Keyword parameter

Q: How deeply does The Well-Grounded Python Developer: How the pros use Python and Flask cover keyword parameter?

Explains defaults, named calls, reordered arguments, and parameter-definition order through function examples, without examining Python implementation internals.

As explained in The Well-Grounded Python Developer: How the pros use Python and Flask

A keyword parameter is a function parameter whose value is supplied by naming the parameter and which may have a default value.

A function call is easier to use when callers can change one optional detail without supplying every other option. In Python, a keyword parameter has a name and a default value. A caller can omit it to use the default or pass a value by name. This makes the function’s inputs part of a clear API contract. section 3.1.2 section 3.2.2

Consider a name-building function with a required first name and optional middle and last names. Its parameter list can be written as `full_name(fname, mname=None, lname=None)`. A caller can supply just a first name or specify `lname` while leaving `mname` at its default. The function must handle omitted names appropriately; a default of `None` does not, by itself, format the result. section 3.2.2

Supply only the option you need

`full_name("John", lname="Smith")` supplies the required `fname` positionally and `lname` by name. `mname` keeps its default of `None`. Named arguments can also be supplied in a different order from the parameter definition. section 3.2.2

Defaults also keep an API compact. Python’s `print` accepts multiple objects and has named options including `sep=' '`, `end='\n'`, and `file=sys.stdout`. A simple call uses those defaults; a call that needs a different separator or destination changes the relevant option rather than requiring a separate printing function. When defining parameters, put required parameters before default-valued ones: after a parameter with a default, following parameters must have defaults too. section 3.1.2 section 3.2.2

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Related concepts

parameterkind of · A model parameter is a numerical value, such as a weight or bias, that is learned or set to determine how a model computes outputs.
positional parametercontrasts with · A positional parameter is a command or function parameter whose value or meaning is determined by its position among supplied arguments rather than an explicit name.
keyword-only parametercontrasts with · A keyword-only parameter is a function parameter that callers must supply by name rather than by positional placement.

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