Can I format decimal places by column with a dictionary?

[Mark17]

This works:

format_mapping = {markets[0]+'_Close':'{:.2f}',markets[1]+'_Close':'{:.1f}',markets[2]+'_Close':'{:.2f}', markets[3]+'_Close':'{:.4f}', markets[4]+'_Close':'{:.4f}'}
for key, value in format_mapping.items():
    merged_df[key] = merged_df[key].apply(value.format)

I did a lot of reading about df.style.format . I don't understand why this did not work:

format_dict = {markets[0]+'_Close':'{:.2f}',markets[1]+'_Close':'{:.1f}',markets[2]+'_Close':'{:.2f}', markets[3]+'_Close':'{:.4f}', markets[4]+'_Close':'{:.4f}'}
merged_df.style.format(format_dict) #WHY DID THIS NOT WORK?

I didn't get a Traceback. I just didn't see any formatting occur. Does it have to do with printing a df rather than a table?
Did I have to do something else to print out the Styler object (which did seem to be there when I typed merged_df.style and/or merged_df.style.format)?

I'm using Python 3.7.9.