how to convert list to str. 'DataFrame' object is not callable error

I've recreated your table with the following code

df_meta_review1 = pd.DataFrame([('a101',['pc', 'game']),('a1001', ['mobile', 'game', 'education'])], columns = ['userID', 'categories'])

Running the first piece of code gives no error. Are you sure you get an error when its run? Below is the resultant table.

  userID                 categories                  category_domain
0   a101                 [pc, game]                   ['pc', 'game']
1  a1001  [mobile, game, education]  ['mobile', 'game', 'education']

If I run the second line, I also get an error. Like so:

TypeError: 'DataFrame' object is not callable

To fix this I remove the round parenthesis from after the data frame name, as the error suggests you we are trying to use the data frame as a function (this is what callable means)

df_meta_review1['category_domain'] = df_meta_review1['categories'].apply(','.join)

This no longer generates an error and the output looks like:

  userID                 categories        category_domain
0   a101                 [pc, game]                pc,game
1  a1001  [mobile, game, education]  mobile,game,education

Which, I think is what you are looking for. For the third line of code I get the error:

TypeError: 'DataFrame' object is not callable 

Again, dropping the round parenthesis from after the data frame name, yields code that executes without error.

df_meta_review1['category_domain'] = ",".join(map(str, df_meta_review1['categories']))

However I don't think it is what you are looking for.

  userID                 categories                                 category_domain
0   a101                 [pc, game]  ['pc', 'game'],['mobile', 'game', 'education']
1  a1001  [mobile, game, education]  ['pc', 'game'],['mobile', 'game', 'education']

You can make use of apply function that loop through the dataframe and define a function to return the results for each row, like this:

def app(row):
    return ",".join(row['categories'])

and pass this to apply function:

df_meta_review1['category_domain'] = df_meta_review1.apply(lambda x: app(x),axis =1)

output:

  userID                 categories          category_domain
0   a101                 [pc, game]                  pc,game
1  a1001  [mobile, game, education]    mobile,game,education