I want to highlight a cell using python with this code.

```
writer = pd.ExcelWriter('4.xlsx', engine='xlsxwriter')
df.to_excel(writer, sheet_name='Sheet1')        
writer.save()

Num_cols = len (df10.TIME1)
print(Num_cols)
for k in range(0, Num_cols):
    a=0
    if df10["TIME1"][k]!=df10["TIME2"][k]:
        b=str(k)
        a='E'+b
        print(a)
        print(df10["TIME1"][k],df10["TIME2"][k])
        writer = pd.ExcelWriter('4.xlsx', engine='xlsxwriter')
        df10.to_excel(writer, sheet_name='Sheet1')
        workbook  = writer.book
        worksheet = writer.sheets['Sheet1']
        worksheet.conditional_format(a, {'type': '2_color_scale'})
        writer.save()

```

But it didn't work with this code.If i use this line

```
worksheet.conditional_format('E3', {'type': '2_color_scale'})

```

instead of this line.E3 is highlighted.

```
worksheet.conditional_format(a, {'type': '2_color_scale'}) 

```

My cell number is variable. I tried also defining "a" value like this `a="'"+a+"'"` but i get error like that `AttributeError: 'NoneType' object has no attribute 'group'`

Thanks for your answer.