strange python lists behaviour while addition

Question

I tried following with python lists

a = [1,2,3]
id(a)
3072380812L
a += [1]
print id(a)
3072380812L  # Same id, which means original list is modified
a = a + [1]
print id(a)
146238764    # Different id, which means new list is allocated and assigned to a

Why is this difference between "var += value" and "var = var + value" for python lists ?

Also see [Different behaviour for list.\_\_iadd\_\_ and list.\_\_add\_\_](http://stackoverflow.com/q/9766387) — Martijn Pieters, Dec 20 '13 at 16:31

Joran Beasley · Answer 1 · 2013-12-20T16:43:48.850

3

+= modifies(if mutable) ... as you have seen, and = assigns also as you have seen ...

both operators are overridable in the classes and their behavior can also be changed at the whims of developers... you could make = do summing if you wanted to ...

edited Dec 20 '13 at 16:43

answered Dec 20 '13 at 16:27

Joran Beasley

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I mean why is this difference ? isn't var += 1 treated as var = var + 1 in all languages ? – Arovit Dec 20 '13 at 16:29
no one modifies, the other assigns ... with ints it equates to the same since they are non mutable – Joran Beasley Dec 20 '13 at 16:32
3

"isn't var += 1 treated as var = var + 1 in all languages?" Python is a language, and it doesn't treat `+=` that way. So, obviously, no... – kindall Dec 20 '13 at 16:33

strange python lists behaviour while addition

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