Use of if-statements for Pandas Dataframe [duplicate]

(Code at the bottom)\

            price_a  price_b companies
2019-01-03       25       50         A
2019-03-03       30       60         B
2019-03-19       23       46         A

Let us say my dataframe above is named 'df'. I would like to extract the entire rows for Company 'A' under the column 'companies'. I am thinking how I could implement the if-statement:

if df['companies'] == 'A':\
    print.....


CODE for df:
p = [25,30,23]
dates = ['2019-01-03', '2019-03-03', '2019-03-19']
df = pd.DataFrame(p, index = dates, columns = ['price_a'])
df['price_b'] = df['price_a'] * 2
df.loc['2019-01-03', 'companies'] = 'A'
df.loc['2019-03-03', 'companies'] = 'B'
df.loc['2019-03-19', 'companies'] = 'A'

print(df)