code: def test_fontconfig_preamble():
      """Test that the preamble is included in the source."""
      plt.rcParams["text.usetex"] = True
  
      src1 = TexManager()._get_tex_source("", fontsize=12)
      plt.rcParams["text.latex.preamble"] = "\\usepackage{txfonts}"
      src2 = TexManager()._get_tex_source("", fontsize=12)
  
      assert src1 != src2
  
file_path: matplotlib/lib/matplotlib/tests/test_texmanager.py
imports: import os
  from pathlib import Path
  import re
  import subprocess
  import sys
  import matplotlib.pyplot
  from matplotlib.texmanager import TexManager
  from matplotlib.testing._markers import needs_usetex
  import pytest
label_desc: Use image node_id 1 for calling a global function with example usage: test_fontconfig_preamble() without return types