Examples
import numpy as np
import pandas as pd
df = pd.DataFrame([[0, 4, 5], [0, 6, 7], [20, 30, 40]],
index=[1, 2, 3], columns=['P', 'Q', 'R'])
df
Get value at specified row/column pair
df.at[2, 'Q']
Set value at specified row/column pair
df.at[2, 'Q'] = 20
df.at[2, 'Q']
Get value within a Series
df.loc[3].at['Q']