Re: exec and global puzzle

Steve Holden Mon, 06 Dec 2004 05:55:09 -0800

Andr? Roberge wrote:

I have the following two files:

#--testexec.py--
def exec_code(co):
    try:
        exec co
    except:
        print "error"

#-- test.py--
import thread
import testexec
import time

code = "def a():\n print 'a'\n\n" +\
       "def c():\n a()\n\nc()"

code2 = "def a():\n print 'a'\n\n" +\
       "def c():\n global a\n a()\n\nc()"

print " exec code - no global"
exec code
print " exec from thread - no global"
thread.start_new(testexec.exec_code, (code,))
time.sleep(1)
print "\n exec code2 - with global"
exec code2
print " exec from thread - with global"
thread.start_new(testexec.exec_code, (code2,))
#-----------------------

Here's the output when I execute test.py:

 exec code - no global
a
 exec from thread - no global
error

 exec code2 - with global
a
 exec from thread - with global
a
#---------
Without the global statement, I get an error when trying to execute
the code.
I don't understand why I need to use the global statement within the
definition of c() in order for it to know what a() is.  If I define
exec_code() within test.py and use it there, I do not get any error,
with or without the use of a global statement.

Andre

I have taken the liberty of restructuring your program slightly, by using a "from" to explicitly import the function you need, and by using extended string literals (""" ... """) to make the code easier to read. I have also included print statements to show the contents of the local and global namespaces after the definition of function a(). After these transformations it looks like this:

import thread
from  testexec import exec_code
import time

code = """\
def a():
  print 'a'

def c():
  a()

c()
"""

code2 = """\
def a():
  print 'a'

def c():
 global a
 a()

c()
"""

print " exec code - no global"
exec code
print " exec from thread - no global"
thread.start_new(exec_code, (code,))
time.sleep(1)
print "\n exec code2 - with global"
exec code2
print " exec from thread - with global"
thread.start_new(exec_code, (code2,))
time.sleep(1)

(OK, I added a final sleep so I saw the output from the second thread).

The reason you are seeing this behavior lies in the behavior of the exec statement. The full syntax for that statement is

exec_stmt  ::=  "exec" expression ["in" expression ["," expression]]

The second and third expressions are mappings that will be used as namespaces. Since you don't provide either, the interpreter uses the current scope (whose contents can be determined using the locals() function) for both namespaces, so it doesn't matter whether the function is added to the local or the global namespace.

When using threads, however (and, by the way, the usual advice is to use the threading module, which has a more convenient API), the namespaces are clearly different. The a() function is being added to the local namespace for the exec.

So ultimately it's to do with namespaces, as many of the more perplexing problems in Python are. I hope this helps establish exactly *why* you see what you do.

regards
 Steve
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