what is the different of  str and object? when  date.series.astype()
#### Code Sample, a copy-pastable example if possible

```python
relatedobjid
5328242781
5328280661
5328282538
5328350087
5328398153
5328452557
5328452719
5328454338
5328463804
5328511640
5328512890
5328517629
5328535452
5328541156

aa=pd.read_clipboard()
##i want the following result 

a1=','.join(aa.relatedobjid) 

print(a1)
'5328242781,5328280661,5328282538,5328350087,5328398153,5328452557,5328452719,5328454338,5328463804,5328511640,5328512890,5328517629,5328535452,5328541156'

##but relatedobjid is int ,so i wang to transform to string

a1=pd.DataFrame(aa.relatedobjid.astype(object))
a2=','.join(aa.relatedobjid)
##but 
Traceback (most recent call last):

  File "<ipython-input-10-9a311f5b92c7>", line 1, in <module>
    a1=','.join(aa.relatedobjid)

TypeError: sequence item 0: expected str instance, int found

##when i input      
a1=pd.DataFrame(aa.relatedobjid.astype(str))
a2=','.join(aa.relatedobjid)

##it is ok    what is the different of    str and object?

```



