syntax glitch with +=

[Denis S. Otkidach]

On Thu, 20 Feb 2003, Jp Calderone wrote:

JC>   Not if you use a global declaration (or if you don't,
JC> since an exception
JC> is raised and the assignment never occurs ;)

You are not right, it changes value in-place (if possible) _and_
assign it to the same variable.

x += y <=> x = x.__iadd__(y)

>>> def add(v):
...     l += [v]
...
>>> l = []
>>> add(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 2, in add
UnboundLocalError: local variable 'l' referenced before
assignment

But this works:

>>> def add(v):
...     local_l = l
...     local_l += [v]
...
>>> add(2)
>>> l
[2]

and even with nested scopes:

>>> def f():
...     l = []
...     def add(v):
...         local_l = l
...         local_l += [v]
...     add(3)
...     print l
...
>>> f()
[3]

-- 
Denis S. Otkidach
http://www.python.ru/      [ru]