title: Pandas: convert dtype 'object' to int
body: <p>I've read an SQL query into Pandas and the values are coming in as dtype 'object', although they are strings, dates and integers. I am able to convert the date 'object' to a Pandas datetime dtype, but I'm getting an error when trying to convert the string and integers.</p>
  
  <p>Here is an example:</p>
  
  <pre><code>&gt;&gt;&gt; import pandas as pd
  &gt;&gt;&gt; df = pd.read_sql_query('select * from my_table', conn)
  &gt;&gt;&gt; df
      id    date          purchase
   1  abc1  2016-05-22    1
   2  abc2  2016-05-29    0
   3  abc3  2016-05-22    2
   4  abc4  2016-05-22    0
  
  &gt;&gt;&gt; df.dtypes
   id          object
   date        object
   purchase    object
   dtype: object
  </code></pre>
  
  <p>Converting the <code>df['date']</code> to a datetime works:</p>
  
  <pre><code>&gt;&gt;&gt; pd.to_datetime(df['date'])
   1  2016-05-22
   2  2016-05-29
   3  2016-05-22
   4  2016-05-22
   Name: date, dtype: datetime64[ns] 
  </code></pre>
  
  <p>But I get an error when trying to convert the <code>df['purchase']</code> to an integer:</p>
  
  <pre><code>&gt;&gt;&gt; df['purchase'].astype(int)
   ....
   pandas/lib.pyx in pandas.lib.astype_intsafe (pandas/lib.c:16667)()
   pandas/src/util.pxd in util.set_value_at (pandas/lib.c:67540)()
  
   TypeError: long() argument must be a string or a number, not 'java.lang.Long'
  </code></pre>
  
  <p>NOTE: I get a similar error when I tried <code>.astype('float')</code></p>
  
  <p>And when trying to convert to a string, nothing seems to happen.</p>
  
  <pre><code>&gt;&gt;&gt; df['id'].apply(str)
   1 abc1
   2 abc2
   3 abc3
   4 abc4
   Name: id, dtype: object
  </code></pre>