How do you include a bold symbol in the axis when exporting a pgf figure using matplotlib?

I am having trouble exporting a plot I made using matplotlib to a pgf. Without the bold symbol in the axis label, there is no issue, but with I get an error.

Here is my code:

import seaborn as sns
import numpy as np
import matplotlib as mpl
mpl.rcParams.update(mpl.rcParamsDefault)
mpl.use('pgf')
mpl.rcParams.update({
    'pgf.texsystem': "pdflatex",
    'font.family': 'serif',
    'text.usetex': True,
    'pgf.rcfonts': False,
    'font.serif': ["Computer Modern Roman"],
})
mpl.rc('text.latex', preamble=r'\usepackage{bm}')
import matplotlib.font_manager
import matplotlib.pyplot as plt

f, ax = plt.subplots(figsize = (8,4))
sns.set_color_codes("colorblind")
b = sns.barplot(x=probs, y=words, color='b')
ax.set(xlabel = r'{Probability of word, $p(w|\boldsymbol{\pi}_{ML})$}',
    ylabel = r'{Word, $w$}' )
b.set_yticklabels(labels=b.get_yticklabels(), va='center')
f.tight_layout()
plt.savefig('graph.pgf', format='pgf')

And the error I get is:

LatexError: LaTeX process halted
! Undefined control sequence.
<argument> ...rd, \(\displaystyle p(w|\boldsymbol 
                                                  {\pi }_{ML})\)}
<*> ...displaystyle p(w|\boldsymbol{\pi}_{ML})\)}}
                                                  
!  ==> Fatal error occurred, no output PDF file produced!
Transcript written on texput.log.

I thought including \usepackage{bm} in my preamble would've circumvented this issue?

Any help would be greatly appreciated!

For the second time this week...hours of trying to debug and searching has lead me to find the solution immediately after posting on stackoverflow.

Solution:

mpl.rcParams.update({
    'pgf.texsystem': "pdflatex",
    'font.family': 'serif',
    'text.usetex': True,
    'pgf.rcfonts': False,
    "pgf.preamble": "\n".join([
         r"\usepackage{bm}",
    ]),
    'font.serif': ["Computer Modern Roman"],
    'text.latex.preamble': pream,
})