human
Solve the following python programming problem given the following code:
def export_as_csv_action(description = None, fields = None, exclude = None, header = True) : 
    "
    This function returns an export csv action
    'fields' and 'exclude' work like in Django ModelForm
    'header' is whether or not to output the column names as the first row
    " 
    def export_as_csv(modeladmin, request, queryset) : 
        '
        Generic csv export admin action.
        based on http://djangosnippets.org/snippets/1697/
        ' 
        opts = modeladmin.model._meta 
        if fields : 
            fieldset = set (fields) 
            field_names = fieldset 
else : 
            if exclude : 
                excludeset = set (exclude) 
                field_names = (field_names - excludeset) 
response = HttpResponse (content_type = 'text/csv') 
        response ['Content-Disposition'] = ('attachment.
filename=%s.csv' % unicode (opts).replace ('.', '_')) 
        writer = csv.writer (response) 
        if header : 
            writer.writerow (list (field_names)) 
for obj in queryset : 
            writer.writerow ([unicode (getattr (obj, field)).encode ('utf-8', 'replace') for field in field_names]) 
return response 
if (fields is None) : 
        description = _ ('Export selected objects as CSV file') 
export_as_csv.short_description = description 
    return export_as_csv 

    
    

    Fix the buggy line: if (fields is None) :
assistant
#Here is Python code to fix the buggy line: if (fields is None) :
.

if (description is None) :