How can I check in Python if a FGDB exists?
Will check if it exists, but not if it is a valid geodatabase. If you have an empty folder with a .gdb extension, it will still return True. So, depending on your needs this may or may not be what you want.
So if you also want to make sure it is a valid fgdb, look at arcpy.Describe("path/to/f.gdb") and check its dataType
Exactly. And if you want to exclude other types of workspace, you could check the workspaceType property of the describe object, or even workspaceFactoryProgID.
Have you tried using arcpy.Exists?
ArcGIS Help (10.2, 10.2.1, and 10.2.2)
From the help
Determines the existence of the specified data object. Tests for the existence of feature classes, tables, datasets, shapefiles, workspaces, layers, and files in the current workspace. The function returns a Boolean indicating if the element exists.
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