string variable as latex in pyplot

It is possible to have Latex formatting in pyplot diagrams, i.e. for title, labels etc.

#some dummy code
plt.plot(x,y,label = r"$a < \gamma$")

It is generated by adding an r before the actual latex string. Now, pyplot accepts variables as inputs for strings, i.e.

#some dummy code
foo = "some fancy label"
plt.plot(x,y, label = foo)

I would like to combine the two worlds, however the only answer I found on SO is for Julia.

How can I make this work?