Trying to change pandas column dtype from str to float
In pandas the object type is used when there is not a clear distinction between the types stored in the column. 

So, I guess that in your column, some objects are float type and some objects are str type. Or maybe, you are also dealing with NaN objects, NaN objects are float objects.

a) Convert the column to string: Are you getting your DataFrame from a CSV or XLS format file? Then at the moment of reading the file, you can specify that that column is an str type or just make the type conversion of the column you are dealing with.

b) After that, you can apply the string changes and/or deal with the NaN objects.

c) Finally, you transform your column into float type`.