Python's variables aren't buckets that contain things; they're pointers that reference objects.
The way Python's variables work can often confuse folks new to Python, both new programmers and folks moving from other languages like C++ or Java.
Variables in Python are not buckets that contain things, but pointers: variables point to objects.
Let's say we have a variable
x which points to a list of 3 numbers:
>>> x = [1, 2, 3]
If we assign
x, this does something kind of interesting:
>>> y = x >>> x == y True
x is equal to the variable
y at this point,
y also have the same
id, meaning they both point to the same memory location.
>>> id(x) 140043174674888 >>> id(y) 140043174674888
This means they both point to the same object.
So if we mutate the object
x points to (by appending to that list)
x will now have
4 in it but so will
>>> x.append(4) >>> x [1, 2, 3, 4] >>> y [1, 2, 3, 4]
The reason this happens is all about the line that we wrote above:
>>> y = x
Assignment statements never copy anything in Python. Assignments take a variable name and point them to an object.
When I say variables are pointers, I mean they're not buckets that contain things.
When you do an assignment, you're pointing the variable name on the left-hand side of the equals sign (
y in this case) to whatever object is referenced on the right-hand side of the equals sign (the list that
x already happens to point to in this case).
So variables in Python are pointers, not buckets that contain things.
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