Как прописать последовательный поиск по DataFrame с условием?

roll = pd.DataFrame([['goods1', 200],['goods2', -100],['goods3', 250]], columns = ['A', 'B'])
 
roll
 
variable = 'goods1'
 
a = []
for i in range(0, len(roll)):
    if roll.loc[i, 'A'] == variable:
        aa = roll.loc[i, 'B']
        a.append(aa*2)
a

aa = roll['B'][roll['A'] == variable]
a = aa**2

aa = roll.B[roll.A== variable]
a = aa**2

roll = pd.DataFrame([['goods1', 200],['goods2', -100],['goods3', 250]], columns = ['A', 'B'])
variable = 'goods1'
 
a = []
for i in range(0, len(roll)):
    if roll.loc[i, 'A'] == variable:
        aa = roll.B[roll.A== variable]
        a = aa*2
a

variables = ['goods1', 'goods2', ...] # замените на множество ({'goods1', 'goods2', ...}) если переменных много, так быстрее будет.
aa = roll.B[roll.A.isin(variables)] ** 2

roll = pd.DataFrame([['goods1', 200],['goods2', -100],['goods3', 250]], columns = ['A', 'B'])
variables1 = ['goods2']
a = []
for i in range(0, len(roll)):
    if roll.loc[i, 'A'] == variables1 :
        aa = roll.B[roll.A.isin(variables1)]
        print(aa*2)
        a.append(aa)
a
[]

roll1 = pd.DataFrame([['goods1', 200],['goods2', -100],['goods3', 250]], columns = ['A', 'B'])
 
variables2 = []
for i in roll1['A']:
    variables2.append(i)
variables2
 
dataset_g = roll.B[roll.A.isin(variables2)]
dataset_g