I know Ruby very well. I believe that I may need to learn Python presently. For those who know both, what concepts are similar between the two, and what are different?

I'm looking for a list similar to a primer I wrote for Learning Lua for JavaScripters: simple things like whitespace significance and looping constructs; the name of nil in Python, and what values are considered "truthy"; is it idiomatic to use the equivalent of map and each , or are mumble somethingaboutlistcomprehensions mumble the norm?

If I get a good variety of answers I'm happy to aggregate them into a community wiki. Or else you all can fight and crib from each other to try to create the one true comprehensive list.