[Matplotlib-users] tex problem

Hi,

I have different questions to use tex with matplotlib.

The first one:

If I'm doing:

rc('text', usetex=True)
xlabel('$\textrm{toto}$')  # bad
xlabel(r'$\textrm{toto}$') # ok

The results are not the same and I don't understand why.

and for:

xlabel('$\textit{toto}$')  #bad
xlabel(r'$\textit{toto}$') #ok

same things, but it's working fine for:

xlabel('$\it{toto}$') #ok
xlabel(r'$\it{toto}$') #ok

but it's not working for \bf:

xlabel('$\textbf{toto}$')  # bad
xlabel(r'$\textbf{toto}$') # ok
xlabel('$\bf{toto}$')        # bad
xlabel(r'$\bf{toto}$')       # ok

And this bring to the second question, I would like to create the xlabel with 
latex stuf inside somewhere in my script:

label='$\textrm{test}_2$'

but that can't work:

xlabel(label) # bad

How to tell that I want the "r" before the chains to have TeX working like 
expected?

Thanks,

N.