Write a Date Formatting Function

EasyPython

Problem

MRM campaign calendars store dates internally as YYYY-MM-DD, but different campaign surfaces may need to display them in a specified field order. Write a function that rearranges the date fields and joins them with the requested separator.

Formal Specification

Implement format_date(date, order, separator), where:

date is a string in the format YYYY-MM-DD.
order is a three-character string containing Y, M, and D exactly once each.
separator is a non-empty string used between output fields.
The date is guaranteed to be valid, and its year, month, and day must remain zero-padded.
Return a string containing the fields in the requested order.

Do not use locale-dependent date formatting. The function should work directly from the date string.

Constraints

date always matches YYYY-MM-DD.
1000 <= YYYY <= 9999.
01 <= MM <= 12.
01 <= DD <= 31, with valid calendar dates guaranteed.
order contains Y, M, and D exactly once each.
1 <= len(separator) <= 3.

Function Signature

def format_date(date, order, separator):

CodePython 3

Run your code to see test output here.

Formal Specification

Implement format_date(date, order, separator), where:

date is a string in the format YYYY-MM-DD.
order is a three-character string containing Y, M, and D exactly once each.
separator is a non-empty string used between output fields.
The date is guaranteed to be valid, and its year, month, and day must remain zero-padded.
Return a string containing the fields in the requested order.

Do not use locale-dependent date formatting. The function should work directly from the date string.