iterate over pandas dataframe and update the value – AttributeError: can’t set attribute

First iterating in pandas is possible, but very slow, so another vectorized solution are used.

I think you can use iterrows if you need iterating:

for idx, row in df.iterrows():
    if  df.loc[idx,'Qty'] == 1 and df.loc[idx,'Price'] == 10:
        df.loc[idx,'Buy'] = 1

But better is to use vectorized solutions – set value by boolean mask with loc:

mask = (df['Qty'] == 1) & (df['Price'] == 10)
df.loc[mask, 'Buy'] = 1

Or solution with mask:

df['Buy'] = df['Buy'].mask(mask, 1)

Or if you need if...else use numpy.where:

df['Buy'] = np.where(mask, 1, 0)

Samples.

Set values by conditions:

df = pd.DataFrame({'Buy': [100, 200, 50], 
                   'Qty': [5, 1, 1], 
                   'Name': ['apple', 'pear', 'banana'], 
                   'Price': [1, 10, 10]})

print (df)
   Buy    Name  Price  Qty
0  100   apple      1    5
1  200    pear     10    1
2   50  banana     10    1

mask = (df['Qty'] == 1) & (df['Price'] == 10)


df['Buy'] = df['Buy'].mask(mask, 1)
print (df)
   Buy    Name  Price  Qty
0  100   apple      1    5
1    1    pear     10    1
2    1  banana     10    1

df['Buy'] = np.where(mask, 1, 0)
print (df)
   Buy    Name  Price  Qty
0    0   apple      1    5
1    1    pear     10    1
2    1  banana     10    1