Concatenating two lists - difference between '+=' and extend() Concatenating two lists - difference between '+=' and extend() python python

Concatenating two lists - difference between '+=' and extend()


The only difference on a bytecode level is that the .extend way involves a function call, which is slightly more expensive in Python than the INPLACE_ADD.

It's really nothing you should be worrying about, unless you're performing this operation billions of times. It is likely, however, that the bottleneck would lie some place else.


You can't use += for non-local variable (variable which is not local for function and also not global)

def main():    l = [1, 2, 3]    def foo():        l.extend([4])    def boo():        l += [5]    foo()    print l    boo()  # this will failmain()

It's because for extend case compiler will load the variable l using LOAD_DEREF instruction, but for += it will use LOAD_FAST - and you get *UnboundLocalError: local variable 'l' referenced before assignment*


You can chain function calls, but you can't += a function call directly:

class A:    def __init__(self):        self.listFoo = [1, 2]        self.listBar = [3, 4]    def get_list(self, which):        if which == "Foo":            return self.listFoo        return self.listBara = A()other_list = [5, 6]a.get_list("Foo").extend(other_list)a.get_list("Foo") += other_list  #SyntaxError: can't assign to function call


matomo