How about this:
/^[^#]*ipdb
The first ^ will anchor the match to the start of the line, [^#] will match
any character except a # (the ^ means to match any character except
those given), and the * repeats this 0 or more times.
See::help /^
:help /[ for some more information.
Note that this will match all lines with ipdb which don't start with a comment, including:
import other_module, ipdb
If you only want to match those starting with import ipdb, you can use:
/^\s*import ipdb
Here ^ will again anchor the match to the start of the line, \s will match any whitespace character, and * repeats this 0 or more times.