"Finance" is such a broad term with respect to IT - you'll find anything if you look hard enough. The following is based on my experience only - I'm sure there will be lots on here with different experiences:

* I've seen more C++/Java/C# than anything else, with a large legacy base of COBOL/RPG (yes, GMI, we're looking at you...).

* Industrial-grade quant libs (for risk, P&L, models that need regulatory approval) tend to be in C++, with the infrastructure to run them increasingly in Java.

* Lots of VBA on the desktop. Every department runs on Excel...

* Python is making inroads (see Quartz at BofA, Athena at JPM), but I suspect that it's still vastly outnumbered (by whatever measure) by C++/Java.

Some firms (and departments in bigger shops) embrace new technology agressively - at my firm I know of large Scala projects, python, big data (mainly Mongo/Hadoop), R and some Haskell.

That said, on my first job in the UK for a large insurance firm, we employed a team that wrote custom CICS machine code for Z-series mainframes. Beat that with yer fancy functional languages...

So no, I wouldn't say that python is dominant.