As programmers we read more than we write. I've started working at a company that uses a couple of "big" Python packages; packages or package-families that have a high KLOC. Case in point: Zope.
My problem is that I have trouble navigating this codebase fast/easily. My current strategy is 

I start reading a module I need to change/understand
I hit an import which I need to know more of
I find out where the source code for that import is by placing a Python debug (pdb) statement after the imports and echoing the module, which tells me it's source file
I navigate to it, in shell or the Vim file explorer.
most of the time the module itself imports more modules and before I know it I've got 10KLOC "on my plate"

Alternatively:

I see a method/class I need to know more of
I do a search (ack-grep) for the definition of that method/class across the whole codebase (which can be a pain because the codebase is partly in ~/.buildout-eggs)
I find one or more pieces of code that define that method/class
I have to deduce which one of them is the one I need to read

This costs a lot of time, which is understandable for a big codebase. But I get the feeling that navigating a large and unknown Python codebase is a common enough problem.
So I'm looking for technical tools or strategic solutions for this problem.
...
I just can't imagine hardcore Python programmers using the strategies outlined above.
I use ipython's ?? command
You just need to figure out how to import the things you want to look for, then add ?? to the end of the module or class or function or method name to view their source code. And the command completion helps on figuring out long names as well.
Navigating a big Python codebase faster