I have a csv file with a column whose type is a dictionary (column 'b' in the example below). But **b** in **df** is a string type even though I define it as the dictionary type. I didn't find the solution to this question. Any suggestions?

```
a = pd.DataFrame({'a': [1,2,3], 'b':[{'a':3, 'haha':4}, {'c':3}, {'d':4}]})
a.to_csv('tmp.csv', index=None)
df = pd.read_csv('tmp.csv', dtype={'b':dict})

```