Insert variable values into a string [duplicate]

I want to introduce a variable [i] into a string in Python.

For example look at the following script. I just want to be able to give a name to the image, for example geo[0].tif ... to geo[i].tif, or if you use an accountant as I can replace a portion of the value chain to generate a counter.

data = self.cmd("r.out.gdal in=rdata out=geo.tif")self.dataOutTIF.setValue("geo.tif")