Python: Difference between list.extend and list.__iadd__
Question:
I thought list.extend and “+=” on list basically do the same thing – extends the list without creating new list.
I expect following code to print [42, 43, 44, 45, 46] but I’m getting UnboundLocalError: local variable 'x' referenced before assignment
Why I’m getting this error? Where is the difference?
def f():
x.extend([43, 44])
def g():
x += ([45, 46])
x = [42]
f()
g()
print x
I tried this in python2.7.3 and python3.4.0.
Answers:
+= gives an object the opportunity to alter the object in-place. But this depends on the type of x, it is not a given that the object is altered in place.
As such, += still needs to re-assign to x; either x.__iadd__() returns x, or a new object is returned; x += something is really translated to:
x = x.__iadd__(something)
Because += includes an assignment, x is marked as a local in g().
x.extend() on the other hand, is not an assignment. The programmer has decided that x is always an object with an .extend() method and uses it directly. Python sees no assignment and x is marked as a global.
I thought list.extend and “+=” on list basically do the same thing – extends the list without creating new list.
I expect following code to print [42, 43, 44, 45, 46] but I’m getting UnboundLocalError: local variable 'x' referenced before assignment
Why I’m getting this error? Where is the difference?
def f():
x.extend([43, 44])
def g():
x += ([45, 46])
x = [42]
f()
g()
print x
I tried this in python2.7.3 and python3.4.0.
+= gives an object the opportunity to alter the object in-place. But this depends on the type of x, it is not a given that the object is altered in place.
As such, += still needs to re-assign to x; either x.__iadd__() returns x, or a new object is returned; x += something is really translated to:
x = x.__iadd__(something)
Because += includes an assignment, x is marked as a local in g().
x.extend() on the other hand, is not an assignment. The programmer has decided that x is always an object with an .extend() method and uses it directly. Python sees no assignment and x is marked as a global.