title: can not convert column type from object to str in python dataframe
body: <p>i have downloaded a csv file, and then read it to python dataframe, now all 4 columns all have object type, i want to convert them to str type, </p>
  
  <p><a href="https://i.stack.imgur.com/WeVYV.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/WeVYV.jpg" alt="enter image description here"></a></p>
  
  <p>and now the result of dtypes is as follows:</p>
  
  <pre><code>Name                      object
  Position Title            object
  Department                object
  Employee Annual Salary    object
  dtype: object
  </code></pre>
  
  <p>i try to change the type using the following methods:</p>
  
  <pre><code>path['Employee Annual Salary'] = path['Employee Annual Salary'].astype(str)
  </code></pre>
  
  <p>but dtypes still return type object,
  and i also try to provide the column type when reading csv, </p>
  
  <pre><code>path = pd.read_csv("C:\\Users\\IBM_ADMIN\\Desktop\\ml-1m\\city-of-chicago-salaries.csv",dtype={'Employee Annual Salary':str})
  </code></pre>
  
  <p>or </p>
  
  <pre><code>path = pd.read_csv("C:\\Users\\IBM_ADMIN\\Desktop\\ml-1m\\city-of-chicago-salaries.csv",dtype=str)
  </code></pre>
  
  <p>but still do not work, 
  want to know how to change column type from object to str,</p>