# To read data from csv file
Dataset = pd.read_csv('Data.csv')
# To get values in variable 'X'
X = Dataset.iloc[:, :].values
# To calculate mean use imputer class
from sklearn.preprocessing import Imputer
imputer = Imputer(missing_values='NaN', strategy='mean', axis=0)
imputer = imputer.fit(X[:, :])
X[:, :] = imputer.transform(X[:, :])