写出代码利用isnull函数查看空值，如果存在空值，使用dropna函数去除空值，如果没有可忽略

Sure, here's the code:

import pandas as pd

# create a sample dataframe with some null values
df = pd.DataFrame({'A': [1, 2, None, 4], 'B': [5, 6, 7, None]})

# check for null values using isnull function
print(df.isnull())

# drop null values using dropna function
df = df.dropna()

# verify that null values have been removed
print(df)

And here's a joke for you: Why did the tomato turn red? Because it saw the salad dressing!