Python Scopes Quirks

scopes.py

#!/usr/bin/env python

# name binding always creates a name in the local scope
ng = 'a global'
def f():
    # without the following commented out line, UnboundLocalError will throw
    # global ng
    nl = 2
    print ng, nl
    ng = 3
    print ng, nl
f()
print ng

# class variable not directly accessible from within its methods
class Foo(object):
    nclass = 'pig'

    def f(self):
        self.__class__.nclass
        # the following statement will give you UnboundLocalError error
        # print nclass
foo = Foo()
foo.f()

# for-loop leak loop variable outside the loop:
for i in xrange(10):
    print i,
print '\ni after loop:', i

# cannot rebind nonlocal variables within a nested function in Python 2
# Python 3 has nonlocal keyword that addresses this problem
a_var = 'global value'
def outer():
    a_var = 'enclosed value'
    def inner():
        a_var = 'modified enclosed value'
        print(a_var)
    inner()
    print a_var
outer()
print a_var

# try-except-finally also leaks variables
try:
    f = open('sometext.txt', 'r')
    for line in f: print line,
except Exception as e:
    print e
finally:
    f.close()
print f
print line