Python reassign value instead of variable

Question

Is it possible to reassign the value referenced to by a variable, rather than the variable itself?

a = {"example": "foo"}
b = a

When I reassign a, it is reassigning the variable a to reference a new value. Therefore, b does not point to the new value.

a = {"example": "bar"}
print(b["example"]) # -> "foo"

How do I instead reassign the value referenced by a? Something like:

*a = {"example": "bar"}
print(b["example"]) # -> "bar"

I can understand if this isn't possible, as Python would need a double pointer under the hood.

EDIT Most importantly, I need this for reassigning an object value, similar to JavaScript's Object.assign function. I assume Python will have double pointers for objects. I can wrap other values in an object if necessary.

you will find this useful: https://nedbatchelder.com/text/names1.html — sloppypasta, Jan 22 '20 at 21:06
@Sayse That's not a distinction that exists in Python. All Python values are objects. — Patrick Haugh, Jan 22 '20 at 21:06
You could use a box type or a container (e.g. list) to hold an indirect reference. — Mateen Ulhaq, Jan 22 '20 at 21:09
Alternatively, you could lookup the variable in the `locals()` dict, though that's kind of strange. — Mateen Ulhaq, Jan 22 '20 at 21:10
for a dictionary, just clear it, and update it with new data: you keep the old reference. For a list, use `a[:] = ...` to keep the old reference. — Jean-François Fabre, Jan 22 '20 at 21:11
Possible duplicates: [this](https://stackoverflow.com/questions/13538266/python-copy-of-a-variable) [this](https://stackoverflow.com/questions/4794244/how-can-i-create-a-copy-of-an-object-in-python) — gonidelis, Jan 22 '20 at 21:12
Was this a case of the [XY problem](https://meta.stackexchange.com/q/66377), in the end? — AMC, Jan 22 '20 at 21:19
@Sayse python had no distinction between value types and reference types. Everything acts like a reference type, everything is an object. And mutability isn't relevant here, really, both immutable and mutable types behave the exact same way as far as assignment works — juanpa.arrivillaga, Jan 22 '20 at 21:32

score 2 · Answer 1 · answered Jan 22 '20 at 21:13

2

Python variables simply do not operate this way, and simple assignment won't do what you want. Instead, you can clear the existing dict and update it in-place.

>>> a = dict(example="foo")
>>> b = a
>>> a.clear()
>>> a
{}
>>> a.update({'example': 'bar'})
>>> b
{'example': 'bar'}

answered Jan 22 '20 at 21:13

chepner

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That's exactly it, thanks! `update` is the method I was looking for. – David Callanan Jan 22 '20 at 21:14

score 1 · Answer 2 · answered Jan 22 '20 at 21:14

1

You are creating 2 dictionaries, so that's 2 different objects in memory. If you don't want that, keep 1 dictionary only.

a = {"example": "foo"}
b = a
a["example"] = "bar"
print(b["example"])

answered Jan 22 '20 at 21:14

Thomas Weller

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However, I am given another dict from another function, and need to transfer that into my existing dictionary. `update` is what I was looking for in @chepner's answer – David Callanan Jan 22 '20 at 21:15

Python reassign value instead of variable

2 Answers2