How to merge cells with Google Sheets API?

I am using the Python client for the Google Sheets API to build a spreadsheet. I am able to create a new sheet and update values in it, but I have been unable to merge cells for the header row that I want.

top_header_format = [
    {'mergeCells': {
        'mergeType': 'MERGE_COLUMNS',
        'range': {
            'endColumnIndex': 3,
            'endRowIndex': 1,
            'sheetId': '112501875',
            'startColumnIndex': 0,
            'startRowIndex': 0
        }
    }},
    {'mergeCells': {
        'mergeType': 'MERGE_COLUMNS',
         'range': {
             'endColumnIndex': 5,
             'endRowIndex': 1,
             'sheetId': '112501875',
             'startColumnIndex': 3,
             'startRowIndex': 0
         }
    }}
]

service.spreadsheets().batchUpdate(
    spreadsheetId=spreadsheet_id,
    body={'requests': top_header_format}
).execute()

This is the code I am running. There are no errors. The response's replies are empty and the spreadsheet doesn't have merged cells. The documentation is here.

Start indexes are inclusive and end indexes are exclusive, so by asking to merge row [0,1) you're saying "i want to merge row 0 into row 0", which is a no-op. You probably want [0,2), e.g:

top_header_format = [
    {'mergeCells': {
        'mergeType': 'MERGE_COLUMNS',
        'range': {
            'endColumnIndex': 4,
            'endRowIndex': 2,
            'sheetId': '112501875',
            'startColumnIndex': 0,
            'startRowIndex': 0
        }
    }},
    {'mergeCells': {
        'mergeType': 'MERGE_COLUMNS',
         'range': {
             'endColumnIndex': 6,
             'endRowIndex': 2,
             'sheetId': '112501875',
             'startColumnIndex': 3,
             'startRowIndex': 0
         }
    }}
]