title: searching rows with specific string from dataframe returns null in python
body: <p>I read pickledata and put into dataframe for further processing. And I found an issue when it comes to choose certain rows that contains special string. 
  Below is the first few lines of dataframe.</p>
  
  <pre><code>                              parent_pid  \
   0            UXXY-C240-M4L
   2          UXXZ-B200-M5-U
   4          UXXZ-B200-M5-U
   6          UXXZ-B200-M5-U
   8          UXXZ-B200-M5-U
                                                        pid
   0      UXXY-F-H19001,UXX-SD480G...
   2      UXX-SD-32G-S,UXX-ML-X64G...
   4      UXX-SD-32G-S,UXX-SD-32G-...
   6      UXX-SD-32G-S,UXX-MR-X32G...
   8      UXX-SD-32G-S,UXX-MR-X32G...
  </code></pre>
  
  <p>when it comes to search rows that contains "UXXZ-B200-M5-U", I used below codes.</p>
  
  <pre><code> df.query('parent_pid == "UXXZ-B200-M5-U"')
  </code></pre>
  
  <p>And below is the return. </p>
  
  <pre><code> Empty DataFrame
   Columns: [parent_pid, pid]
   Index: []
  </code></pre>
  
  <p>I used many different ways to search rows with this string, it returns the same.
  whitespace in the columns doesn't seem to matter.</p>
  
  <pre><code> df[df["parent_pid"].isin(["UXXZ-B200-M5-U"])]
   df.filter(like="UXXZ-B200-M5-U").columns
  </code></pre>
  
  <p>Does anyone know what's the issue here? </p>