Create a Pandas Dataframe by appending one row at a time

How do I create an empty DataFrame, then add rows, one by one?

I created an empty DataFrame:

df = pd.DataFrame(columns=('lib', 'qty1', 'qty2'))

Then I can add a new row at the end and fill a single field with:

df = df._set_value(index=len(df), col="qty1", value=10.0)

It works for only one field at a time. What is a better way to add new row to df?