[Python] Format string template defined outside of loop? [duplicate]

I'm getting odd results when defining a format string outside of a loop. I want to define a variable containing my format string like:

MYSTRING = f"filepath/{year}/{month}"


If I define it outside the loop I get repeated same value. If I copy/paste that string inside the loop it works fine.

This one goes all the way back to 2023 as expected:

last_n_months = 12
months = date.today().year * 12 + date.today().month - 1 # Months since year 0 minus 1
tuples = [((months - i) // 12, (months - i) % 12 + 1) for i in range(last_n_months)]

for item in tuples:
year = str(item[0])
month = str(item[1])
# string used in directly with the format() function
print(f"xyz/Year={year}/Month={month}".format())

------------------------
xyz/Year=2024/Month=11
xyz/Year=2024/Month=10
xyz/Year=2024/Month=9
xyz/Year=2024/Month=8
xyz/Year=2024/Month=7
xyz/Year=2024/Month=6
xyz/Year=2024/Month=5
xyz/Year=2024/Month=4
xyz/Year=2024/Month=3
xyz/Year=2024/Month=2
xyz/Year=2024/Month=1
xyz/Year=2023/Month=12


But this one repeats the same year/month for each:

# the format string
myformatstring = f"xyz/Year={year}/Month={month}"

last_n_months = 12
months = date.today().year * 12 + date.today().month - 1 # Months since year 0 minus 1
tuples = [((months - i) // 12, (months - i) % 12 + 1) for i in range(last_n_months)]

for item in tuples:
year = str(item[0])
month = str(item[1])
# use the format string variable from outside the loop
print(myformatstring.format())

------------------------
xyz/Year=2023/Month=12
xyz/Year=2023/Month=12
xyz/Year=2023/Month=12
xyz/Year=2023/Month=12
xyz/Year=2023/Month=12
xyz/Year=2023/Month=12
xyz/Year=2023/Month=12
xyz/Year=2023/Month=12
xyz/Year=2023/Month=12
xyz/Year=2023/Month=12
xyz/Year=2023/Month=12
xyz/Year=2023/Month=12


Any ideas?

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