Fix Python – 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 d….