import pandas as pd
columns = ["col1", "col2"]
left = pd.DataFrame([[1, 2]], columns=columns)
right = pd.DataFrame(columns=columns)
res = left.merge(right, on="col1", how="outer")
print(res) # 空表没事儿
res = right.merge(left, on="col1", how="outer")
print(res) # 空表没事儿