from datetime import datetime
from dateutil.relativedelta import relativedelta

one_year_ago = (datetime.now() - relativedelta(years=1)).isoformat()[:-3] + "Z"

def lambda_handler(event, context):

    #Function in question
    def test_func(last_time: str = one_year_ago) -> str:
        return "last_time: " + last_time

    print(one_year_ago)

    #Where the function is called
    print(test_func("2019-09-20T06:28:26.000Z"))
    print(test_func())

    return 0


lambda_handler("","")

Execution result

2018-12-08T20:22:57.183Z
last_time: 2019-09-20T06:28:26.000Z
last_time: 2018-12-08T20:22:57.183Z

According to AWS Lambda's Best Practices, it is better to separate the logic. Because it was a thing, I corrected it as follows.

from datetime import datetime
from dateutil.relativedelta import relativedelta

one_year_ago: str = ""

def lambda_handler(event, context):
    global one_year_ago
    one_year_ago = (datetime.now() - relativedelta(years=1)).isoformat()[:-3] + "Z"
    print(one_year_ago)
    
    #Where the function is called
    print(test_func("2019-09-20T06:28:26.000Z"))
    print(test_func())
    return 0

#Function in question
def test_func(last_time: str = one_year_ago) -> str:
    return "last_time: " + last_time


lambda_handler("","")

--Other than the Lambda handler part, it is called only at the time of cold start [^ 1], so it is in the handler to ask for one year ago.

• Therefore, the first code itself does not work properly. --Define the variable with an empty string in advance (otherwise it will be "not defined").

What happened

If you specify a date, it works, but if you do not specify it, an error occurs. The default value seems to be disabled.

Execution result

2018-12-08T20:15:21.611Z
last_time: 2019-09-20T06:28:26.000Z
last_time: 

• The actual code was throwing a boto3 request, and the error InvalidParameterValueException [^ 2] was output.

Conclusion

The default argument is evaluated only the first time, so the first empty string has been set. https://note.nkmk.me/python-argument-default/

I think it's not cool, but I avoided it by doing the following for the time being.
Code

from datetime import datetime
from dateutil.relativedelta import relativedelta

one_year_ago: str = ""

def lambda_handler(event, context):
    global one_year_ago
    one_year_ago = (datetime.now() - relativedelta(years=1)).isoformat()[:-3] + "Z"
    print(one_year_ago)

    #Where the function is called
    print(test_func("2019-09-20T06:28:26.000Z"))
    print(test_func())
    return 0

#Function in question
def test_func(last_time: str = "one_year_ago") -> str:
    if last_time == "one_year_ago":
        last_time = one_year_ago
    return "last_time: " + last_time


lambda_handler("","")

Execution result

2018-12-08T20:21:13.942Z
last_time: 2019-09-20T06:28:26.000Z
last_time: 2018-12-08T20:21:13.942Z

Digression

――I think there seems to be a more beautiful way of writing. ――There is a feeling of being forced to ask for a year ago. ..

  one_year_ago = (datetime.now() - relativedelta(years=1)).isoformat()[:-3] + "Z"

--Actually, I want to store the inquiry history to AWS support in DynamoDB, I'm writing this code.

  def cases_get(last_time: str = "one_year_ago") -> list:
    if last_time == "one_year_ago":
        last_time = one_year_ago
    cases_detail: list = []
    for lang in ("ja", "en"):
        for page in cases_paginator.paginate(
            includeResolvedCases=True,
            maxResults=100,
            language=lang,
            includeCommunications=False,
            afterTime=last_time,
        ):
            for cases in page["cases"]:
                cases["displayId"] = int(cases["displayId"])
                cases["yearIndex"] = int(cases["timeCreated"][:4])
                cases_detail.append(cases)

    return cases_detail