I'm not female, but I saw lots of single (local) females traveling on the subway (and trams) - and it seemed perfectly safe. Just don't go looking too much like a tourist, or flashing around signs of "wealth" to draw attention to yourself.

The trams can get PACKED at certain times -if you must use them then, just make sure you are pickpocket proof.

I found it like any other city - usual precautions, be and look confident and not lost, and be on the lookout for scams.

The one time I did feel I was being scammed (and not in Prague) was after crossing the border (from Germany) by train, and the Czech train conductor bugging me a couple of times for change. I have a feeling I'd have been getting scammed.