What does None in a function mean / optional fields

Question

I am trying to use the function below. Mostly when i call this function I just pass in the report_type but a few of the report calls wants a start_date. Questions

Does the None in the definition of function mean that the field is optional

def request_report(self, report_type, start_date=None, end_date=None, marketplaceids=()):
data = dict(Action='RequestReport',
            ReportType=report_type,
            StartDate=start_date,
            EndDate=end_date)
data.update(utils.enumerate_param('MarketplaceIdList.Id.', marketplaceids))
return self.make_request(data)

I normally call the function with one of the lines below depending on if I am passing a start_date. Other then having a series of 'if' statements for each request type is there a good way to call this function and only pass a specific variable if it is populated? Over time I will be adding more optional parameters and done want a giant 'if' statement
```
requested_report=report_api.request_report(report_type=report_name, start_date=starting_date)
requested_report=report_api.request_report(report_type=report_name)
```

So how do you determine that `starting_date` is not needed? Isn't that an `if` statement too? — Martijn Pieters, Aug 11 '18 at 15:15
for each report type that I request I have a row in a mysql database. So I added a column start_date. Plan to check is string is not empty and then use the code from your answer below — personalt, Aug 12 '18 at 00:37

Martijn Pieters · Accepted Answer · 2018-08-11T15:24:08.480

The =... part gives a parameter a default value. Here that's None, and that does mean the parameter is optional when calling the function. If you don't specify it, the default is used. See Default Argument Values in the Python tutorial, as well as the reference documentation for function definitions:

When one or more parameters have the form parameter = expression, the function is said to have “default parameter values.” For a parameter with a default value, the corresponding argument may be omitted from a call, in which case the parameter’s default value is substituted.

When calling such a function, you could pass in the same value as the default, to this specific function that makes no difference; so when there is no starting_date, you can pass in None:

start_date = starting_date or None  # when determining the starting date, default to None
requested_report=report_api.request_report(
    report_type=report_name,
    start_date=starting_date
)

Another option is to set any optional keyword arguments in a dictionary, then pass those in with the ** call syntax to unpack the dictionary as keyword arguments:

kwargs = {}
if ...:  # test that decides if start_date is needed
    kwargs['start_date'] = starting_date  # expression to set the start date

requested_report=report_api.request_report(
    report_type=report_name, **kwargs)

It is fine for the kwargs dictionary to be empty.

there is so much helpful knowledge here. – personalt Aug 12 '18 at 00:19 — personalt, Aug 12 '18 at 00:19

Kishor Pawar · Answer 2 · 2018-08-11T15:31:10.013

0

It is called a default parameter.

Default parameter values are evaluated when the function definition is executed

Here is the doc on method definition.
Here is additional information

edited Aug 11 '18 at 15:31

answered Aug 11 '18 at 15:17

Kishor Pawar

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*No, It doesn't mean optional*? So what, in your book, would optional *mean*? It **is** optional in the call, in that you don't have to specify a value for that argument. – Martijn Pieters Aug 11 '18 at 15:22
I agree, let me change my answer. – Kishor Pawar Aug 11 '18 at 15:22

What does None in a function mean / optional fields

2 Answers2