Bold font weight for LaTeX axes label in matplotlib

In matplotlib you can make the text of an axis label bold by

plt.xlabel('foo',fontweight='bold')

You can also use LaTeX with the right backend

plt.xlabel(r'$\phi$')

When you combine them however, the math text is not bold anymore

plt.xlabel(r'$\phi$',fontweight='bold')

Nor do the following LaTeX commands seem to have any effect

plt.xlabel(r'$\bf \phi$')
plt.xlabel(r'$\mathbf{\phi}$')

How can I make a bold $\phi$ in my axis label?

Unfortunately you can't bold symbols using the bold font, see this question on tex.stackexchange.

As the answer suggests, you could use \boldsymbol to bold phi:

r'$\boldsymbol{\phi}$'

You'll need to load amsmath into the TeX preamble:

matplotlib.rc('text', usetex=True)
matplotlib.rcParams['text.latex.preamble']=[r"\usepackage{amsmath}"]

If you intend to have consistently bolded fonts throughout the plot, the best way may be to enable latex and add \boldmath to your preamble:

# Optionally set font to Computer Modern to avoid common missing font errors
matplotlib.rc('font', family='serif', serif='cm10')

matplotlib.rc('text', usetex=True)
matplotlib.rcParams['text.latex.preamble'] = [r'\boldmath']

Then your axis or figure labels can have any mathematical latex expression and still be bold:

plt.xlabel(r'$\frac{\phi + x}{2}$')

However, for portions of labels that are not mathematical, you'll need to explicitly set them as bold:

plt.ylabel(r'\textbf{Counts of} $\lambda$'}

In case anyone stumbles across this from Google like I did, another way that doesn't require adjusting the rc preamble (and conflicting with non-latex text) is:

ax.set_ylabel(r"$\mathbf{\partial y / \partial x}$")