Python pandas: fill a dataframe row by row

df['y'] will set a column

since you want to set a row, use .loc

Note that .ix is equivalent here, yours failed because you tried to assign a dictionary
to each element of the row y probably not what you want; converting to a Series tells pandas
that you want to align the input (for example you then don’t have to to specify all of the elements)

In [6]: import pandas as pd

In [7]: df = pd.DataFrame(columns=['a','b','c','d'], index=['x','y','z'])

In [8]: df.loc['y'] = pd.Series({'a':1, 'b':5, 'c':2, 'd':3})

In [9]: df
Out[9]: 
     a    b    c    d
x  NaN  NaN  NaN  NaN
y    1    5    2    3
z  NaN  NaN  NaN  NaN