Hello,
I have problem with the font of my Pd. I have installed Pd 0.50.2 and the
font it is using is Courier 10 instead of Deja Vu Sans Mono. So I get this:
*WARNING: Font family 'DejaVuSansMono.ttf' not found, using default
(Courier 10 Pitch)*

The Pd GUI uses the Tk library which accesses the system to find what fonts are available. If the deja vu package is installed and the font is accessible by other applications, it could be a problem with Tk and/or the naming of the font on your platform. Pd actually does very little itself other than ask Tk to get a font from the system called "DejaVu Sans Mono" which it then uses to estimate sizing for the object boxes.

Not a big deal but I would like to understand why and how to have it right.

This is a bug, either with some configuration on your system, with Tk, or possible with Pd.

I teach so I would like my students to see I use the same font as them ;)

This is also why deja vu sans mono is used for all platforms: consistency of the canvas between platforms by default.

The funny thing is that I had this problem, I deleted all directories I
could find about Pd, did:
*sudo apt-get --purge remove puredata*
*sudo apt-get autoremove*
*sudo apt-get --purge remove puredata-core* (because with *dpkg list* and
it was still there*)*
Then I reinstalled this version of Pd, compiled from here
http://msp.ucsd.edu/Software/pd-0.50-2.src.tar.gz
<https://www.google.com/url?q=http://msp.ucsd.edu/Software/pd-0.50-2.src.tar.gz&sa=D&source=hangouts&ust=1588244565555000&usg=AFQjCNHgBYMjxM-j3AHvu1vZfm5zN7KhMw>

then compiled with
*./autogen.sh*

*make *
*sudo make install*

Rebuilding Pd does nothing in this case as the font finding & loading is all done via the tcl/pd-gui.tcl startup script. Finding fonts is not done in C on Linux or macOS and only via a small custom Tcl plugin on Windows.

Then on top of this I tried
*sudo apt-get install fonts-dejavu*
with no effect, in the terminal I read it is already there in the *font
folder.*

As the font/README.txt file states:

"DVSM is usually installed with the system & is a dependency for the puredata

package. The included font file in this directory are not installed with Pd.

The fallback font when not found is Courier."

The files in the font directory are only used for Windows and macOS as they need to be distributed with Pd to be available without requiring manual installation.