x = 3
x = 4
Is the second line an assignment statement or a new variable binding?
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1"variable binding" is fancy words for "assign the [..] variable".user2864740– user28647402014-10-24 08:13:27 +00:00Commented Oct 24, 2014 at 8:13
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1Can you explain what you think "assignment statement" and "new variable binding" mean?BrenBarn– BrenBarn2014-10-24 08:22:23 +00:00Commented Oct 24, 2014 at 8:22
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what I mean is that does python mutates the value of the x variable or does it throws away the first x, make a new variable and bind the new value to it.Amen– Amen2014-10-24 08:31:05 +00:00Commented Oct 24, 2014 at 8:31
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2 Answers
There is no difference. Assigning to a name in Python is the same whether or not the name already existed.
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In (very basic) C terms, when you assign a new value to a variable this is what happens:
x = malloc(some object struct)
If I'm interpreting your question correctly, you're asking what happens when you reassign x - this:
A. *x = some other value
or this:
B. x = malloc(something else)
The correct answer is B, because the object the variable points to can be also referred to somewhere else and changing it might affect other parts of the program in an unpredictable way. Therefore, Python unbinds the variable name from the old structure (decreasing its "reference counter"), allocates a new structure and binds the name to this new one. Once reference counter of a structure becomes zero, it becomes garbage and will be freed at some point.
Of course, this workflow is highly optimized internally, and details may vary depending on the object itself, specific interpreter (CPython, Jython etc) and from version to version. As userland python programmers, we only have a guarantee that
x = old_object
and then
x = new_object
doesn't affect "old_object" in any way.
