This fix may lead to some backward compatibility issues with outdated versions of mapnik and proj. I am not sure if it is an issue, because most popular distributions ship mapnik versions that support newer proj API thus incompatible with current mapproxy source code.

However, if we don't want to deal with this compatibility break there is a way to generate valid proj4 string from EPSG code with pyproj and using it for mapnik configuration

proj_string = pyproj.Proj('EPSG:XXXX').to_proj4()

Hello, sorry for the very late response. If we can use a variant that does not break compatibility, I would prefer that one. We can then merge this in the near future.

What about something like this? It needs an import from pyproj that I see nowhere else, but that library seems to be a requirement for mapproxy.

from pyproj import PROJ_VERSION
#....
# Check major proj version and adjust srs definition accordingly
if PROJ_VERSION[0] >= 6:
    m.srs = str(query.srs.srs_code.lower())
else:
    m.srs = '+init=%s' % str(query.srs.srs_code.lower())