I have a dataframe in which one of the columns has a uniform datatype of string, and for a specific string I wanted to check if it was in that column. I found the simple comparison gave 'False', but after checking the dataframe itself I verified that the specific string was indeed in that column. On the other hand, if I convert the selected column to a set I get the expected behaviour. 

```
>>> 'string' in df['col_name']
False
>>> 'string' in set(df['col_name'])
True

```

1. Why do pandas dataframes have this behaviour?
2. Is the a conventional way to check if an object is in a pandas dataframe column?