[issue24800] Incorrect handling of local variables in comprehensions with exec()

[Peter Eastman]

New submission from Peter Eastman:

The following script demonstrates a bug in the exec() function in Python 3.4.  
(It works correctly in 2.7).

script = """
print(a)
print([a for i in range(5)])
"""
exec(script, globals(), {"a":5})

It produces the following output:

5
Traceback (most recent call last):
  File "test.py", line 5, in 
exec(script, globals(), {"a":5})
  File "", line 3, in 
  File "", line 3, in 
NameError: name 'a' is not defined

The variable "a" is getting passed to the script, as expected: printing it out 
works correctly.  But if you use it in a comprehension, the interpreter claims 
it does not exist.

--
messages: 248064
nosy: Peter Eastman
priority: normal
severity: normal
status: open
title: Incorrect handling of local variables in comprehensions with exec()
type: behavior
versions: Python 3.4

___
Python tracker 
<http://bugs.python.org/issue24800>
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue24800] Incorrect handling of local variables in comprehensions with exec()

[Peter Eastman]

Peter Eastman added the comment:

I don't believe that explanation is correct.  You can just as easily get the 
same problem without explicitly passing a map to exec().  For example:

def f():
script = """
print(a)
print([a for i in range(5)])
"""
a = 5
exec(script)

f()

The documentation for exec() states, "In all cases, if the optional parts are 
omitted, the code is executed in the current scope."  Therefore the code above 
should be exactly equivalent to the following:

def f():
a = 5
print(a)
print([a for i in range(5)])

f()

But the latter works and the former doesn't.  Contrary to the documentation, 
the code is clearly not being executed in the same scope.

--
resolution: not a bug -> 
status: closed -> open

___
Python tracker 
<http://bugs.python.org/issue24800>
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue24800] Incorrect handling of local variables in comprehensions with exec()

[Peter Eastman]

Peter Eastman added the comment:

Then fix the documentation.  This behavior directly contradicts the 
documentation of the exec() function.  The question is not what scope the 
comprehension runs in, it's what scope the script runs in.  See my third 
example.  A comprehension in the f() function has no problem seeing local 
variables defined in that function.  If the script were running into the same 
scope as that function, then comprehensions inside the script would also see 
those variables.  They don't, clearly demonstrating that the script does *not* 
run in the same scope, and contradicting the documentation.

--
resolution: not a bug -> 
status: closed -> open

___
Python tracker 
<http://bugs.python.org/issue24800>
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com