title: Merging 2 files together seems to be doing a "cross" instead of "left"
body: <p>Context: I'm trying to merge 2 dataframes with similar columns based on a large unique identifier</p>
  <p>The main_file dataframe has 125.428 rows and 26 columns and the new_file has 111.597 rows and 24 columns. Both have the same column called <code>concat_pyton</code> to merge the data on</p>
  <p>I was doing this:</p>
  <pre><code>main_file.merge(new_file, how='left',on='concat_python')
  </code></pre>
  <p>or even this</p>
  <pre><code>digital_concatenated = pd.DataFrame()
  digital_concatenated = reduce(lambda  left,right: pd.merge(left,right,on=['concat_python'], how='left'), dfs)
  </code></pre>
  <p>But this gives me a dataframe with 1.547.301 rows and 49 columns which is basically repeating all the columns adding suffixes _x and _y. I just want one final dataframe merged on a unique identifier called <code>concat_python</code> with the columns based on the main_file and adding any additional rows and/or columns that may come from the new_file which are not present on the main_file</p>
  <p>Thank you for your help!</p>