I'm definitely not saying python is anywhere near perfect, and yes it certainly has design flaws (as does any complex system). But obviously it's done a lot of things right to gain the reach that it has.
Indeed it has. It is a very accessible (and even moreso "available") language.
But it has so many odd (and frankly wrong) peculiarities that big projects tend to become more difficult more quickly because of the mess that is most Python code. Some of the footguns are so rediculous that you cannot believe they exist; and even more astounding is the apologist arguments for why they should be the way they are. This one is my favorite: https://stackoverflow.com/questions/1132941/least-astonishme...
Since it is so accessible and has such a rich ecosystem, someone with little skill can cobble together a system which works well enough to get to production. But that junior developer pay rate cost (and ease of hiring) will mean that as the project matures and more and more features are added, the code more rapidly becomes incomprehensible. Everything being mutable, no types (except for bolted on, visually noisy, non-runtime linters), OOP missing some of the key information hiding features, awkward or half-baked functional features, and time/space performance problems all combine to make Python a terrible choice for non-prototype level projects.
That’s not a confusing language design. It’s a very simple case of mutability that new programmers stumble over because they don’t yet know how to reason about mutability.
But go ahead, tell us what should have been done instead here? Make it impossible to pass mutable arguments to functions? Or make it impossible to use mutable variables as default?
This should illustrate the problem:
-------------------------------------
Python 3.10.6 (main, Aug 11 2022, 13:36:31) [Clang 13.1.6 (clang-1316.0.21.2.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> def visit_city(city_name, visited=[]):
... visited.append(city_name)
... return visited
...
>>> a = visit_city("Boston")
>>> a
['Boston']
>>> a = visit_city("New York", a)
>>> a
['Boston', 'New York']
>>> b = visit_city("Dallas")
>>> b
['Boston', 'New York', 'Dallas']
>>>
--------------------------------
Any reasonable developer would expect b to => ['Dallas']. You absolutely would not expect the value of b to have been influenced by previous calls to visit_city().
Python apologists only offer one positive for this: "It is useful for memoization". That is a very edge case use, and it was not the intention when default arguments were implemented. It was an accidental side use of a misfeature.
No they are not, I’d offer you an example, but you gave one yourself. Are you confused as to what mutable and immutable means?
> You absolutely would not expect the value of b to have been influenced by previous calls to visit_city().
Yes they would. Python is not a functional language. If you have A=[1,2,3] and call A.append(4), what do you expect A to be? Same if you have a function with argument “mylist” and a default set to A, what do you expect to happen to A if you call mylist.append(4) inside that function body on a call that didn’t pass another list?
> Python apologists only offer one positive for this: "It is useful for memoization".
No, that is not the only use, if you couldn’t pass mutable arguments you would cripple the language. And if you couldn’t pass mutable defaults you’d have an odd inconsistent handling of them.
> if you couldn’t pass mutable arguments you would cripple the language
This is not about passing mutable arguments. This is about having a default value for a parameter be mutable (unreliable).
Yes the arguments in Python functions are mutable. That's not unexpected. What is unexpected is that the default value assigned to a non-provided argument is not consistent. In my example I show that the parameter "visited" has a default value of [] if no argument is passed in.
Off the top of my head, Ruby, Javascript, and Swift offer the ability to have default parameter values, and they are all predictably immutable (defaults). Python is the odd one here.
I will say no more about this here, because I'm beating a dead horse.
$a = []
def func(b=$a)
b.append 5
end
func
func
# $a is [5, 5]
Python has actual default values which while sometimes surprising when those values are mutable it is just as much a valid design choice as expressions and both permit mutation.
Default expressions have a different equally crazy footgun side effects.
$a = []
def func(b=$a.append(1))
b.append 2
end
func
func
# $a is [1, 2, 1, 2]Yes, you can stuff anything, even a reference, into the right hand side of a default parameter definition. But in 17 years of using Ruby, I've never done this, never needed to, and never seen anyone else do it. Granted I don't read source of libraries or Rails (except occasionally), so maybe _somewhere_ someone does this. I've read a few Ruby books over the years and never seen this illustrated as a thing to do, even in the Ruby Cookbook book.
(added) To be fair, one thing I have seen which is confusing to people new to programming and applies in Ruby, is if you define a constant EMPTY = [] elsewhere, and you use EMPTY as your default value for a parameter, you will get the same behavior as Python had with just []. That's because EMPTY is a constant reference to an array, but the array is mutable. This would be true for any language which is not immutable. Change the reference? No, that's immutable. Change the contents of the thing it references? Yes.
However, [] would appear to any sane person as an immediate new allocation of an empty array, not an unnamed reference to one spot in memory which gets an empty array allocated for it the first time it is encountered.
In fact, to be very literal, in the Python example the default value is only [] (empty array) once - the first time. In my visit_city() example, after the first run it is now ["some city"]. But if you read that code, and even if you are interactively debugging, the code itself of course does not change. The code still tells you, "Hey, here's a default empty array []". So this utterly fails the principle of least surprise. We expect when we read code that something which appears to be a literal value (or an expression not based on a variable) will evaluate to exactly what it appears to be.
At this point we'll just have to disagree on whether the Python behavior is "sometimes surprising" or "supremely stupid". I would simply put it this way: the use cases for Ruby's (and Swift's and Javascript's and probably any other langauge which offers default function params) fits the more common need, and that Python's opposite behavior is a footgun in a land where people very rarely intentionally want to shoot at feet.
Your final Ruby example is obvious in what it will do, and not shocking at all. And you had to go out of your way to allow that behavior. Nobody does this. No beginner would see this, in a guide or a book. No beginner would accidentally decide to do this.
But beginner and experienced developers alike would come to Python and do def foo(arg=[]):.
No you don’t. And calling a mutable value “not consistent” because it’s not immutable is just changing the words used after your mistake of immutable vs mutable. You haven’t done what you think you’ve done because you don’t understand that [] is not “a new empty list every time it is referenced”, but is just a single mutable list. So you’ve provided a default mutable value, which initially is [], but by the nature of being mutable might change in the future. And that is what gets used if there is no value provided. It’s exactly the same behavior as if you define a named variable with an initially empty list, and pass that list to the default. If you want it to be initiated to [] every time no argument is passed, then you need to initiate a new mutable list as the argument on each call. Just like you wouldn’t be able to define a named list once as empty, and expect every call of .append() to give you a new list with only one element. You’ve passed a single list as the default, not a list imitator, or some crazy “spawns new empty lists on references” object.
And yes, you could have default arguments only support immutable values, then you would not be able to pass in a whole range of objects in defaults.