, geom_point, ggplot, ggtitle, guides, labs, lims, scale_x_continuous, stage, stat_identity, theme, theme_gray, xlab, xlim, ylab
  from plotnine.exceptions import PlotnineError, PlotnineWarning
  from plotnine.mapping.aes import is_valid_aesthetic
label_desc: Use image node_id 12 for calling a global function with example usage: test_add_aes() without return types