list comprehension in exec with empty locals: NameError

Question

Consider the following snippet:

def bar():
    return 1
print([bar() for _ in range(5)])

It gives an expected output [1, 1, 1, 1, 1]<

Answer 1

As Hendrik Makait found out, the exec documentation says that

If exec gets two separate objects as globals and locals, the code will be executed as if it were embedded in a class definition.

You can get the same behaviour by embedding the code into a class definition:

class Foo:
    def bar():
        return 1
    print([bar() for _ in range(5)])

Run it in Python 3 and you will get

Traceback (most recent call last):
  File "foo.py", line 9, in 
    class Foo:
  File "foo.py", line 15, in Foo
    print({bar() for _ in range(5)})
  File "foo.py", line 15, in 
    print({bar() for _ in range(5)})
NameError: global name 'bar' is not defined

The reason for the error is as Hendrik said that a new implicit local scope is created for list comprehensions. However Python only ever looks names up in 2 scopes: global or local. Since neither the global nor the new local scope contains the name bar, you get the NameError.

The code works in Python 2, because list comprehensions have a bug in Python 2 in that they do not create a new scope, and thus they leak variables into their current local scope:

class Foo:
    [1 for a in range(5)]
    print(locals()['a'])

Run it in Python 2 and the output is 4. The variable a is now within the locals in the class body, and retains the value from the last iteration. In Python 3 you will get a KeyError.

You can get the same error in Python 2 too though, if you use a generator expression, or a dictionary/set comprehension:

class Foo:
    def bar():
        return 1
    print({bar() for _ in range(5)})

The error can be produced also by just using simply

class Foo: 
    bar = 42
    class Bar:
        print(bar)

This is unlike

def foo():
    bar = 42
    def baz():
        print(bar)
    baz()

because upon execution of foo, Python makes baz into a closure, which will access the bar variable via a special bytecode instruction.