title: pd.concat in pandas is giving a TypeError: cannot concatenate object of type '<class 'str'>'; only Series and DataFrame objs are valid
body: <p>I have 2 dataframes. I am slicing them to create 2 sub set dataframes. After subsets are created, I want to concat those 2 subsets and then analyse the concatenated subset.</p>
  
  <p>1st subset dataframe</p>
  
  <pre><code>year_d_df = df_calender[['year']]
  </code></pre>
  
  <p>2nd subset dataframe</p>
  
  <pre><code>daily_df_train = pd.DataFrame(df_train.loc[:,'d_1':].sum(), columns = ['Count'])
  </code></pre>
  
  <p>Renaming 2nd subset dataframe indexes to make them same as sub set dataframe 1. This is a precondition to concatenation. Indexes must be same otherwise NaN values are inserted as part of concatenation process.</p>
  
  <pre><code>idx=0
  for value in daily_df_train.index:
      if idx &lt; 1914:
          daily_df_train.rename({value: idx}, axis = 'index', inplace=True)
          idx += 1
  </code></pre>
  
  <p>Above 2 subsets are created. If I do a type on them, they return as dataframe. But when I try to concatenate them, I get the TypeError. </p>
  
  <p>Concatenating subset dataframes:</p>
  
  <pre><code>yearly_trend = pd.concat(['daily_df_train','years_d_df'])
  
  TypeError                                 Traceback (most recent call last)
  &lt;ipython-input-101-e5e68495a055&gt; in &lt;module&gt;
          yearly_trend = pd.concat(['daily_df_train','years_d_df'])
  
  ----Error
  ~\Anaconda3\lib\site-packages\pandas\core\reshape\concat.py
  
  TypeError: cannot concatenate object of type '&lt;class 'str'&gt;'; only Series and DataFrame objs are valid
  </code></pre>
  
  <p>Both subsets are dataframe. They both have 1 column each which is a int64 type. Then, why is this code generating the TypeError?</p>