Named replacement fields (the {...} parts in a format string) match against keyword arguments to the .format() method, and not positional arguments.

Keyword arguments are like keys in a dictionary; order doesn't matter, as they are matched against a name.

If you wanted to match against positional arguments, use numbers:

"{0} {1}".format(10, 20)

In Python 2.7 and up, you can omit the numbers; the {} replacement fields are then auto-numbered in order of appearance in the formatting string:

"{} {}".format(10, 20) 

The formatting string can match against both positional and keyword arguments, and can use arguments multiple times:

"{1} {ham} {0} {foo} {1}".format(10, 20, foo='bar', ham='spam')

Quoting from the format string specification:

The field_name itself begins with an arg_name that is either a number or a keyword. If it’s a number, it refers to a positional argument, and if it’s a keyword, it refers to a named keyword argument.

Emphasis mine.

If you are creating a large formatting string, it is often much more readable and maintainable to use named replacement fields, so you don't have to keep counting out the arguments and figure out what argument goes where into the resulting string.

You can also use the **keywords calling syntax to apply an existing dictionary to a format, making it easy to turn a CSV file into formatted output:

import csv

fields = ('category', 'code', 'price', 'description', 'link', 'picture', 'plans')
table_row = '''\
    <tr>
      <td><img src="{picture}"></td>
      <td><a href="{link}">{description}</a> ({price:.2f})</td>
   </tr>
'''

with open(filename, 'rb') as infile:
    reader = csv.DictReader(infile, fieldnames=fields, delimiter='\t')
    for row in reader:
        row['price'] = float(row['price'])  # needed to make `.2f` formatting work
        print table_row.format(**row)

Here, picture, link, description and price are all keys in the row dictionary, and it is much easier to see what happens when I apply the row to the formatting string.

You've misunderstood what '{:0{N}d}'.format(d, N=N) does. The d in the format spec doesn't refer to the variable d; it means print an integer in base 10. Python matches that field with the d argument positionally.

When you try that with an f-string, f'{:0{N}d}', f-strings have no positional arguments. Python doesn't know what to format for that field. You need to fix your original bug and place d in front of the : so Python knows it's supposed to format d there.

For reference, the correct version with format would have been '{d:0{N}}'.format(d=d, N=N).