🔒 Default argument persistence | Python | Page 1

muted minnow Mar 4, 2026, 1:23 AM

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Good evening,

I’m seeing unexpected state persistence with a function like this:

def add_item(item, items=[]):
    items.append(item)
    return items

Calling it twice:

add_item("apple")
add_item("banana")

returns ["apple", "banana"] on the second call instead of ["banana"]. I expected the default list to be empty each time. Why does it persist and what's the canonical way to do this ?

echo turtleBOT Mar 4, 2026, 1:23 AM

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muted minnow Good evening, I’m seeing unexpected state persistence with a function like this...

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@muted minnow

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iron leaf Mar 4, 2026, 1:24 AM

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the default is created and assigned exactly once: when the function is defined

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that's actually a common foot gun and linters would flag it

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the usual thing is to have a sentinel value like None as the default, and if items is none when called create a new list

muted minnow Mar 4, 2026, 1:25 AM

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Noted. Thank you very much

haughty rivet Mar 4, 2026, 1:28 AM

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muted minnow Good evening, I’m seeing unexpected state persistence with a function like this...

Yes. This is a standard gotcha for new Python people.

The default argument is evaluated when you define the function. SO changes to that object persist.

The standard idiom is this:

def f(arg=None):
    if arg is None:
        arg = []

Not needed if the ddefault is imutable, such as a tuple or a string.

quaint badge Mar 4, 2026, 1:38 AM

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Reference: https://stackoverflow.com/questions/1132941/

if you don't actually want this, then better not to expect a mutable parameter (default or not) in the first place, and better not to mutate it (i.e. if you return a value, then only do that). https://en.wikipedia.org/wiki/Command–query_separation

The standard library strongly abides by this principle, which is also exactly why e.g. you can't chain .append calls.

Stack Overflow

"Least Astonishment" and the Mutable Default Argument

def foo(a=[]):
a.append(5)
return a

Python novices expect this function called with no parameter to always return a list with only one element: [5]. The result is different and astonishin...

Command–query separation

Command-query separation (CQS) is a principle of imperative computer programming. It was devised by Bertrand Meyer as part of his pioneering work on the Eiffel programming language.
It states that every method should either be a command that performs an action, or a query that returns data to the caller, but not both. In other words, asking a qu...

fickle island Mar 4, 2026, 2:03 AM

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quaint badge Reference: https://stackoverflow.com/questions/1132941/ if you don't actually w...

that isn't even nearly true for python and it wouldn't be practical either. when you need atomicity you more or less is forced to do both an action and return data with the same call

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take .pop() for example on lists, dicts and sets
there are many more examples where you need to do this

echo turtleBOT Mar 4, 2026, 3:05 AM

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#🔒 Default argument persistence