Q:

Pandas: change data type of Series to String

I use Pandas 'ver 0.12.0' with Python 2.7 and have a dataframe as below:
df = pd.DataFrame({'id' : [123,512,'zhub1', 12354.3, 129, 753, 295, 610],
                    'colour': ['black', 'white','white','white',
                            'black', 'black', 'white', 'white'],
                    'shape': ['round', 'triangular', 'triangular','triangular','square',
                                        'triangular','round','triangular']
                    },  columns= ['id','colour', 'shape'])

The id Series consists of some integers and strings. Its dtype by default is object. I want to convert all contents of id to strings. I tried astype(str), which produces the output below.
df['id'].astype(str)
0    1
1    5
2    z
3    1
4    1
5    7
6    2
7    6

1) How can I convert all elements of id to String? 
2) I will eventually use id for indexing for dataframes. Would having String indices in a dataframe slow things down, compared to having an integer index?

A:

You can convert all elements of id to str using apply
df.id.apply(str)

0        123
1        512
2      zhub1
3    12354.3
4        129
5        753
6        295
7        610

Edit by OP:
I think the issue was related to the Python version (2.7.), this worked:
df['id'].astype(basestring)
0        123
1        512
2      zhub1
3    12354.3
4        129
5        753
6        295
7        610
Name: id, dtype: object

A:

You must assign it, like this:-
df['id']= df['id'].astype(str)

A:

A new answer to reflect the most current practices: as of version 1.0.1, neither astype('str') nor astype(str) work. 
As per the documentation, a Series can be converted to the string datatype in the following ways:
df['id'] = df['id'].astype("string")

df['id'] = pandas.Series(df['id'], dtype="string")

df['id'] = pandas.Series(df['id'], dtype=pandas.StringDtype)

