Question

Can someone explain to me why the following code produces the exception it does?

>>> class CallableKlass(object):
    def __init__(self, callible):
        self.callible = callible
    def __call__(self, arg):
        return self.callible(arg)


>>> class Klass(object):
    d = {'foo': 'bar'}
    m = CallableKlass(lambda x: d[x])


>>> Klass.m('foo')

Traceback (most recent call last):
  File "<pyshell#10>", line 1, in <module>
    Klass.m('foo')
  File "<pyshell#5>", line 5, in __call__
    return self.callible(arg)
  File "<pyshell#9>", line 3, in <lambda>
    m = CallableKlass(lambda x: d[x])
NameError: global name 'd' is not defined

Answer 1

The class namespace (stuff defined directly in the class body) is not accessible from within functions defined in that namespace. A lambda is just a function, so this applies to lambdas as well. Your CallableKlass is a red herring. The behavior is the same in this simpler case:

>>> class Foo(object):
...     d = {'foo': 'bar'}
...     (lambda stuff: d[stuff])('foo')
Traceback (most recent call last):
  File "<pyshell#3>", line 1, in <module>
    class Foo(object):
  File "<pyshell#3>", line 3, in Foo
    (lambda stuff: d[stuff])('foo')
  File "<pyshell#3>", line 3, in <lambda>
    (lambda stuff: d[stuff])('foo')
NameError: global name 'd' is not defined
>>> class Foo(object):
...     d = {'foo': 'bar'}
...     def f(stuff):
...         d[stuff]
...     f('foo')
Traceback (most recent call last):
  File "<pyshell#4>", line 1, in <module>
    class Foo(object):
  File "<pyshell#4>", line 5, in Foo
    f('foo')
  File "<pyshell#4>", line 4, in f
    d[stuff]
NameError: global name 'd' is not defined