I want to set the value of a pandas column as a list of strings. However, my efforts to do so didn't succeed because pandas take the column value as an iterable and I get a: `ValueError: Must have equal len keys and value when setting with an iterable`. 

Here is an MWE

```
>> df = pd.DataFrame({'col1': [1, 2, 3], 'col2': [4, 5, 6]})
>> df
col1    col2
0   1   4
1   2   5
2   3   6

>> df['new_col'] = None
>> df.loc[df.col1 == 1, 'new_col'] = ['a', 'b']
ValueError: Must have equal len keys and value when setting with an iterable

```

I tried to set the `dtype` as `list` using `df.new_col = df.new_col.astype(list)` and that didn't work either. 

I am wondering what would be the correct approach here. 

---

EDIT

The answer provided here: [Python pandas insert list into a cell](https://stackoverflow.com/q/26483254/937153) using `at` didn't work for me either.