I am trying to understand code that uses the following expression

f"{current_file_start_time.year}{current_file_start_time.month:02}{current_file_start_time.day:02}"

Here is a minimum working example, pasted from a Spyder console:

from datetime import datetime, timezone
current_file_start_time = datetime.now(timezone.utc)
current_file_start_time.month
   Out[20]: 4
type(_)
   Out[21]: int
 f"{current_file_start_time.year}{current_file_start_time.month:02}{current_file_start_time.day:02}"
   Out[22]: '20240410'

More specifically, The month 04 is due to the component {current_file_start_time.month:02}. The relevant rules seem to be described by this reference page:

format_spec     ::=  [[fill]align][sign]["z"]["#"]["0"][width][grouping_option]["." precision][type]
fill            ::=  <any character>
align           ::=  "<" | ">" | "=" | "^"
sign            ::=  "+" | "-" | " "
width           ::=  digit+
grouping_option ::=  "_" | ","
precision       ::=  digit+
type            ::=  "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "n" | "o" | "s" | "x" | "X" | "%"

The leading 0 in {current_file_start_time.month:02} seem to be matchable to several components of format_spec above, namely: (i) [fill]; (ii) ["0"]; and (iii) and the leading digit of [width]. I can’t find a description of component (ii), perhaps it’s a special case of using "0" for fill.

What are the rules that determine whether 0 in {current_file_start_time.month:02} is used for fill, "0", or the leading digit of width? While it may not matter much in this example, I would like to not guess when composing other format specifiers?

I am using Python 3.9.

>Solution :

Reading further down in the specification:

When no explicit alignment is given, preceding the width field by a zero (‘0’) character enables sign-aware zero-padding for numeric types. This is equivalent to a fill character of ‘0’ with an alignment type of ‘=’.

So 02 it represents ["0"][width] in the format spec.

format_spec ::= [[fill]align][sign]["z"]["#"]["0"][width][grouping_option]["." precision][type]

It can’t be fill because [[fill]align] means if fill is present, align is also present.