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How Python function arguments behave
When a function is called, its parameter becomes a local name referring to the object supplied by the caller. The caller’s variable and the parameter are separate names; they may refer to the same object, but there is no alias between the names themselves. The Python 3.14.8 Programming FAQ puts it plainly: “Remember that arguments are passed by assignment in Python.”
This explains both common observations: assigning a different object to a parameter does not replace the caller’s binding, while changing a shared mutable object in place can be seen outside the function.
Rebinding a parameter does not reassign the caller’s variable
def rebind(value):
value = ["new"]
items = ["old"]
rebind(items)
print(items) # ['old']At the call, both items and the local name value refer to the original list. The assignment inside rebind makes value refer to a new list instead. It does not change what items refers to, so the caller still sees ['old'].
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Mutating a shared object can be visible to the caller
def mutate(value):
value.append("new")
items = ["old"]
mutate(items)
print(items) # ['old', 'new']Here, value and items refer to the same list when append runs. The method changes that list in place, so the change is visible through the caller’s name too. The result comes from sharing a mutable object—not from the function changing the caller’s variable binding.
Why mutability matters—and what it does not mean
Mutability describes whether an object can be changed in place; it does not select a different argument-passing rule. Python binds parameters to objects the same way regardless of whether those objects are mutable. With an immutable object, an operation that appears to produce a changed value instead yields a different object or value; rebinding the parameter still leaves the caller’s name alone. With a mutable object, an in-place operation can alter the shared object.
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Containers can also combine these cases: an immutable container can refer to a mutable object, and that contained object may still be changed. The Python 3.13.16 data model reference describes Python objects and mutability. The useful question is not “Is this argument passed by reference because it is mutable?” but “Is this operation rebinding a local name, or mutating an object the caller also refers to?”
Is Python call by value or call by reference?
Neither label, used without explanation, captures the behavior particularly well. In the ordinary output-parameter sense, Python is not call by reference: a function cannot rebind its parameter and thereby rebind the caller’s variable. The FAQ’s term is passed by assignment. Another teaching formulation, used in SciPy lecture notes, is that parameters are references to objects passed by value. Both descriptions point to the same practical distinction: a parameter refers to an object, but the parameter name is not the caller’s name.
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So avoid saying “mutable arguments are passed by reference and immutable arguments are passed by value.” That makes it sound as though Python changes its calling convention depending on the argument’s type. It does not; mutability only determines whether an in-place change to the object is possible.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to return updated values from a function
If a function computes replacement values, return them and assign them to the caller’s names. The Python FAQ says returning a tuple is almost always the clearest way to return multiple results:
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def updated(a, b):
return "new-value", b + 1
x, y = updated(x, y)The call-site assignment makes the caller’s rebinding explicit. A function can also communicate a result by mutating a passed mutable object, but do that when changing that object is an intentional part of the function’s behavior—not as a workaround for expecting call by reference.
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A quick way to reason about a function call
- Parameter reassigned? The local name now refers to a different object; the caller’s name is unchanged.
- Shared mutable object changed in place? The caller can observe the changed object through its own name.
- Need to replace the caller’s values? Return the new values and assign the result at the call site.
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