[issue14704] NameError Issue in Multiprocessing

[Mark Dickinson]

Mark Dickinson  added the comment:

Thanks for the report.

This is expected behaviour.  It isn't actually anything to do with 
multiprocessing;  it's to do with invoking exec from within a function scope.  
You can see the same effect with code like this:


code = """\
def show_name():
print my_name
show_name()
"""

def run():
my_name = "me"
exec code

run()


See

http://docs.python.org/reference/executionmodel.html#interaction-with-dynamic-features

for more explanation.

--
nosy: +mark.dickinson
resolution:  -> invalid
status: open -> closed

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[issue14704] NameError Issue in Multiprocessing

[David M. Rogers]

New submission from David M. Rogers :

Python Devs,

  There is an issue relating to variable lookup using exec from within 
multiprocessing's fork()-ed process.  I'm attempting to use the forked process 
as a generic remote python shell, but exec is unable to reach variables from 
within functions.  This issue makes it impossible to define a function which 
uses un-passed variables defined in the remote process.

  The simplest way to reproduce the error is:

--- err.py ---
from multiprocessing import Process, Pipe

def run_remote(con, name):
  my_name = name
  for i in range(2):
code = con.recv()
exec code

me, he = Pipe()
p = Process(target=run_remote,
args=(he, "Sono Inglese de Gerrards Cross."))
p.start()

me.send("print my_name") # works

me.send("""
def show_name():
  print my_name
show_name() # doesn't work
""")
--- end err.py ---

This program prints:
$ python2.6 err.py
Sono Inglese de Gerrards Cross.
Process Process-1:
Traceback (most recent call last):
  File "/sw/lib/python2.6/multiprocessing/process.py", line 232, in _bootstrap
self.run()
  File "/sw/lib/python2.6/multiprocessing/process.py", line 88, in run
self._target(*self._args, **self._kwargs)
  File "err.py", line 7, in run_remote
exec code
  File "", line 4, in 
  File "", line 3, in show_name
NameError: global name 'my_name' is not defined

I'm using Mac OSX (10.6.8) and
Python 2.6.5 (r265:79063, Sep 23 2010, 14:05:02) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin

The issue (with the same traceback) also occurs for:
Python 2.7 (r27:82500, Sep 29 2010, 15:34:46) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin


Using exactly the same set of exec calls locally results in the correct 
behavior.

--- noerr.py ---
my_name = "Sono Inglese de Gerrards Cross."
exec "print my_name"

exec """
def show_name():
  print my_name
show_name()
"""
--- end noerr.py ---

--
components: None
messages: 159764
nosy: frobnitzem
priority: normal
severity: normal
status: open
title: NameError Issue in Multiprocessing
versions: Python 2.6

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