Python class scoping rules

Question

EDIT: Looks like this is a very old \"bug\" or, actually, feature. See, e.g., this mail

I am trying to understand the Python scoping rules. More pr

Answer 1

First focus on the case of a closure -- a function within a function:

x = "xtop"
y = "ytop"
def func():
    x = "xlocal"
    y = "ylocal"
    def inner():
 #       global y
        print(x)
        print(y)
        y='inner y'
        print(y)
    inner()  

Note the commented out global in inner If you run this, it replicates the UnboundLocalError you got. Why?

Run dis.dis on it:

>>> import dis
>>> dis.dis(func)
  6           0 LOAD_CONST               1 ('xlocal')
              3 STORE_DEREF              0 (x)

  7           6 LOAD_CONST               2 ('ylocal')
              9 STORE_FAST               0 (y)

  8          12 LOAD_CLOSURE             0 (x)
             15 BUILD_TUPLE              1
             18 LOAD_CONST               3 ()
             21 LOAD_CONST               4 ('func..inner')
             24 MAKE_CLOSURE             0
             27 STORE_FAST               1 (inner)

 14          30 LOAD_FAST                1 (inner)
             33 CALL_FUNCTION            0 (0 positional, 0 keyword pair)
             36 POP_TOP
             37 LOAD_CONST               0 (None)
             40 RETURN_VALUE

Note the different access mode of x vs y inside of func. The use of y='inner y' inside of inner has created the UnboundLocalError

Now uncomment global y inside of inner. Now you have unambiguously create y to be the top global version until resigned as y='inner y'

With global uncommented, prints:

xlocal
ytop
inner y

You can get a more sensible result with:

x = "xtop"
y = "ytop"
def func():
    global y, x
    print(x,y)
    x = "xlocal"
    y = "ylocal"
    def inner():
        global y
        print(x,y)
        y = 'inner y'
        print(x,y)
    inner()    

Prints:

xtop ytop
xlocal ylocal
xlocal inner y

---

The analysis of the closure class is complicated by instance vs class variables and what / when a naked class (with no instance) is being executed.

The bottom line is the same: If you reference a name outside the local namespace and then assign to the same name locally you get a surprising result.

The 'fix' is the same: use the global keyword:

x = "xtop"
y = "ytop"
def func():
    global x, y
    x = "xlocal"
    y = "ylocal"
    class Inner:
        print(x, y)
        y = 'Inner_y'
        print(x, y) 

Prints:

xlocal ylocal
xlocal Inner_y

You can read more about Python 3 scope rules in PEP 3104