Build the full path filename in Python

I need to pass a file path name to a module. How do I build the file path from a directory name, base filename, and a file format string?

The directory may or may not exist at the time of call.

For example:

dir_name="/home/me/dev/my_reports"
base_filename="daily_report"
format="pdf"

I need to create a string '/home/me/dev/my_reports/daily_report.pdf'

Concatenating the pieces manually doesn’t seem to be a good way. I tried os.path.join:

join(dir_name,base_filename,format)

but it gives

/home/me/dev/my_reports/daily_report/pdf