title: SQLAlchemy warning Textual column expression should be explicitly declared?
body: <p>I keep getting this warning and no matter what can't seem to get rid of it (besides surpressing it):   </p>
  
  <pre><code>C:\...\site-packages\sqlalchemy\sql\elements.py:4390: SAWarning:
  Textual column expression 'column_name' should be explicitly declared
  with text('column_name'), or use column('column_name') for more
  specificity 
  
  if guess_is_literal else "column"
  </code></pre>
  
  <p>I build a list of Column() objects (column name + data type) in one metadata context, and later in another metadata context create a table using this list. While this works, it does give this warning. I've tried:</p>
  
  <ul>
  <li>storing it as a "quotedname"</li>
  <li>casting the column to a "ColumnClause", using column()</li>
  <li>casting the column to a "TextClause", using text()</li>
  <li>casting the column to a String, using str()</li>
  </ul>
  
  <p>No matter what, I still get the warning.</p>
  
  <p>Here are a few snippets of the Python code:</p>
  
  <pre><code>for col_name in self.cols_source:
              print(meta.tables[self.table_name].c[col_name].name)
              print(type(meta.tables[self.table_name].c[col_name].name))          #quotedname
              print(type(column(meta.tables[self.table_name].c[col_name].name)))  #ColumnClause
              print(type(text(meta.tables[self.table_name].c[col_name].name)))    #TextClause
              print(type(str(meta.tables[self.table_name].c[col_name].name)))     #Str
  
              #source_query_cols.append( Column( name=meta.tables[self.table_name].c[col_name].name, type_=meta.tables[self.table_name].c[col_name].type ))
              #source_query_cols.append( Column( name=column(meta.tables[self.table_name].c[col_name].name), type_=meta.tables[self.table_name].c[col_name].type ))
              #source_query_cols.append( Column( name=text(meta.tables[self.table_name].c[col_name].name), type_=meta.tables[self.table_name].c[col_name].type ))
              source_query_cols.append( Column( name=str(meta.tables[self.table_name].c[col_name].name), type_=meta.tables[self.table_name].c[col_name].type ))
  </code></pre>