My favourite language feature

I’ve been writing mostly Python and C for a while. This means that the features I’m used to are mostly Python ones, because C doesn’t really have any features so it’s hard to get used to.

And this means that when I write other languages I’m surprised because there are things that are ubiquitous in Python that I’ve forgotten aren’t ubiquitous everywhere. Most of them aren’t that important, but there was one that popped out at me that on thinking about I decided that I really liked and thought it was a shame that not every language implemented it.

That feature is named arguments with defaults.

It’s definitely not a Python specific feature – there are a bunch of other languages that have it – but Python is the language where I’ve really gotten used to how useful they are, and it’s far from ubiquitous.

I don’t really like Python’s implementation of it that much to be honest, but most of the flaws in it are easy enough to work around (and PEP 3102 makes life much better, or would if I didn’t have to write Python 2 compatible code), and when you do it makes it much easier to create extremely useful and flexible APIs. See my usage in hypothesis.strategies for example. e.g. the lists has 6 (ok, 5 really. The first one is only defaulted for legacy reasons) default options, all of which are useful and all of which play well together. Trying to do something similar with overloading or with different function names would be a complete horror, and really the best way to do it without default arguments is some sort of builder object which emulates them.

In my ideal world, I think this is how named arguments with defaults would work:

  1. There is a hard separation between named and positional arguments. The names of your arguments are not significant unless you declare them as named arguments, and named arguments cannot be passed positionally. A function may take both positional and named arguments, there’s just a clear delineation between the two. This is basically essential if you want it to be possible to make forwards compatible APIs.
  2. Named arguments are not required to have defaults.
  3. Positional arguments cannot have defaults (I’m not heart set on this, but it seems like a feature that is very much of limited utility and it’s cleaner to not have it)
  4. Defaults arguments are evaluated as expressions in the defining scope (not the calling scope) each time they are used. None of this ‘You can’t use [] as a default argument because it’s only evaluated once and then you’re sharing a mutable object’ nonsense from Python.
  5. Default arguments may not depend on other named argument values. Sorry. I know this is useful, but it messes with evaluation order in the calling scope really hard and it just doesn’t make sense as a feature.
  6. Optional: Default arguments may depend on positional argument values. This seems like an acceptable compromise for the preceding.

That’s pretty much all I have to say on the subject of named arguments with defaults: They’re great, more APIs should make extensive use of them where possible, and more languages should implement them.

Runners up

There are a bunch of other features that I think are great. Some of them made it on to my wish list but a lot didn’t for the same reason they didn’t claim top place for this post: They’ve already won. Support isn’t always ubiquitous, but it’s close enough that languages that don’t support them are weird and backwards outliers. Examples include:

  1. Namespaces with renaming on import available (If you don’t have renaming on import then you don’t have namespaces, you have implementation hiding and are still shoving APIs into the global scope).
  2. Local named and anonymous function definitions (“lambdas” or “closures”, but also nested functions).
  3. Parametrized types and functions over them (for statically typed languages).

I’d probably take any of those over named functions with default, but fortunately i mostly don’t have to.

This entry was posted in programming, Python on by .

One thought on “My favourite language feature

  1. Pingback: David MacIver: My favourite language feature | techcodesoft

Comments are closed.