Any explanation of exec's behavior? [duplicate]

Question

Looking for good explanation of why this code raises SyntaxError.

def echo(x):
    return x

def foo(s):
    d = {}
    exec(s, {}, d)
    return dict((x,y) for x,y in d.items())

def bar(s):
    d = {}
    exec(s, {}, d)
    return dict((x, echo(y)) for x,y in d.items()) # comment this to compile

s = 'a=1'
foo(s)

---

  File "test.py", line 11
    exec(s, {}, d)
SyntaxError: unqualified exec is not allowed in function 'bar' it contains a
             nested function with free variables

Answer 1

In Python 2.x, exec statements may not appear inside functions that have local "functions" with free variables. A generator expression implicitly defines some kind of "function" (or more precisely, a code object) for the code that should be executed in every iteration. In foo(), this code only contains references to x and y, which are local names inside the generator expression. In bar(), the code also contains a reference to the free variable echo, which disqualifies bar() for the use of exec.

Also note that your exec statements are probably supposed to read

exec s in {}, d

which would turn them into qualified exec statements, making the code valid.

Note that your code would work in Python 3.x. exec() has been turned into a function and can no longer modify the local variables of the enclosing function, thus making the above restriction on the usage of exec unnecessary.