It's keywords as sentences. It basically allows a function signature to be spelled out as a sentence with infix operators. These days you can see the lineage of

It's keywords as sentences. It basically allows a function signature to be spelled out as a sentence with infix operators. These days you can see the lineage of smalltalk in keyword arguments. For example, python vs. smalltalk: obj.do_thing("string", with_factor=8, log_to=logger) obj do_thing: "string" with_factor: 8 log_to: logger One of those reads as a sentence a little better. But to be fair python learned this from languages that had already taken this idea from smalltalk. Like most older languages smalltalk's implementation is more powerful than the bastardization that other languages learned from it. Library interfaces in smalltalk are built around this syntax feature. Note how the function call looks the same as the keyword argument. This shared syntax makes the language read better. It makes function calls and keywords look like a normal infix language (e.g. the math expressions in most languages). Where parens are used like any other infix language (rather than be overloaded for function calls): (obj do_thing: "string" with_factor: 8) log_to: logger would then call ".log_to(logger)" on the result of the parenthesized expression. The same way in (2 + 4) * 5 multiplies the result of the parens by 5. A single syntax structure for all these concepts - rather than mixing multiple syntax - allows it to express new ideas in the same syntax as everything else. Like lisp does.