In Python, I’ve seen two variable values swapped using this syntax:
left, right = right, left
Is this considered the standard way to swap two variable values or is there some other means by which two variables are by convention most usually swapped?
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Answer
Python evaluates expressions from left to right. Notice that while evaluating an assignment, the right-hand side is evaluated before the left-hand side.
That means the following for the expression a,b = b,a
:
- The right-hand side
b,a
is evaluated, that is to say, a tuple of two elements is created in the memory. The two elements are the objects designated by the identifiersb
anda
, that were existing before the instruction is encountered during the execution of the program. - Just after the creation of this tuple, no assignment of this tuple object has still been made, but it doesn’t matter, Python internally knows where it is.
- Then, the left-hand side is evaluated, that is to say, the tuple is assigned to the left-hand side.
- As the left-hand side is composed of two identifiers, the tuple is unpacked in order that the first identifier
a
be assigned to the first element of the tuple (which is the object that was formerly b before the swap because it had nameb
)
and the second identifierb
is assigned to the second element of the tuple (which is the object that was formerly a before the swap because its identifiers wasa
)
This mechanism has effectively swapped the objects assigned to the identifiers a
and b
So, to answer your question: YES, it’s the standard way to swap two identifiers on two objects.
By the way, the objects are not variables, they are objects.