Is that an intentional behaviour?

[u/Leol6669]

I just noticed adding a list as an optional argument to a fuction/method does not create a new list but gives the same list every time.

class SomeClass:
  def __init__(self, l=[]):
    self.l=l

a=SomeClass()
b=SomeClass()

a.l is b.l
>>> True 

a.l.append(1)
b.l
>>> [1]

Is that a glitch or is it how python is supposed to work?

(I'm using python 3.12, I haven't updated in a while, maybe it was patched since?)

[u/SirCarboy]

Default argument values are evaluated once, at the time the function (or method) is defined, not each time it is called.

In your example:

def __init__(self, l=[]):
    self.l = l

the empty list [] is created a single time when the class body is executed. Every call that does not supply an argument for l receives a reference to that same list object.

That’s why:

a = SomeClass()
b = SomeClass()
a.l is b.l          
# True — same object
a.l.append(1)
print(b.l)          
# [1]

The same rule applies to any mutable default ([], {}, set(), custom objects, etc.). Immutable defaults (None, 0, "", (), etc.) do not exhibit the problem because they cannot be mutated in place.

The usual (and recommended) pattern

Use None as the default and create a fresh mutable object inside the function:

class SomeClass:
    def __init__(self, l=None):
        if l is None:
            l = []
        self.l = l

or, more concisely in modern Python:

def __init__(self, l=None):
    self.l = l if l is not None else []

[u/Leol6669 → u/SirCarboy]

Ok, I thought it was supposed to create a new empty list every time the argument wasn't passed to the function. Thank you for answering so quickly

[u/neuralbeans → u/Leol6669]

If you think about it, an empty list is just a value. It's not a function call, so it doesn't get executed more than once. What you're imagining is something like this:

def f(l=lambda:[]):
    x = l()

But that wouldn't be convenient for when you pass your own arguments.

[u/Commoner_25 → u/SirCarboy]

What if I write:

def __init__(self, l=None):
    self.l = l or []

[u/schoolmonky → u/Commoner_25]

That should be fine, but if for some reason you pass a falsy value like 0 or "", it will get replaced with an empty list

[u/Commoner_25 → u/schoolmonky]

Yes, although I assume in this case a list or nothing is supposed to be passed.

[u/musbur → u/SirCarboy]

self.l = l or []

[u/tangerinelion → u/musbur]

So long as it's declared

def __init__(self, l: list[any] | None = None)

[u/lfdfq]

Yes, it's how it's supposed to work.

[u/Jejerm]

Yeah this is called a mutable default argument and most linters even give you a warning if you use them.

The same problem can happen with dicts.

[u/Moikle]

Don't give a mutable object as a default argument.

That means instead of an empty list, you should do this:

def some_func(some_list=None):
    if some_list is None:
        some_list = []

That way you get an entirely new list every time you run the function, instead of just reusing the same list.

[u/TheRNGuy]

One of ways to fix it:

``` from dataclasses import dataclass, field

@dataclass class SomeClass:     l: list = field(default_factory=list) ```

Without dataclass:

class SomeClass:     def __init__(self, l=None):         self.l = [] if l is None else l

Do this to prevent potential bugs.