A formatting expression, that is an expression of the form the_format.format(args) or format(the_format, args), can use explicitly numbered fields, like {1}, or implicitly numbered fields, such as {}, but it cannot use both. Doing so will raise a ValueError.

Use either explicitly numbered fields or implicitly numbered fields, but be consistent.

In the following example the formatting uses both implicit, {}, and explicit, {1}, numbering for fields, which is illegal.

Python Library Reference: String Formatting.